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4 Oct. 29, 1946.
s. D. EILENBERGER
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2,410,275 ‘
SIGNALING BY SUB-MODULATION
Filed Dec. 13, 1941
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INVENTOR
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0a. 29, 1946.
s; 0. EILENBERGER
2,410,275 _
SIGNALING BY SUB-MODULATION
Filed Dec. 13, 1941
3 Sheets-Sheet 2
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INVENTQR
.BY
ATTORNEY
2,410,275
f’atented Oct. 29, 19:46
UNITED STATES PArENrorFici:
SIGNALING BY SUBMODULATION
Stanley D. Eilenberger, Kenosha, Wis., assignor,
by mesne assignments, of sixty-?ve percent to
S. E. Steen, Kenosha, Wis., seventeen and one-_
half percent to L. G. Voorhees, Macedonia,
Ohio, and seventeen and one-half percent tov
Howard W. Taft, Balboa Heights, 0. Z.
Application December 13,1941, Serial No. 422,915
22 Claims. (01. 250—6)
1
2
This invention relates to a methodof signaling ~
where the main energy is con?ned within the
carrier. It is particularly applicable to the trans
mission of intelligence by wire and radio and
wave in which the modulation factor is increased
in ‘successive steps prior to demodulation.
Sixth, to provide a method of reception of a
radio-frequency carrier‘
wave in which the modulation factor is increased
frequency-modulated
when so applied, results in an economy of the
frequency space required for such transmission. . ,?. at the receiver by reducing the mean carrier fre
Brie?y the invention consists of a method of
frequency modulation in which the carrier fre
" quency to a value less than the frequency of the
signal wave recovered by demodulation.
Seventh; to provide a method of communication
10 by frequency modulation by which the modula
nal frequency. The carrier so modulated may be- . tion factor may be controlled at the receiver.
quency is varied through a frequency range nar
rower than the bandwidth of the modulating sig
the radio-frequency carrier bylwhich radio trans-_
4.
Eighth, ‘to provide a method of communication
mission is effected, a low-frequency carrier of a
which‘ sha'll’provide a high degree of freedom
mean frequency less than the frequency of the
from all undesired signals, such as noise, static,
modulating signal, or an intermediate carrier 15 etc.
from which the low-frequency carrier is produced.
Ninth, to provide a ‘method of secret commu
The low-frequency carrier may be transmitted
H“ nication.
-
directly over wire lines or used to modulate a
Tenth, to provide a method of controlling the
higher carrier frequency for radio transmission
depth ‘of modulation in a frequency-modulation
as hereinafter described. In such a frequency 20 system so that the e?iciency of recovering the
modulated wave the highest density of energy
original signal’ at the receiver may be uniform
will always be in the carrier wave.
ly high.
'
'
'
, Among the numerous objects of this invention
Eleventh, to provide a method of demodulation
are:
in a frequency modulated system so that a small
First, to provide a method of producing a modu
lated signal wave where the main energy is con
?ned to the carrier and the amplitude of the side
bands is inversely proportional to the signal fre
quency, thus reducing the interference poten
tialities of the sidebands so that the frequency
channel required by this modulated signal wave
duringtransmission by means of wire line or
radio shall be much smaller than the original
signal bandwidth, thus providing a means of
communication in a very narrow channel.
'Second, to provide a method of frequency
modulation of a radio-frequency carrier where
the bandwidth occupied by the main energy of
themodulated radio-frequency carrier is less than
the bandwidth of the original signal wave.
.
radio-frequency
Twelfth, to provide a method of frequency
modulation in which a high order of frequency
stability is obtained.
.
Thirteenth, to provide a method of combin
ing two or more of the above advantages into a
single system.
-
It is understood that a basic purpose of this
invention is to provide a means of controlling the
35 energy concentration ratio in an electric wave
asset forth in object one above, and that such
control is always obtained in combination with
one or more other features of this invention,
For the purposes of this application, the term
sub-modulation is understood to mean a condi
Third, to provide a method of reception ofa
frequency-modulated
25 deviation ratio at the transmitter is made to ap
pear as a large deviation ratio at the receiver.
carrier
tion where the lowest frequency of the modulat
ing signal wave is higher than the carrier wave;
this low frequency carrier is referred to as a
subcarrier, and the‘ process of modulating such a
and. frequency range.
‘
45 carrier is referred to as sub-modulation, Such
7 .Fourth, to provide a method of reception of a
a sub-modulated subcarrieris always obtained
frequency modulated radio-frequency , carrier
in combination with one or more other features
wave in which the modulation factor is increased
of this invention.
'
wave as described, that will permit full recovery
of the original signal wave both as to amplitude
prior to demodulation.
.
The following mathematical analysis of the
Fifth, to provide a method of reception of a 50 frequency modulation ‘process and the accom
frequency-modulated radio-frequency carrier
panying explanation of the manner in which
2,410,275
4
various factors are treated in this invention will
serve to show how this invention differs from
Equation 3 represents a familiar expression of a
modulated wave containing both the carrier and
prior frequency modulation methods, where
l\T:21.-n denotes the signal frequency.
F=21rf, the frequency modulated carrier.
Ar the amplitude of the unmodulated carrier F.
the two side band frequencies at a distance cor
C11
m the modulation index de?ned by the equation.
responding to the signal frequency. In contrast
to amplitude modulation, the side band compo
nents have opposite signs and are 90° out of
phase with the carrier. The amplitude of the
side bands is in the ?rst approximation propor
tional to the modulation index as de?ned above.
10 Hence, for any given width of the frequency
swing between the two edge frequencies fa and ft,
the amplitude of the side bands will become
where fa and it are the edge frequencies between
which the carrier F is shifted at the rate of the
smaller as the signal frequency becomes higher.
This is obvious because the modulation index 112
for a given frequency swing is inversely propor
signal frequency.
A frequency modulated wave is represented by
the following equation:
'
A=Af cos [FM-12L sin Ni]
tional to the signal frequency.
It can therefore be stated: in a frequency mod
(1)
ulating system where the modulation index is
small the main energy will be con?ned to the
It is well known that Equation 1 can be ex
panded in a series to be represented by the 20 carrier and furthermore, the amplitude of the
side bands will become lower with increasing
Bessel function
signal frequency. In contrast .to ordinary am
71:00
plitude modulation where the amplitude of the
A=l:lo(m) sin Ft+22[;,,(m)
cos 2nFt+
12:1
side bands is only determined by the depth of
2211 (QM-1) (m) sin (2n,—l)Ft:|A; (1a)
25 modulation and is independent of the value of
In:
In this series the term with the coeflicient Io rep
resents the carrier corresponding to the Bessel
the signal frequency and where, therefore, the
interference of adjacent channels is solely de
pendent upon the highest frequency present in
the sideband,v a frequency modulated system in
function of the zero order.
The side bands are 30 which the modulation index m is small presents
a case wherein the amplitude of the interfering
of the Bessel function of the ?rst and nth order.
signal from the adjacent channels will continu
It can be shown that if the argument m becomes
ously decrease as the frequency limits of the side
small then the series converges very rapidly and
band are approached. In the commonly used
all the terms having the coefficients of the higher 35 ‘systems of frequency modulation, the modula
order function will disappear. In such a case
tion index is always larger than unity, and therethe coefficient In of the ?rst term representing
fore, the energy distribution as given by the
represented by the terms having the coefficients
the carrier will contain the greater part of the
expansion of the frequency modulated wave into
energy contained in the entire series.
the Bessel function series shown in Equation
For purposes of illustration Equation 1 can be 40 1a, shows that where m is larger than unity, a
greater part of the energy will be contained in
transformed into
the ?rst and nth order terms representing the
sidebands. In a system according to this inven
tion, m is always smaller than unity, and there
It may be assumed that m is small, for instance,
fore, the greater part of the energy will remain
smaller than 0.3. In this case
in the carrier.
