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Nov. 5, 1946.
’ w.- A, F11-_cH
2,410,489
NON-LINEAR FREQUENCY MODULATION SIGNALING SYSTEM
Filed July 19, 1944
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ATTORNEY I
Nov. 5, 1946.
w. A. FITCH
2,410,489 y _`
NON-LINEAR FREQUENCY MODULATION SIGNALING SYSTEM
Filed July 19, 1944
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Nov. 5, 1946.
. W. A. FITCH
' 2,410,489 ,
NON-LINEAR FREQUENCY MODULATION SIGNALING SYSTEM
Filed July 19., 1944 `
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RANGE
INVENTOR
WlLLIAM A. Fri-CH
~BY
ATTO RNEV
l
Patented Nov. 5v, ‘1.946î
2,410,489
UNITED STATES
PATENT
OFFICE -
2,410,489
N ONLIN EAR FREQUENCY MODULATION
SIGNALÍNG SYSTEM
William A. Fitch, Schenectady, N. Y., assigner to
Radio Corporation of America, a corporation
of Delaware
1
Application July 19, 1944, Serial No. 545,576
13 Claims. (Cl. Z50-6)
2
This application relates to signaling systems
wherein the timing of oscillatory energy is modu
counter-distortion is obtained in the arrangement
of Fig. 8.
lated in accordance with potentials or currents
In the conventional frequency modulation
representing signals of any type, such as voice,
transmitter the antenna current may be repre
television, facsimile, etc.
5 sented by the following expression.
The general object of the present invention is
(l)z'=Ao sin (wot-I-mf sin ut)
the, reduction of noise in the system, so that at
the receiver output little or no noise appears.
K
In its broadest aspect my invention then is a
method of and means for reducing the noise out
mf: fcifo
kf: modulation factor
put in the response of a frequency modulation re
ceiver by means of a distortion of the conventional
Ícff0=frequency deviation
f0=the radio frequency
frequency characteristic of the receiver discriminator, and the utilization of an opposite or
counter distortion of the discriminator character l5
fw =the audio frequency
istic of the transmitter to give perfect fidelity.
The modulation factor in the typical frequency
modulation transmitter varies linearly with the
The manner in which the noise reduction is
accomplished in my improved system will not be
strength of the audio modulating voltage. Hence
described in detail. In this description reference
will be made to the attached drawings, wherein
20 the frequency deviation varies linearly with the
strength of the audio modulating voltage. Thus
Fig. 1 illustrates the relation of the audio modu
to'vtake a typical example assume
lating voltages to frequency deviation in known
frequency modulation systems.
fo=40,000,000 cycles
Fig. 2 illustrates the relation of the audio modu
lcf(max) :.0025
lating voltages to the frequency deviation in a
From the values given above the maximum
timing modulation system arranged and oper 25
deviation is seen to be 100 kc. Assume that 10
ated in accordance with my invention.
volts of audio voltage at the output to the modu
Fig. 3 illustrates by a curve the characteristic
lator gives this deviation of 100 kc. A chart
of the counter-correction used at the receiver,
and also shows the improvement in noise reduc 30 showing the frequency deviation versus modulat
ing voltage amplitude for the typical frequency
tion gained by the use of my system.
modulation
transmitter is as shown in Fig. 1.
Fig. 4 illustrates by block diagram the essential
features of a frequency modulation transmitter
system, arranged in accordance with my inven
tion.
_
Fig. 5 illustrates details of the distorting or
modulation potential modifying -circuit which may
Note that the deviation is substantially directly
proportional to modulation voltage amplitude.
In my improved system the deviation does not
35 vary linearly with the audio modulationg voltage
but follows a curve such as, for example, the
curve of Fig. 2. In this curve it is again assumed
be used in my system to obtain the desired modi
the
maximum deviation is 100 kc. and occurs at
ñcation of the modulating potentials used for
10 Volts modulation voltage at the modulator
timing modulating the carrier.