A=Af[cos (Ft) cos (“gen Nt)——sin (Ft) sin (N01 (2)
In the discussion above no statement was made
7-3 Sin N't
will be smaller than
m
2
itself because the sine function is under all cir
cumstances smaller than unity. Hence, the value
of the cosine function
(3
m
__
.
concerning the relationship between the frequen
cies F and N. Under ordinary circumstances
the frequency F is largev in comparison to fre
quency N. However, it can be easily shown that
if the frequency F modulated with the frequency
N should form part of a beat frequency, an ex
pression similar to Equation 3, for the amplitude
of the resulting beat frequency B can be obtained
in which
0S ( 2 sin N t)
.
can be approximated very well with unity with
out making any great error. For similar reasons,
60
if m is small, the function
sin
The resulting beat frequency signal thus has the
same characteristics as the original frequency
modulated signal and contains the same signal
frequencies in the form of sidebands. By proper
selections of the values F and F1, a condition can
be arrived at in which E is small in comparison
sin N t)
can be written as
in substituting the sine function by the angle
to N.
s
itself. With this approximation Equation 2 will
The invention will be best understood from a
be transformed into
70 consideration of the following detailed descrip
A=Af[cos Ft—% sin (Ft) sin (N01:
. Afcos (Fz)+@2A, [cos (pimps-3251, [cos (F—N)t]
(3)
tion in view of the accompanying drawings form
ing a part of the speci?cation; nevertheless, it is
to be understood that the invention is not con
?ned to the disclosure, being susceptible of such
changes and modi?cations as de?ne no material
2,410,275 -
5
Plate 6 is coupled to control grid ,8 through a
departure from the salient features of the inven
tion as expressed in the appended claims.
variable compression condenser l8 having a‘
maximum capacity of approximately ‘100 paf. '
‘ In the drawings:
and resistor l9'which~ is approximately 150,000
ohms. 20 is a resistance of .5 megohm and 4 is‘
also a resistance of .5-megohm. I2 is a'cathode
radio frequency wave in accordance with this
resistorof 300 ohms and I3 is a cathode ‘by-pass‘
invention.
'
condenser of .01 ,uf. I4 is a series screen r'e
Figure 2 shows in block form a receiver for use
sistor of 30,000 ohms which is by-passed by mica‘
with the transmitter of Fig. 1.
Figure 3 shows in block form a radio transmit 150K condenser l6 of .01 pf. and also electrolytic con
denser !5 having a capacity of 8 cf. 2| is a
ter for the amplitude modulation of the radio
variable compression mica condenser having a
wave by the new modulating wave produced in
accordance with this invention.
maximum capacity of 30 lief” while 3 is the com
‘Figure 1 shows in block form a radio transmit
ter arranged to produce a frequency-modulated
' Figure 4 shows in block form a receiver for use
mon ground connection and ‘AZ-23 are connec
with the transmitter of Fig. 3.
Figure.5 shows in block form a modi?cation of
apart of the transmitter of Fig. 3.
tions to the oscillator tuned circuit. The audio
frequency signal applied to the modulator grid 7
‘ Figure 6 illustrates in detail an arrangement
of the reactance ‘modulator tube 9 will cause a‘
phase shift in the resistance capacity network
which may be used for modulator unit C in the
made 7 up by condensers
l8—2l and resistors
transmitters of ‘Figs. 1, 3, 5 and unit C1, in the 203 l9—-20, which will be proportional to the ampli
tude of the signal applied, and the number of
receiver of Fig. 2.
'
shifts per second will be proportional to the fre
Figure 7 illustrates the frequency versus volt
quency of the signal applied. This exerts a tun-v
age characteristic of discriminator unit R in the
ing effect on controlled oscillator D.
'
, ’
receivers of Figs. 2 and 4.
Figure 8 illustrates a detailed arrangement of‘ 25 Modulator unit C is arranged to shift the fre-~
quency of oscillator D from its mean frequency
discriminator unit R in the receivers in Figs. 2
and 4.
of, for example, 1,000,000 C. P. S. through a range
Figure 9 illustrates in detail an arrangement
of 20 C. P. S. plus and 20 C. P. S. minus, a total
carrier shift of 40 C. P. S. , equal to a maximum
for use in ?lter units G of Figs. 2, 4 and 5.
Figure 10 illustrates the frequency response 30 carrier frequency of 1,000,020 ‘and a minimum
carrier frequency of 999,980 C. P. S. The maxi
curve of ?lter unit G and discriminator unit R.
mum shift will then correspond to the maximum
Figure 11 is a curve showing the distribution
amplitude of the original signal, and the total
of the energy in the sidebands for two cases, with
number of shifts per second will be equal to the
a total frequency deviation in one direction of 100
cycles and 1000 cycles as a function of the signal 35 frequency of the original signal, the entire opera;
frequency.
tion being identical with that commonly em
ployed in conventional frequency modulation, ex
cept that the total carrier shift is extremely small,
Figure 12 is a similar energy distribution curve
showing the energy content in the ‘sidebands if
as compared to the usual shift of '75 kc. plus and
the deviation from the carrier is 20 cycles in one
direction.
40 '75 kc. minus, or a total shift of 150 kc. The nar
_
Figure 13 illustrates the percentage change of
row deviation frequency-modulated radio-fre
quency wave is then ampli?ed by a radio-fre
quency ampli?er I and connected to the antenna
. the instantaneous values of the unmodulated
carrier due to modulation.
_ Figure 14 shows an exaggerated curve repre
senting one quarter cycle of a frequency modu
lated wave modulated with a, signal frequency ten
times the carrier frequency.
'
Figure 15 shows the attenuation of the inter
fering signals originating in channels adjacent to
the carrier as a function of carrier separation.
. Referring more more particularly to Fig. 1, A is
a source of original signal, which may be a micro
phone having an original audio frequency input
covering the range of 250 to 3000 C. P. S., while B
is an audio frequency ampli?er of conventional
type, which may be any ampli?er such as is in
_ K.
L is a conventional power supply for the
transmitter.
-
’
Referring to the receiver block schematic in
Fig. 2, K represents the antenna‘, vN the R. F.
ampli?er, O the convertor, all of which are con
, ventional; P represents the I. F. ampli?er which,
for example, is tuned to a resonant frequency
of 465,000 C. P. S. N and P are tuned to accept
the carrier and both sidebands as in conventional
practice. E represents a ?xed R. F. oscillator,
tuned to a ?xed frequency of 464,900 C. P. S.,
55 which is designed to have a relatively high am
plitude, in comparison to the output of I. F. am
pli?er P, which it is beating with. With static
common use; C is a modulator unit which might
conditions'at the transmitter of Fig. 1, the mean
be of the magnetically controlled condenser type,
beat frequency developed in the mixer-ampli?er
or of the so-called reactance modulator type
illustrated by Fig. 6. The object of modulator 60 unit F is 100 C. P. S., which is the beat between
unit C is to provide a capacitive or inductive var
iation as a padder tuning element of oscillator D.
The reactance vmodulator circuit illustrated by
Fig. 6 accomplishes modulation electronically
and is the preferred form. The circuit shown by 65
Fig. 6 is approximately the same as used in cer
tain frequency modulation’ systems of COIlVBll-r.
the ?xed oscillator unit E and the I. F. output of
unit P. As the transmitter of Fig. 1 is modulated;
causing a frequency shift of 40 cycles maximum
(with a signal at unit A of maximum amplitude),
the I. F. at the output of unit P shifts by a like
amount,‘ thus causing a similar shift in the beat
note frequency developed in unit F, from a mini
tional type, where the'ampli?ed audio frequency
mum value of 80 to a maximum value of 120
signal from the output of ampli?er unit B is sup
plied to the input connections I and 2 and is
applied to modulator grid 1 of vacuum tube 9
which‘ might be‘ commercial type 6L7. B is the
C. P. s., the total shift being dependent on the
amplitude of the original signal, and the number
of shifts per second being dependent on'the fre-v
quency of the original signal, as has been previ
ously shown for the carrier and I. F. waves. This
shifting frequency, 80 to 120 C. P. S., is now acting
as an audio-frequency carrier. Thisfrequency~
plate terminal of vacuum tube 9 which is fed
through radio frequency choke 5 from a source
of voltage II, which is approximately 250 volts.
7
$2,410,275
8
modulated audio-frequency carrier is then passed
from the unmodulated wave in percent of the
through ?lter G to attenuate impressed frequen
cies having a frequency greater than 120 C. P. S.,
such as harmonics and the other modulation prod
ucts of the mixer-ampli?er F. This audio fre
quency carrier wave is hereinafter referred to as
instantaneous value of the unmodulated-wave can
of the unmodulatedv wave as a basis of compari
son. Figure 13 representing the percentage
the subcarrier wave.
change of a 100 cycle carrier modulated with a
be calculated simply by forming the difference be
tween the two and taking the instantaneous value
signal frequency of 1000 cycles and with a modu
Fig. 9 illustrates in detail a parallel resonant
lation index of 0.01 was calculated in such a way.
?lter which I have used in practice as unit G of
Fig. 2. Referring more particularly to Fig. 9, 10 It might be seen that in the ?rst period of
0.00025 second the maximum deviation is only
59 and 50, represent the input terminals, from
minus 0.08 percent against the unmodulated
ampli?er-mixer unit F; B! is a transformer,
wave. This deviation reaches its maximum value
whichmight well have 2. turns ratio 1:1 and a
of plus and minus 3.3 percent at; a distance equal
secondary, inductance 63, which might have a
value of approximately 4 henrys; condenser 65 15 ing a quarter cycle of the signal frequency be
fore and after the quarter cycle of the carrier
would then have an approximate value of .25 ,lf.,
frequency.
so that the resonant circuit represented by in—
ductance B3 and capacity 55 would be approxi
If the resonant circuit G shall follow the mod
ulation according to Equation 5, the Q factor
mately in resonance at 100 C. P. S. Primary in
ductance 62 could have a like value of inductance, 20 of the circuit as de?ned by the Equation 6 must
be chosen accordingly.
and might also be tuned to resonance with capac
ity 64, if so desired, although it is not essential
that the primary inductance be in resonance;
vacuum tube 12 is used as a coupling element.
68 represents the common ground connection, 25 In (6), Rp denotes the equivalent parallel loss re
sistance of the circuit in ohms, C the capacity in
while 66 and 6'.’ represent the output terminal
farads and L the inductivity in henrys. If at any
connection to frequency discriminator R, or to
moment the source of energy supply in circuit G
succeeding stages of filter G. As previously men
should be disconnected, the voltage E across the
tioned such a, ?lter as illustrated by Fig. 9 will
attenuate all impressed frequencies having a fre 30 circuit would not fall to zero immediately, but
due to the stored energy in the circuit would drop
quency greater than the pass band character
according to the expression
istic of the tuned resonant circuit to a far greater
degree than it will attenuate the high signal fre
;_—B£
quency modulation component of the 100 cycle
E: Eoe 2Q Sin (Bt+¢)
(7)
carrier Wave, which it is, desired to pass on to fre 35
in
which
e
denotes
the
base
of
the
natural
loga
quency discriminator unit R.
rithms.
The following will clarify the action of a par
allel resonant circuit as used in ?lter G and illus
trated by Fig. 9, in response to an impressed fre
quency modulated wave in which the modulating
signal frequency is of a, higher order than the
carrier frequency. Filter unit G is resonant at
If the circuit is connected to a source
of energy with the terminal voltage E0 sin Bt,
maximum amplitude will not be reached imme
diately, but again according to the function
Obviously, if any changes due to modulation
the carrier frequency, which, in the example
should take place in the instantaneous value of
given, is 100 cycles. In a frequency modulated
45 the carrier wave, then the Q factor and the con
wave represented by the equation
stants of the circuit must be such that if, for in
stance, the circuit was instantaneously discon
nected from its source of voltage, the voltage
would change by a higher percentage than that
the value of the unmodulated function Ab cos ‘.Bt
will coincide with the value of the modulated 50 indicated by the modulation index m. For in
stance, for the values shown in Fig. 13, this would
wave for any time t at which Nt=21rnt is an
mean that at the point of largest difference be
integer multiple of 1r. Between those points of
tween modulated and unmodulated wave form
coincidence the modulated wave will deviate from
the resonant voltage of the circuit must decay by
the values of the unmodulated wave determined
more than 3.3% of its actual value within a time
by the magnitude of the modulation index m.
A=Ab<cos Bt-l-WZL sin Ni)
(5)
If B is small in comparison to N, and m is small,
then the frequency modulated wave correspond
ing to Equation 5 will attain a form as shown in
Fig. 14. In this ?gure a quarter cycle of the car
rier frequency B is shown modulated by a fre
quency N equalling ten times B, where 69 is the
unmodulated carrier wave 13 and ‘I0 is the modu
lated wave form, ‘H representing ?xed points
where the modulated and unmodulated waves
coincide; the modulation index m is then 0.01.
At any point where modulation occurs the slope
of the wave will change to a slope corresponding
to a higher or lower frequency by an amount de
termined ‘by the depth of modulation. The maxi
mum deviation can be expected at the point where
the slope of the unmodulated wave is a maxi
mum. For a cosine function this will be at a,
quarter cycle of the, wave B. If the function in
Equation 5 is computed for a given set of values
13, N and m, the deviation of the modulated Wave
of 0.00025 second. In a generalized form the con
dition
—1rB/
60
cos Bt
cos Bt-cos (3H1; sin Ni) [
Q N]
—_P1r
P~T< 1*” Q
(9)
65 must be ful?lled if the signal frequency n shall
be passed. In the above expression p denotes the
ratio between signal and carrier frequency. For
instance, with a carrier frequency of 100 cycles
resonant with the circuit G and a modulation in
70 deX of 0.01, the maximum signal frequency which
could be passed is 1000 cycles provided the Q of
the circuit is not higher than 8.7. If the Q is re
duced to, for instance, 2.9, the maximum fre
quency which the circuit will transmit as a fre
75 quency modulated signal willbe increased to 3000
2,410,275
10
cycles. This system, is, for inherent reasons and
for its practical purposes, limited to low values
of modulation index. For instance, if the modu
carrier frequency deviates from its mean fre
quency, and that the audio frequencies recovered
depend upon the rate at which the carrier fre
lation index should be increased to 0.1, the max
quency deviates from the mean frequency.
imum signal frequency which could be passed
In agreement with the mathematical discus
sion above, Fig. 10 shows average physical char
with a carrier and a resonant circuit G of 100
acteristics of units G and R. In this figure the
cycles and a Q of 8.7 would be limited to 110 cycles.
line A represents the attenuation of the resonant
The output of ?lter G is connected to the input
filter G as a function of frequency, taking the
24 and 25 of the discriminator unit R, which- is
a conventional demodulator circuit commonly 10 carrier resonant frequency as zero level and ap
plies to frequencies not associated with the de
called a frequency discriminator, tuned to a mean
sired frequency-modulated subcarrier. 'Curve A
frequency of 100 C. P. S. The values of the circuit
was arrived at by impressing a constant ampli
elements required for the proper functioning of
tude variable frequency across input terminals 59
the demodulator in the desired frequency range
and 60, and measuring ?lter response across out
are disclosed in connection with the detailed de
put terminals 66 and 67. Referring to the same
scription of Fig. 8, where two iron core trans
zero level, curve vB indicates the frequency re
formers 26 and 21, with the primaries 28 and 29
sponse of the ?lter G to sidebands associated with
connected in series by connecting link 30 have
the frequency-modulated subcarrier and of the
their secondaries 3! and 32 tuned to the frequen
cies 80 and 120 cycles by means of condensers 33 20 recovered audio signal across the output termi
nals 5i and 58 of the discriminator unit R.
having an approximate value of l ,uf., and 34,
Figure 10 shows the practical results obtained
having an approximate value of .15 ,uf., provided
with this invention in recovering the original
secondaries 3! and 32 have an approximate like
modulating signal, the frequency of which is
value of inductance of 4.5 h. The common junc
tion 35 of the two tuned secondaries is linked to a 25 higher than that of the carrier frequency.