_ 40 input._ The deviation for all intermediate values
Fig. 6 illustrates by block diagram a receiver
arranged in accordance with my invention and
of modulation, however, is greater than in usual
systems. The modulation factor lcf is no longer
linear with respect to the modulation strength.
Fig. '7 illustrates a modulation distorting device
for use at thereceiver to accomplish counter 45 The deviation varies at a rate higher than
linearly with respect to the modulation rate.
distortion of the modulation to correct the distor
Assuming deviation to be plotted on the VY”
tion used at the transmitter for noise reduction
axis and modulation voltage on the “X” axis, the
purposes.
`
second derivative of “Y” with respect to “X”
In Fig. 8 I have shown an arrangement for pro
including counter-distorting means, while
ducing the counter-distortion in the intermediate
frequency circuit of the receiver, for example, the
frequency discriminator circuit.
50
(i. e. 21X
_
Figs. 9, 10, 11 and 12 comprise characteristic
curves of the circuit of Fig. 8. These curves are
should be negative.
`
At the receiver then, I use a modulation correc
tion network having a characteristic opposite to
used in explaining the manner in which the 55 that of Fig. 2. Such a characteristic is shown
2,410,489
3
in Fig. 3 by curve B. Here the audio output does
not vary linearly with respect to the frequency
deviation; it varies at a rate that is less than
linearly with respect to the deviation. Assuming
deviation to be plotted on the “Y” axis and modu
lation voltage on the “X” axis, the second deriva
tive of “Y” with respect to “X”
C i@
i. e. dx
should be positive.
4
age when the hon-linear impedance 26 is discon
nected. ' When the non-linear impedance 2B is
connected in the circuit, the output voltage is
practically the same as above for small values of
audio voltage input. However, for large values
of audio voltage input, the impedance which tube
lil works into decreases due to the characteristics
'oi non-linear impedance 2t and hence the out
put is reduced. Of course the change in resist
ance of 26 with current is gradual. By adjusting
the value of resistor 24 the shape of curve, Fig,
-2, may be changed as desired.
Inspection >of these curves shows that an ad
The receiver may be arranged as illustrated in
vantage of the receiver discriminator character
Fig. 6, wherein «the distorting device is connected
istic shown at B in Fig. 3, is that for noise having 15 between the discriminator and detector and the
a low frequency deviation the signal to noise
output. The receiver may include a radio fre
ratio is improved by the ratio AC/BC, `which is
quency ampliiier and converter including a
an improvement of the order of 2 to 1.
A
source of oscillations at
and intermediate
My improved signaling method, as described
frequency amplifier and limiter in 32, a discrim
hereinbefore, and means for carrying out the 20 inator and detector in 3d, and the receiver mod
same, will be apparent to those skilled in the
ulation distorting correcting device in 35. As to
art without further description.
the discrimina-tor and detector, it may be of any
However, in Fig. 4 I have illustrated by rec
well known approved type. Preferably `a dis
tangles the essential features of a timing modu
criminator and detector o_f the type illustrated in
lation system arranged in ‘accordance with my
25 Crosby U. S. Patent #2,229,640, dated January
invention.
28, 1941, or Seeley U. S. Patent #2,121,103, dated
The input which for convenience has been des
June 2l, 1938, or yConrad U. S. Patent #2,057,640,
ignated audio input, but may be inputs of other
dated October 13, 1936, is used here.
types and of frequencies other than audio, is sup
The distorting device in 35 may be as illus
plied to a modulation distorting device 6, which
trated in Fig. 7. The modulation distorting de
has a characteristic as illustrated in Fig. 2. The
vice illustrated in Fig. 'l is essentially an audio
so-modiñed modulation current or potentials is
amplifier using a pentode tube 28 of the remote
then applied to a frequency modulator 8, Where
cutoff type. These tubes .are commonly used for
in a: carrier is modulated as to timing in accord
automatic volume control and have an output
ance with the modiñe'd modulating potentials.