Figs. 11 and 12 indicate to what extent the
ground connection 36. The potential ends of the
energy can be concentrated in the carrier, if the
tuned secondaries are connected to grids 39 and
modulation index is chosen properly. Curve A
40 of two triode ampli?er tubes 44 and 45 with
shows the total energy content represented in all
a common cathode 4! connected to ground
sidebands as a function of the signal frequency
through the common cathode bias resistor 31. An
for a case in which the maximum frequency
outlet 38 is provided to furnish control voltage
deviation from the main carrier frequency is
for the automatic frequency control unit C1.
selected as 100 cycles. The total energy content
The plates 42 and 43 of the tubes 44 and 45 are
of all sidebands is less than 10 percent, in com
connected through condensers 5| and 52 to the
parison to the large percentage of the energy
primary 53 of an audio output transformer 54,
present in the sidebands if the frequency devia
the secondary 55 of which is feeding potenti
tion is increased to 1000 cycles, as indicated in
ometer 56. The balance of the discriminator
curve B of Fig. 11. A similar curve showing the
circuit is obtained by adjusting the slider 49 of
energy content of all sidebands for the practical
the potentiometer 4B, which might have a resist
ance of 25,000 ohms, in series with the two equal 40 case where the frequency deviation is 20 cycles
is shown in Fig. 12, which indicates that the
resistances 46 and 4'! of approximately 100,000
maximum energy content of all sidebands is al
ohms. Plate voltage is supplied to the tubes 44
ways less than 2'% for modulating signals above
and 45 through the connection 50. Under condi
250 cycles.
tions of balance, the audio frequency carrier volt
In the example given above, the signal is moved
age becomes 0, between plates 42 and 43, only the 45
through a value 20% higher than resonance and
recovered signal voltage appearing at these points.
20% lower than resonance. This is many times
The variable output taken off from the poten
greater than is¢possib1e in a radio frequency cir
' tiometer 56 across the terminals 5'! and 58 is fed
cuit. For example, if the I. F. was 4.3 megacycles
either to a correcting network NET or across the
high pass ?lter S to the audio frequency ampli- -
?er T. Typical working characteristics of such
a circuit are shown in Fig. 7, which represents an
idealized input-frequency output voltage charac
teristic such as possessed by a discriminator. of
the type shown in Fig. 8. Input frequencies are
represented by the abscissae, and output voltages
are represented by ordinates on an arbitrary scale
of +1 to -1. With an input carrier frequency
of 100 C. P. S., which corresponds to a point of
zero amplitude of the modulating signal wave,
the characteristic shows an output voltage of
and, the total carrier shift in a, frequency modu
lated system was 100 kc., the discriminator cir
cuit would be forced to operate on a relatively
small departure from resonance. If K1‘ denotes
the total frequency shift for maximum amplitude
of the signal, then K is de?ned as the modulation
factor, indicating the percentage change of the
mean carrier frequency. In the present case, the
extremes above and below resonance are a high
percentage of the mean frequency, and, there
fore, it is-possible to develop good efficiency in
recovering the audio frequency energy repre
sented by the frequency swing between 80 and
zero. As the signal Wave approaches maximum
120 cycles. In order to maintain the frequency .
amplitude in the negative direction, the carrier
of the subcarrier frequency always exactly at the
frequency increases toward 120 C. P. S., and the
output voltage approaches a value of -1. As the 65 same value, a conventional automatic frequency
modulating signal wave varies from negative
control circuit C1 might be provided which‘ is
maximum to zero, completing the half cycle, the
controlled by the discriminator unit R. Unit
C1 is essentially the same as unit C in Fig. 1.
frequency decreases from 120 C. P. S. to 100
Any deviation from the correct Value of the sub
C. P. S. and the output voltage varies from —1 to
0. On the next half cycle of the modulating sig 70 carrier frequency will disturb the balance of the
discriminator unit R, thus furnishing a control
nal wave the frequency decreases to 80 C‘. P. S.
voltage to unit C1, of such polarity and magni
and returns to 100 C. P. S., yielding a like half '
tude that the correct value of the subcarrier fre
cycle (0, +1, 0) of recovered voltage at the re
quency is restored. This might be important for
ceiver. It is thus obvious that the recovered audio
the reason that the automatic frequency control
voltage is dependent upon the amount that the
11
12
unit is compensating forall fluctuations in fre
quency due to instability of the oscillators either
on thetransmitting or on the receiving end, thus
maintaining a constant subcarrier frequency.
1900 cycles, which would also be rejected "by
?lter G. 'It should be noted that ?lter G is
shown as a single stage in Fig. 9, but it is obvious
that any number of such stages could be used in
Automatic volume control, ‘in its conventional -
cascade, preferably separated by coupling tubes.
form, would also be desirable, in order to main
tain'a relatively constant amplitude at the out
put of I. F. ampli?er P. No circuits are shown
forautomatic volume control, as this would be
In the example given, the carriers are separated
by 1000 C. P. S. ; the receiver of- Fig. 2 must vhave
suflicient selectivity in ?lter circuits G and dis
criminator R to accept the desired carrier and
arranged in accordance with such circuits as com 10 reject the undesired adjacent carriers; it has
monly used. The output of R is passed through
previously been shown that the sidebands of these
:unit S, which is a conventional high-pass ?lter
undesired carriers are of such low amplitude that
with a cut-off frequency of 120 cycles. This re
they cannot interfere with the desired carrier,
moves any remaining frequencies below 120 cycles,
even though the carriers were much closer than
which, up to this, point have acted as an audio
1000 C. P. S. given in this example; the necessary
frequency carrier for the ‘original signal, also
carrier separation is strictly a function of the
any additional undesired low frequencies mak
selectivity of circuits G and R; this selectivity
ing the overall system substantially noise free,
could be increased by inserting a standard band
by a combination'of ?lters. The output of unit
rejection ?lter between the mixer unit F and ?lter
S is coupled into a conventional audio frequency
unit G, at the points marked ER in Fig. 2; such
ampli?er T, and reproduced on a conventional
a band rejection v?lter would be conventional in
reproducer U. These units may be any type de
design, and would be tuned to reject the un
sired and are'not illustrated in detail. The sys
desired carrier; such a ?lter would need have a
tem, as described above, with the example given,
rejection band only 40 cycles wide, as the un
would not reproduce original audio frequencies 25 desired carrier must always have its main energy
below 120 cycles, and might discriminate against
within the narrow band through which it is modu
frequencies just above this pass band, but would
lated; if it is desired to reject more than one
reproduce all audio frequencies lying between 250
interfering carrier, several band rejection ?lters
might be connected in cascade, each tuned to a
and ‘3,000 without discrimination, in well de
30 different frequency, as is conventional practice.
signed circuits.
Referring now to Fig. '3, units A. B. C and D
It is obvious that the receiver of Fig. 2 may be
are identical with the corresponding units of Fig.
turned to resonance with the desired carrier in
1 and perform the same functions, producing at
the ordinary manner, .by'tuning the radio fre
the input to mixer-ampli?er F a signal frequency
quency circuits of R. F. ampli?er-N and the oscil
lator circuits of converter 0; this is true even 35 frequency-modulated wave of 40 C. P. S. frequency
swing. This is mixed with a constant frequency
though the next adjacent carriers were relatively
wave from the oscillator E to produce a fre
close to the desired carrier. For example, if
quency modulated low-frequency wave having a
the transmitter of Fig. 1 is transmitting at the
mean frequency of 100 cycles, which is passed
frequency of 1000 kc. and this signal is frequency
modulated plus and minus 20 cycles, the R. F. 40 through low pass ?lter J, the purpose of which is
to pass all frequencies below the highest signal
and convertor circuits in the receiver of Fig. 2
must also be tuned to 1000 kc. in order to pro
frequency, for instance-3000 C. P‘.'S., and to ‘re
.move all superfluous combination frequencies
duce the 100 cycle subcarrier beat note at the in
and harmonics generated in the mixer circuit F.
put to ?lter G; if the next adjacent carrier was
separated by 1 k0,, this carrier would also be 45 The frequency-modulated low-frequency wave is
then ampli?ed by audio-frequency ampli?er H
ampli?ed by the R. F. ampli?er N and L. F.
and caused to modulate the amplitude of a radio
ampli?er P and ‘would therefore appear in the
frequency wave supplied to the modulator M by
mixer unit F and also beat with ?xed oscillator
the constant frequency oscillator E2. The am
E, which has a frequency of 464,900 C. P. 8.; this
plitude-modulated wave is ampli?ed ‘by ‘a'high
‘is so because'the pass band of N and P must be
frequency ampli?er I and fed to‘the antenna K.
wide enough to pass all side-bands oi‘ the desired
carrier; the undesired carriers, being separated
from the desired carrier by 1000 C. P. S. would
produce a subcarrier beat frequency, at the in
put to‘?lter G, of 900 or 1100 C‘. P. 5., depending
upon whether-the undesired carrier was above or
As before, L is a power'supply for the radio
transmitter. The ?nal signal may be On any
carrier frequency desired and unit E2 may be a
crystal controlled oscillator or any other type
desired for a particular ‘class of service. The
wave radiated from antenna K normally will have
"below the desired carrier. The two carriers, sepa
a band-width equal'tortwice the highest frequency
rated by 1000 cycles will not beat with themselves
of the frequency-modulated low-frequency car
to produce a spurious beat frequency ,of 1000
cycles, provided the input characteristics of mixer 60 rier passed by the ?lter J. However. this can
be reduced by approximately one-half by the use
unit F are linear; the design of mixer circuits
of ?lters between modulator M and the ampli?er
having such characteristics is well known in the
I to effect single-sideband transmission in the
art. Thedesired carrier, having a subcarrier beat
conventional manner.
note of 1000. P. S., is accepted and passed by
Referring to the receiver block diagram, as il
'?lter'G and the signal recovered by discrimina
lustrated in Fig. 4, as the receiving unit for the
‘tor R, as ‘both of these circuits are resonant at
radio transmiter of'Fig. 3, K represents the an
‘100 C. P. ‘3.; the undesired carriers, having sub
tenna; N a radio frequency ampli?er; ‘P an inter
carrier beat notes of 900 or 1100 cycles would be
mediate frequency ampli?er; O the convertor; Q
rejected by both ?lter G and discriminator unit 70 the demodulator or detector circuit. All of these
R, and therefore, no interference would be pro
circuits are conventional and maybe of any type
duced with the desired carrier. It is obvious
desired, such as those in common use. No ‘de
.that .the nextadjacent carriers, separated from
:the desiredcarrierby 2..kc. above or below, would
‘alsoxproduce subcarrienbeat .notes of 2100 and
tailed schematics are shown for these units.
.As before, however, the pass band for the radio
,frequency and inter-mediate frequency ampli
2,410,275
v13
?ers should accept the carrier and ?rst order
sidebands.
The output of I. F. ampli?er P is connected to
the input of ampli?er-mixer unit F, where it is
mixed with the constant frequency wave from
oscillator E which is adjusted to the same fre
quency as the output of I. F. ampli?er. P, which,
14
both Figs. 2 and 4. In well designed circuits this
equalizing network should not be necessary but
should such a network be desired for any reason.
the design would follow conventional practice.
The ?delity of either system would be the maxi-1
mum possible with the circuits used.
If the system is to be operated as a wire line
system, as in telephone service, the audio input
in the example given, is 465 kc., so that the re
from the originating telephone is fed into the
sultant beat note is zero; inasmuch as the output
of I. F. ampli?er P was amplitude modulated by 10 audio-frequency ampli?er B, of Fig. 3. The out
put of audio-frequency ampli?er H, which is the
low-frequency frequency-modulated carrier wave
of 80 to 120 C. P. S., is fed into the wire line. At
signal, the effect of beating the output of I. F. am
the receiving point the line can be connected di
pli?er P with its own frequency is essentially the
same as demodulation, and only the 100 cycle 15 rectly to the input of a demodulator R, or if it is
desired to eliminate noise or signals lying in other
subcarrier frequency with its associated frequency
frequency ranges on the line, the line willbe con
modulated sideband will remain, the carrier hav
nected to the input of ?lter G. Following the
ing been eliminated by heating with its own fre
the original 100 cycle subcarrier, which in turn
had been frequency modulated by the original
quency. Unit C1 is a reactance tuning unit ar
ranged for automatic frequency control in a man
ner similar to the arrangement shown for Fig. 2,
so that oscillator E may be held at a constant
frequency, thus resulting in a continuous zero
beat, even though the frequeency at the output
of unit P might vary. In the example of Fig. 2
reactance tuning unit C1 received its control volt
age from discriminator unit R; in the present
instance the control voltage is derived directly
from mixer unit F; it is wel1 known that in any
mixer stage where two frequencies are present,
the DC. plate current will rise sharply as zero
beat is approached, reaching a maximum at zero
beat; by utilizing the voltage drop across a series
plate resistor in the mixer tube of unit F. a con
trol voltage is developed which will cause reac
tance' tuning unit C1 to maintain the frequency of
oscillator E so that Zero beat always results. This
method of obtaining automatic frequency con
trol is well known in the art. The output of am
?lter will be a demodulator R and such conven
tional audio-frequency networks as are required
to convey the recovered audio-frequency wave to
the receiving telephone.
As applied to carrier current telephony, the fre
quency-modulated low-frequency carrier of 80 to
120 C. P. S. can be used in lieu of the original
audio-frequencies to modulate a plurality of car
rier frequencies each separated from the next by
an amount su?icient to permit selection of the
individual 40 C. P. S. carrier channels by proper
?lters at the receiving point. This process and
the equipment necessary for carrying it outform
the subject of a companion application and will
not be described in detail herein.
The methods described of modulating an audio
frequency subcarrier wave, in themselves provide
a new means of secret communication, as it is self
evident that the frequency-modulated 100 cycle
audio-frequency carrier wave or the radio wave
amplitude-modulated thereby would not provide
pli?er-mixer unit F is connected to the input 40 an intelligent signal on any existing equipment,
and that the subcarrier wave in itself would pro
of ?lter G, and from this point on the operation
Vide an ef?cient masking signal, effectively con
of the circuit is the same as shown for Fig. 2; as
cealing the intelligence contained in the frequency
previously shown, additional frequencies equal to
modulation of this subcarrier wave; only a spe
the carrier separation will be present: these will
be attenuated in the manner previously described 45 cial receiver of the type illustrated in block sche
matic form by Fig. 4 would recover the. original
in detail; as in the case of Fig. 2 a band rejection
audio-frequency signal. It is also obvious that
?lter tuned to the interfering carrier frequencies
existing equipment would not recover the intelli
might be inserted at the points marked BR, if
gence transmitted by the system of Fig. 1, due
this was desirable or necessary.
to the small modulation index; only a receiver
The tuning method just described is for use
of the type shown in block schematic form by Fig.
when two or more adjacent channels are oper
v2 would recover the original signal. It should be
ated close to each other, all channels using a sys
further noted that any existing system of privacy
tem similar to that illustrated by the transmitter
in communication, such as the many forms of
of Fig. 3 and the receiver of Fig. 4. However, such
frequency inversion, might be combined with the
a system might be operated as a single channel
methods herein described to provide additional
between two other carriers of a different type,
new methods of secret communication.