35 characteristic similar to Fig. 3. By adjusting the
The carrier is then ampliñed or .frequency mul
value of resistor 2t (Fig. 5) curve, Fig. 2, may be
tiplied, etc., as desired, in a power amplifier it
made complementary to the curve of Fig. 3. In
and utilized. The frequency modulator in 8 may
operation the 'input of tube 28 is coupled to the
be of ’the type illustrated, for example, in Crosby
detector output of unit '3.15 (Fig. 6), and the out
U. S. Patent #2,279,659, dated April 14, 1942, or 40 put of tube 23 is supplied to a utilization circuit.
of the type wherein the phase of a carrier of ñXed
The counter-distortion may be accomplished by
frequency is modulated by potentials corrected
modifying the response of radio frequency cir~
in such a manner that the output derived by fre
cuits, as, for example, the characteristic of the
quency multiplying the phase modulated carrier
discriminator circuit. The essential feature is
has the characteristics ci frequency modulation
that the relation between the modulation and de
or sinf'iilar characteristics.
viation be changed in the transmitter, as I have
The correction network in 5l may be arranged
disclosed, and a compensating or restoring
in vvarious manners. For example, this network
change made at the receiver.
may »comprise an arrangement as illustrated in
In Fig/8 I have shown a frequency discrimi
Fig. 5. In Fig. 5, the modulation input is `sup
nator circuit arranged to accomplish this
plied to the input electrodes of a tube I4. The 50 counter-distortion. This circuit consists of two
input electrodes are shunted by a resistance i6
resonant series circuits A and B, one tuned above
of high impedance >whichlsuppli'es .grid bias due
and the other below the _frequency modulated
to grid rectification. The bias supplied by I6
carrier band. The two series circuits are in par
may be supplemented by the source I8 shunted
allel
across the tuned stage 42, which may «be
55
by modulation potential >‘by-pass condenser BP.
coupled to vthe tuned stage 'du oi ßthe intermedi-v
The output electrodes are shunted by a non-lin
ate Vfrequency amplifier. 'Tuned circuitïâï vthen
ear impedance 26 and a high resistance’Z'll. The
supplies
the frequency modulated I. F. to these
impedance 2i? has a characteristic such that its
ries circuits A and B. `Each of .the circuits A and
resistance varies inversely with the applied volt
age. A material'known as "Thyrite” has such a 60 B comprises an inductance and a capacity'in se
ries. Each circuit also contains a series resist*
characteristic. The output thus is loaded lightly
ance, #lil and 46, which is of-high value as com
at low levels and heavy at high levels. The anode
pared to the maximum »reactance of the circuit
source 26 is shunted by a modulation frequency
icy-pass condenser BP, The output from the sys
tem is supplied to the frequency or phase modu
lator in 8.
There are a number of materials which may
be used for the non-linear impedance 26. The
device should have a high "resistance with low
values of current, and a low resistance for high
values of current. Copper oxide rectifiers have
a characteristic like this. The operation -of this
non-linear impedance is as follows. Tube I4 is
an ampliñer .having an audio output voltage
which is practically linear with audio input volt-
within the band.
The two series circuits inl parallel -are connected
to one -end of circuit 42 by resistances 44 `and 46.
The high frequency circuit is completed by con
necting the other end oí the tuned circuit I2
through >resistance 45 to a resistance divider in
70 shunt to the anodes 152 and 54 of .the diode 50.
The anode 52 of the diode -iscoupled-to a point on
the series circuit A lbya radio frequencycoupling
condenser 5 I- and a series resistance 53 to include
aportion of the series resonantcircuit A in shunt
75 to the electrodes of this diode. The anode 54 of
65
2,410,489
5
the other diode is similarly coupled to a point on
the series circuit B by a radio frequency coupling
condenser 55 and resistance 51.
Due to the preponderance of resistance in the
circuits, the currents in the two branches A and
B are essentially constant and equal, regardless
of frequency. Hence, across the reactive part of
tude, and modulating the timing of oscillatory
energy in accordance with the modified modu
lating
potentials.
.
'
r
:
Y
' `2. The method of signaling which includes
these steps, generating oscillations of carrier wave
frequency, generating currents of a predeter
mined amplitude range characteristic of signals,
deviating the timing ofthe oscillations in ac
cordance with the signal currents, and modifying
the amplitude of the signal currentsintermediate
the upper and lower ends of the amplitude range
each branch appears a voltage which varies al
most linearly with frequency, reaching zero on
the frequency midway between the resonant fre
quencies of the branches A and B and rising from
zero value on either side of the said midway fre
quency. These two voltages are separately de
tected in the double diode 5i! and the outputs of
the detectors are fed in phase opposition to the
resistances 5i) and 62, one end thereof being
grounded. The corrected output is taken from
these resistances. Thus, at the center of the band
the detectors are producing equal direct current
voltages across the load resistances 6i) and 62,
and the direct current potential across the total
load is zero since these equal outputs oppose. If
the frequency increases, the output of one de
tector increases and the output of the other de
tector decreases, causing the total output voltage
to go negative by an amount proportional to the
thereof in such a manner that the timing devia
tion of the oscillations is expanded throughout a
corresponding
range.
~.
-
`
3. The method of signaling which includes
these steps, generating oscillations of carrier
wave frequency, generating currents representing
signals the amplitude of which vary from a mini
mum to a maximum value, deviating the timing
of the oscillations in accordance with the signal,
modifying the amplitude of the signal currents
in such a manner that the timing deviation of
the oscillations is expanded for signal amplitudes
intermediate said minimum and maximum Values,
transmitting said oscillations so modulated, varid
subjecting the same to an amplification and de
frequency deviation. In similar manner a de
crease of frequency will cause the output Voltage
modulation process wherein the resulting signal
current amplitude is compressed with respect to
timing deviations intermediate maximum and
to go positive. This pushpull type of detector not
only balances out the harmonic distortion which 30 minimum deviations.v
'
would 'occur due to the departure from linearity
4. The method of demodulating timing modu
in the series resonant circuit, but also causes any
lated oscillations of the character recited in claim
amplitude modulation to be bucked out.
2 which includes these steps, amplifying the tim
The customary adjustment for the two series
ing modulated oscillations and deriving from the
resonant circuits A and B is shown in Fig. 9. -
ampliñed oscillations signal currents the ampli
When the outputs of the two diodes are combined,
and bucking, the output curve is linear with fre
quency, as illustrated in Fig. 10.k
In accordance with my invention, adjustment
tudes of which are compressed with respect to
deviations intermediate the maximum and mini-V
mum deviations.
5. In a communication system, the method of
is made such as to introduce counter-distortion. 40 signaling which includes these steps, expanding
To do this I adjust the resonant frequencies of
the two series resonant circuits further apart
the amplitudes of modulating potentials for am
plitudes intermediate minimum and maximum
so that the resonance curves are situated as i1
values thereof, modulating the timing of oscil
lustrated in Fig~ l1. This is done by tuning the
reactances and/0r resistances of the series cir
cuits A and B. Now when the outputs of the
detectors are combined bucking in the double
diode the output will appear as in Fig. 12. This
will result in a discriminator output similar to
that shown inFig. 3 of the drawings. In this ar
rangement abscissas designate frequency devia
tion, while the ordinates represent discriminator
output.
As an example of the frequencies involved, it
latory energy in accordance with the modified
modulating potentials, transmitting the timing
modulated energy, amplifying and demodulating
the timing modulated carrier energy while dis
torting the character thereof i‘n a, sense opposite
to the modification of the same at the trans
mitter.