such as conventional amplitude modulated car
A simpli?cation of the method is illustrated
riers, so that this type of tuning, that is. the zero
by the block diagram of Fig. 5. As in Figs. 1
beat method achieved by the use of oscillator E,
was unnecessary. In this case the operation can
and 3, A is a microphone, B is an audio-frequency
be simpli?ed by entirely eliminating units E. F 60 ampli?er, and C is a modulator. Controlled
oscillator D has a mean frequency of 100 cycles
and C1, and connecting the output of I. F. ampli
which is varied between the limits of 80 and ‘120
?er P directly to a conventional demodulator unit
C. P. S. to produce directly the low-frequency
Q, as illustrated by the dotted lines, so that the
frequency-modulated wave that. is produced by
original subcarrier frequency of 100 cycles is re—
covered by recti?cation; the output of demodu 65 the beat method in Fig. 3. Since a greater tuning
effect is required to vary the frequency of a 100
lator Q would then be connected directly to the
C. P. S. oscillator between the limits of 80 and 120
input of ?lter G, and from this point on the op
C. P. S. than to vary the frequency of a high
eration of the receiver is as shown above. As
frequency oscillator through a range of 40 C. P. S.,
shown for the receiver of Fig. 2, automatic volume
control might be added in the conventional man 70 a magnetic condenser might be more effective
than the modulator‘ of Fig. 6. Alternatively,
her, which would be desirable for reasons pre
viously shown.
If desired, or necessary, an equalizing network,
modulator unit C‘ might consist of a- standard
“varistor” as commonly used in telephone com
munication, so designed as to provide a resistive
designed to correct the signal frequency response
might be inserted at the points market NET, in 75 variation to the frequency determining resistors
2,410,275
15
of a resistance tuned oscillator.
The so called
“varistor” generally consists of a plurality of
copper-oxide recti?er discs arranged in the form
of a Wheatstone bridge and so poled that the
passage of a pulsating current through two of
the opposite junctures will result in a variation
in the resistance presented to a circuit connected
to the two conjugate junctures. The end re
sults secured by this or other alternate methods
16
low carrier level, taken at the input to low-fre
quency discriminator unit R and shows the maxi
mum possible amplitude which the sidebands
may attain, in terms of interference with an act
jacent channel, Where the two channels are sep
arated by a mean frequency of 250 cycles and
both carriers have the same amplitude. ‘ Figure
15 was prepared from data contained in the
energy distribution curve of Fig. 12 and the ?lter
of tuning local oscillator D would be the same 10 unit G attenuation curve of Fig. 10. In actual
as those already described for modulator unit
physical practice, the carrier separation would
C and illustrated herein by Fig. 6. The output of
oscillator D passes through a ?lter G and ampli
?er H to the wire line or radio modulator as
above explained in connection with Fig. 3.
It should be noted that no accepted de?nition
exists for the depth of modulation in a frequency
modulated system. It has been variously de?ned
as (a) the modulation index is 1 when the total
shift in carrier frequency is equal to the fre
quency of the modulating signal, and (b) an ar—
bitrary factor where the total shift in carrier fre
quency is considered 100% modulation, regard
less of the actual value of this carrier shift. It
will appear logical that a more reasonable meas
ure of modulation factor in a frequency modu
lated wave would be the relation of the total fre
quency deviation to the mean carrier frequency;
this is so because in any resonant circuit of given
decrement a very de?nite frequency deviation 30
from the resonant frequency would be necessary
in order to create the maximum amplitude change
possible, which in a frequency discriminator cir
probably exceed 250 cycles and would be lim
ited only by the ability of the receiver to select
the desired carrier frequency. For any carrier
separation of 250 cycles or greater, interference
from the sideband frequencies would be neg
ligible.
Two alternate methods of producing a radio
Wave with the energy concentrated in the ‘carrier
wave have been described in detail, the method of
Figs. 1 and 2 being preferred for radio applica
tion. The method of Figs. 3 and 4, however, While
being more complex for radio purposes, illus
trates better the basic method which results in a
frequency-modulated wave of lower mean fre
quency than the lowest modulating signal fre
quency, with concentration of energy in the car
rier, which permits an economy of frequency
space for both wire and radio transmission, This
is so because the sideband frequencies do not
have the power to interfere with closely adjacent
carrier frequencies, even though all sideband fre
quencies are present. Therefore, an economy of
cuit such as unit R of Figs. 2 and 4, would cor‘
respond to- maximum efficiency. For the pur 35 frequency space requiredv is realized by placing
adjacent carrier frequencies closer together than
poses of this description the latter de?nition is
the bandwidth occupied by their sideband fre
used as a measure of modulation factor of a
quencies, and allowing such sideband frequencies
frequency modulated wave, which I have previ
to overlap these closely adjacent carrier fre
ously denoted as K, wherein K represents the
quencies. Both methods are inherently noise free
percentage change of the carrier frequency.
40 to a high degree, due to the action of ?lter unit
From the de?nition of modulation factor given
G in attenuating all amplitude modulation not
immediately above, it is self-evident that there
present
in the original frequency modulation,
is no relation between percentage frequency de
while at the same time the original frequency
viation produced by the frequency modulation in
modulation is passed with only small insertion
the transmitters of Figs. 1 and 3, and the carrier
loss, thus rejecting, to a high degree, such am
frequency, nor is there any relation between this
plitude
modulation as may have been produced
percentage frequency deviation and the ?nal L. F.
by natural or man made static, etc. Inherently,
frequency in the receiver of Figs. 2 and 4. The
a high order of frequency stability is obtained,
carrier and L. F. frequencies might be anything
due to the very small frequency deviation caused
desired, without affecting the ?nal modulation
by modulation; it is well known in the art that
factor or the overall e?iciency of this system. '
carrier frequency stability decreases as the car
The only basically important percentage relation
rier frequency deviation increases. A high order
ship between frequency deviation and the mean
of ei?ciency in signal recovery is obtained, by
resonant frequency exists at the input to fre
the use of a very low subcarrier frequency, so that
quency discriminator unit R of Figs. 2 and 4.
the small frequency deviation caused by modula
In the system of Figs. 1 and 2 this relation
tion is a high percentage of the frequency of this
ship is under control in the receiver of Fig. 2.
low frequency subcarrier; therefore, a high
This is so because the resonant frequency, that is,
modulation factor is always obtained in the sub
the mean beat frequency produced between the
carrier wave prior to demodulation.
output of I. F. ampli?er unit P and ?xed oscil
lator E may be made any value desired.
In the 60
example given the mean resonant beat frequency
is 100 cycles ‘and the total frequency deviation
produced by frequency-modulation at the trans
mitter is from 80 cycles to 120 cycles.
’ The foregoing description clearly indicates that
in the system of Figs. 1 and 2 the depth of modu
lation or modulation factor is always under con
trol in the design of the receiver of Fig. 2, andv
that this modulation factor may always be made
any desired value, so that in well designed cir- '
suits, the frequency ratio between maximum fre
quency deviation and the mean resonant beat
frequency is such that maximum amplitude is
developed in the discriminator unit R.
~
' Figure 15 represents the D. B. attenuation be— 75
The above examples are for the purpose of illus
trating some of the methods and means by which
the broad purposes of the invention may be car
ried out and are not to be deemed as restrictive
in any manner. Other modi?cations and alterna
tives will occur to those skilled in the art without
departing from the scope of the invention as de
?ned by the following claims.
I claim:
1. The method of signaling, which comprises
the steps of frequency modulating at a modula
tion index smaller than unity, a high frequency
carrier with a signal containing the intelligence
to be transmitted, transmitting said frequency
modulated high frequency carrier to a receiving
point, beating the received frequency modulated
2,410,275
1:17
,18
carrier to produce a frequency modulated audio
frequency‘ subcarrier, the mean frequency of
which shall be lower than the lowest signal fre
5. The method of receiving an amplitude modu
lated wave modulated by a frequency modulated
subcarrier wave as de?ned in claim 5, which com
quency, so selecting a band offrequencies in-_
cluding the subcarrier that “the subcarrier and
its associated ‘sidebands are substantially un
prises the steps of receiving the amplitude modu
lated wave, demodulating the desired amplitude
modulated wave to recoverithe frequency modu
attenuated while frequencies lying within the fre
quency band occupied by the sidebands and not.
lated subcarrier wave, so selecting a band of fre
quencies including the subcarrier that the sub
associated with the subcarrier are attenuated
carrier and its associated sidebands are substan
- below a desired interference level, and demodulat-v 10 tially unattenuated while frequencies lying with
ing the frequency modulated audio frequency
subcarrier to recover the original signal.
ing the frequency band occupied by the sidebands
and not associated with the subcarrierare at
' 2. The method of signaling, which comprises
tenuated "below a desired interference level,‘ and
the steps of frequency modulating 'at a modula
demodulating the recovered frequency modulated
tion index smaller than unity, a high frequency 15 subcarrier‘wave to recover the original signal.
carrier with a signal containing the intelligence
6. The method of signaling, which comprises
to be transmitted, mixing said frequency modu
the steps of frequency modulating at a modula
lated carrier with a constant frequency to pro
tion index smaller than unity, a high frequency
duce a new frequency modulated carrier wave of
carrier with a signal containing the intelligence
audio frequency, the mean, frequency of which
to be transmitted, transmitting said frequency
shall be lower than the lowest signal frequency,
modulated high frequency carrier to a receiving
transmitting said audio frequency frequency
point, beating the received frequency modulated
modulated-subcarrier‘wave to a receiving point,
carrier to produce a frequency modulated beat
so selecting a band of frequencies including the
frequency, the mean frequency of which shall be
subcarrier that the subcarrier and its associated
lower than the lowest signal frequency, to act
sidebands are substantially unattenuated while
as an audio frequency frequency-modulated sub
frequencies lying within the frequency band oc
cupied by the sidebands and not associated with
carrier, rejecting all undesired subcarrier fre
quencies produced from vadjacent channels and
selecting only the desired subcarrier frequency
and demodulating said audio frequency fre
the subcarrier are attenuated below a desired in
terference level, and demodulating the subcarrier
to recover the original signal.
quency-modulated subcarrier to recover the orig
3. The method of signaling, which comprises
inal signal.
'
the steps of frequency modulating at a modula
'7, The method of signaling, which comprises
tion index smaller than unity, a high frequency
the steps of frequency modulating a high fre-v
carrier with a signal containing the intelligence 35 quency carrier with a signal containing the intel
to be transmitted, mixing said frequency modu
ligence to be transmitted, at a modulation index
lated carrier with a constant frequency to pro
smaller than unity, mixing said frequency modu
duce a new frequency modulated subcarrier wave
of audio frequency, the mean frequency of which
lated carrier with a constant frequency wave-to
produce a new frequency modulated carrier wave
shall be lower than the lowest signal frequency,
impressing said frequency modulated audio fre
of audio frequency, the mean frequency of which
shall be lower than the lowest signal frequency,
quency subcarrier as a modulating signal on a
transmitting the frequency modulated audio fre
second high frequency carrier by amplitude
quency subcarrier to a receiving point, rejecting
'modulation, transmitting said second high fre
all undesired carriers and selecting only the de
quency carrier as an amplitude modulated high 45 sired carrier and demodulating the frequency
frequency signal modulated with the audio fre
modulated audio frequency subcarrier, to recover
quency frequency-modulated subcarrier to a re
the original signal.
ceiving point, selecting the desired carrier fre
quency and rejecting all undesired carrier fre
quencies, demodulating the received amplitude
modulated high frequency signal to recover the
desired frequency modulated audio frequency sub
carrier and demodulating said frequency modu
lated audio frequency subcarrier to recover the
8. The method of signaling, which comprises
the steps ‘of frequency modulating a low fre
ouency carrier with a signal containing “the intel
ligence to be transmitted, at a modulation index
smaller than unity, where the low frequency
carrier has a mean frequency lower than the low
est signal frequency, transmitting said low fre-‘
original signal.
55 quency frequency-modulated carrier to'a receiv
4. The method of producing an amplitude
ing point, selecting the desired low frequency'
modulated carrier wave,'having the main energy
carrier and rejecting all others, and demodulating
con?ned to a narrower band of frequencies than
the frequency modulated low frequency carrierto
that-occupied by the signal containing the intel
recover the original signal.
' '
ligence to be transmitted, which comprises the
9. The method of signaling, which comprises
steps of producing a signal of given bandwidth,
the steps of frequency modulating a low fre
frequency modulating a ?rsthigh frequency car
quency carrier with a signal containing the intelé
rier with said signal, at ‘a modulation index‘
ligence to be transmitted, at a modulation index
smaller than unity, beating the so modulated
smaller than unity, where the low frequency car
?rst carrier wave with a constant frequency wave, 65 rierhas a mean frequency lower than the lowest
thus producing a frequency modulated audio fre
signal frequency, impressing said frequency
_ quency subcarrier wave accompanied with side
modulated low frequency carrier as a modulating
band frequencies corresponding to the signal fre
signal on a high frequency carrier by ‘amplitude
quencies, whereby the amplitudes of the sideband
modulation, transmitting said'amplitude modu
frequencies of said subcarrier wave are small as
lated high frequency carrier to a receiving’ point,
compared to the amplitudes of the audio fre
quency subcarrier itself, and amplitude modulat
reiecting all undesired carriers and selecting only
the desired carrier, demodulating the'received
amplitude modulated high frequency carrier by
ing a second high frequency carrier wave with
said frequency modulated audio-frequency sub
carrier wave.
'
recti?cation to recover the frequency modulated
75 low frequency carrier, and demodulating the fre
2,410,275
19
'20
quency modulated low frequency carrier to re
separated .by a, narrower frequency band than the
cover the original signal.
bandwidth occupied by ‘the sideband frequencies
10. The method of receiving a frequency modu
lated high frequency carrier, modulated with a
of any adjacent modulated carriers, which com
prises the steps of producing a frequency modu
signal containing the intelligence to be trans
lated ‘carrier modulated with a signal frequency
containing ‘the intelligence to be transmitted at a
mitted, at a modulation index smaller than unity,
modulation index smaller than unity, and asso
which comprises the steps of beating the received
ciated with sidebands the energy content of which
frequency modulated carrier to produce a fre
is small in comparison to the energy content of
quency-modulated beat frequency, the mean fre
quency of which shall be lower than the lowest 10 the carrier; and in which the amplitude of any
frequency component in the sideband becomes
signal frequency, to act as a frequency modulated
audio frequency subcarrier, ?ltering the frequency
modulated subcarrier to attenuate all amplitude
modulation products not originally present in the
frequency modulated carrier wave, which have a
frequency greater than the highest frequency
reached by the frequency modulated subcarrier
wave clue to modulation by the original signal,
also to attenuate all other subcarrier frequencies
having a frequency differing from the resonant
frequency of the ?lter, demodulating the ?ltered
frequency modulated subcarrier wave to recover
smaller the farther said component is separated
from its associated carrier, transmitting said. fre
quency modulated carrier to a receiving point,
beating the received frequency modulated carrier
to produce a frequency modulated beat frequency,
the mean frequency of which shall be lower than
the lowest signal frequency, to act as a frequency
modulated audio frequency subcarrier, impressing
the same on an audio frequency frequency-dis
criminator circuit to recover the original signal.
14. The method of separating a desired fre
quency modulated carrier from a number of simi
the original signal, ?ltering the recovered signal
lar modulated carriers received in a common
to attenuate all residual components with a fre
quency lower than the highest frequency reached 25 channel, all modulated at‘ a modulation index
smaller than unit, in which the frequency be
by the frequency modulated subcarrier due to
tween any tWo adjacent carriers is smaller than
frequency modulation by the original signal and
the maximum bandwidth of the side bands asso
reproducing the ?ltered signal.
ciated with any one of the frequency modulated
11. A method of privacy in communication,
which comprises the steps of frequency modulat 30 carriers, which comprises beating the received fre
quency modulated carriers to produce a plurality
ing a ?rst high frequency carrier with the intelli
of frequency modulated audio frequency subcar
gence to be transmitted, at a modulation index
rier waves, one of which having a mean frequency
smaller than unity, beating the frequency modu
lower than the lowest signal frequency, passing
lated ?rst high frequency carrier with a constant
frequency wave, to produce a frequency modu 35 this last-mentioned subcarrier wave through a
lated beat frequency which shall act as an audio
?lter circuit resonant at the mean frequency of
frequency subcarrier, in which the amplitude of
the subcarrier and frequency discriminating said
?ltered frequency modulated audio frequency
the‘ subcarrier at its'mean frequency is large as
compared to the amplitude of the sidebands,
amplitude modulating a second high frequency
carrier with this frequency modulated audio fre
quency subcarrier, transmitting the amplitude
subcarrier wave to recover the original signal.
l5pIn a receiver for the reception of a fre
quency modulated carrier modulated with a signal
frequency containing the intelligence transmitted,
modulated second high frequency carrier to a re
at a modulation, index smaller than unity, in
ceiving point, during which transmission the fre
quency modulated audio frequency subcarrier
cluding a constant frequency oscillator producing
a ?nal frequency modulated beat frequency sub
acts as a masking signal to prevent recovery of
carrier having a mean frequency lower than the
the original signal by unauthorized persons, de
modulating the amplitude modulated high fre
lowest signal frequency, a parallel resonant cir
cuit, the resonant frequency of which is equal to
quency carrier to recover the frequency modu
said frequency modulated subcarrier mean fre
lated audio frequency subcarrier and demodulat
quency, the Q factor of said circuit satisfying the
ing the recovered frequency modulated subcarrier
condition
'
wave to recover the original signal.
12. The method of privacy in communication,
which comprises the steps of frequency modu
lating a high frequency carrier with the intelli
gence to be transmitted at a modulation index
smaller than unity so that the frequency modu
in which Fb denotes the resonant frequency and
PS the highest signal frequency and a frequency
discriminator circuit including two resonant cir
lated high frequency carrier will appear as a con
stant frequency, transmitting this frequency
cuits with the frequencies F1 and F2, above and
modulated carrier to a receiving point, beating 60 below the frequency Fb, satisfying the same con
dition when F1 or F2 are substituted for Fb.
the received carrier to produce a frequency modu
16. In a receiver for the reception of a carrier
lated audio frequency subcarrier, the mean fre
I wave amplitude modulated by a frequency modu
quency of which shall be lower than the lowest
signal frequency, so selecting a band of fre
lated subcarrier having a modulation index less
quencies including the subcarrier that the sub 65 than unity, a resonant ?lter circuit tuned to the
mean frequency of the subcarrier and having a Q
carrier- and its associated sidebands are substan
factor which satis?es the condition
tially unattenuated while frequencies lying with
in the frequency band occupied by the sidebands
and not associated with the subcarrier are at
tenuated below a desired interference level, and 70
demodulating the frequency modulated audio fre
quency subcarrier so that authorized persons may
"recover the original signal.
' 13. The method of signaling by modulated car
cos Fct
for any given signal frequency F5 and subcarrier
mean frequency Fe, a frequency discriminator cir
cuit following the resonant ?lter circuit and in
rier currents in which the carrier frequencies are 75 cluding resonant circuits tuned to slightly differ
2,410,275
21
22
ent frequencies F1 and F2, above and below the
subcarrier mean frequency F0 and satisfying the
condition for their respective Q factors, when F1
subcarrier on. a high frequency carrier by am
or F2 are substituted for Fe.
point, beating the received amplitude modulated
17. In a transmitting circuit for the transmis
sion of modulated carrier currents, a high fre
quency oscillator, a reactance modulator modu
lating said oscillator, a constant frequency os
plitude modulation, transmitting said amplitude
modulated high frequency carrier to a receiving
carrier with a constant frequency wave the fre
quency of which is equal to the received carrier
frequency, thus producing a direct current com
ponent corresponding to zero beat frequency and
sideband components corresponding to the fre—
cillator separated from the mean frequency of
the ?rst oscillator by a frequency which is less 10 quency modulated low frequency subcarrier, so
selecting a band of frequencies including the sub
than the lowest modulating frequency, a mixer
carrier that the subcarrier and its associated
circuit associated with both the aforesaid oscil
sidebands are substantially unattenuated while
lators, a low pass ?lter circuit following said
frequencies lying within the frequency band oc
mixer circuit to pass all the difference frequencies
cupied by the sidebands and not associated with
produced in the mixer circuit.
the subcarrier are attenuated below a desired in
18. In a transmitting circuit for the transmis
terference level, and demodulating the frequency
sion of modulated carrier currents, a reactance
modulated audio frequency subcarrier to recover
the original signal.
21. The method of receiving an amplitude mod
stant frequency oscillator separated from the 20
ulated high frequency wave, modulated by a fre
mean frequency of the ?rst oscillator by a fre
quency modulated subcarrier wave having a mean
quency which is less than the.lowest modulating
frequency lower than the lowest signal frequency
frequency, a mixer circuit associated with both
modulator, a high frequency oscillator frequency
modulated by said reactance modulator, a con
aforesaid oscillators, a low pass ?lter to pass all
and frequency modulated at a modulation index
di?erence frequencies produced in said mixer cir 25 lower than unity, which comprises beating the
received amplitude modulated carrier with a con
cuit and a high frequency oscillator amplitude
stant frequency wave the frequency of which is
modulated with the output of said mixer circuit
equal to the received carrier frequency, thus pro
through said low pass ?lter.
ducing a direct current component corresponding
19. The method of receiving an amplitude
modulated wave modulated by a frequency modu 30 to zero beat frequency and sideband components
corresponding to the frequency modulated low
lated audio frequency subcarrier wave as de?ned
frequency subcarrier, so selecting a band of fre
in claim 5, which comprises the steps of receiv
ing the amplitude modulated wave, beating the
quencies including the subcarrier that the sub
carrier and its associated sidebands are sub
stant frequency wave the frequency of which is 35 stantially unattenuated While frequencies lying
received amplitude modulated carrier with a con
equal to the received carrier frequency, thus pro
ducing a direct current component correspond
ing to zero beat frequency and sideband compo
within the frequency band occupied by the side
bands and not associated with the subcarrier are
attenuated below a desired interference level, and
demodulating the frequency modulated audio fre
nents corresponding to the frequency modulated
audio frequency subcarrier, so selecting a band 40 quency subcarrier to recover the original signal.
22. The method of signaling, which comprises
of frequencies including the subcarrier that the
the steps of frequency modulating at a modula
subcarrier and its associated sidebands are sub
stantially unattenuated while frequencies lying
tion index smaller than unity, a high frequency
carrier with a signal containing the intelligence
within the frequency band occupied‘ by the side
bands and not associated with the subcarrier are 45 to be transmitted, transmitting said frequency
modulated high frequency carrier to a receiving
attenuated below a desired interference level, and
demodulating the frequency modulated audio fre
point, beating the received frequency modulated
quency subcarrier to recover the original signal.
carrier to produce a frequency modulated beat
20. The method of signaling, which comprises
frequency, the mean frequency of which shall be
the steps of frequency modulating a low fre 50 lower than the lowest signal frequency, to act as
an audio frequency frequency-modulated sub
quency carrier with a signal containing the in
carrier, and demodulating said audio frequency
telligence to be transmitted, at a modulation in
dex smaller than unity, the low fraquency car
frequency-modulated subcarrier to recover the
rier having a mean frequency lower than the
original signal.
lowest signal frequency, impressing said frequency 55
STANLEY D. EILENBERGER.
modulated low frequency carrier as a modulating
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