6. The method of signaling which includes
these steps, generating oscillatory energy, gen
erating modulating currents, modifying the am
plitude of the modulation currents in such a
may be assumed that the mean frequency of the 55 manner that the modified modulation amplitude
intermediate frequency fed to tuned circuit 4U
is substantially zero for minimum modulation
and thence to tuned circuit 42 is about 800 kc.
amplitude and a ñxed value for maximum modu
rI'hen circuit A might, if conventional, be tuned
lation amplitude but is expanded for modulation
to about 690 kc. and circuit B might be tuned to
amplitudes intermediate said minimum and
about 910 kc., as indicated in Fig. 9. In accord 60 maximum values, and modulating the timing of
ance with my invention, however, A is series tuned
oscillatory energy in accordance with the modi
fied modulating potentials.
to about 660 kc. and B tovabout 940 kc., as indi
cated in Fig. 11. Then the circuit elements of
'7. The method of receiving timing modulated
A and B may be of the values indicated in Fig. 8.
oscillatory energy of the character described in
It will be understood that for other I. F. input fre 65 claim 6 which includes these steps, amplifyingy
quencies, other circuit values may be used, and
the said timing modulated oscillatory energy, de
also at the input frequency used as an example
tecting the modulations thereonto derive the
the element values given may be varied considera
modulation components, and subjecting the mod
bly in practice Without altering the results ob
ulation components to a modificationA which
tained in accordance with my invention.
counteracts the modification of `the modulation
I claim:
at the transmitter.
_
1. The method of signaling which includes8. In a communication system, the method of
these steps, amplifying modulating potentials to
a degree such that they vary in amplitude greater
than linearly with respect to their initial ampli
signaling which includes these steps, at the
transmitter expanding the amplitudes of modu
75 lation potentials through a range intermediate
2,410,489
7
8
a -frequency discriminator and detector circuit
coupled to said ampliñer, the detector and dis
criminator having a characteristic such that the
demodulation components vary less than linearly
in amplitude With respect to deviations of said
oscillatory energy intermediate maximum and
modulating the timing modulated oscillatory en
minimum deviations thereof.
ergy to derive the modulation components, and
12. A receiver for timing modulated oscillatory
compressing the derived modulation components
energy wherein the timing is- modulated greater
throughout a ‘range intermediate the minimum
than linearly for a range of amplitudes of vsignal
and maximum amplitude values of the derived
ing currents, a frequency discriminator and de
modulation components.
tector circuit excited by said oscillatory energy,
9. A modulation system comprising apparatus
an output circuit, and a modulation ampliñer
wherein oscillatory energy the timing of which
coupling said output circuit to said detector, the
is to be modulated in accordance with signals ap
pears, a source of signal current covering a pre 15 modulation amplifier having a characteristic
such that the demodulation components in the
determined range of intensities coupled ‘by modu
output circuit vary less than linearly in ampli
lating means to said apparatus, and signal cur
tude with respect to deviations of said oscilla
rent modifying means in said coupling such that
tory energy corresponding to said range of am
the timing modulation of the oscillatory energy
is expanded with respect to intermediate values 20 plitudes.
13. The method of signaling which includes
of signal current intensity.
Vthese steps, generating oscillations of carrier
10. A system as recited in claim 9, wherein
the minimum >and maximum amplitude values,
modulatingthe timing of oscillatory energy in
accordance with the so-expanded modulating
potentials, .transmitting the timing modulated
oscillatory energy, receiving amplifying and de
said last named means comprises an electron
Wave frequency, generating currents of a prede
discharge device having input electrodes coupled
termined amplitude range characteristic of sig
nals, amplifying the generated currents to a de
gree such that they vary in amplitude linearly
with respect to their original minimum and max
imum amplitude Values and greater than linearly
electrodes.
for amplitude values intermediate said minimum
ll` A receiver for timing moulated oscillatory
energy wherein the timing is modulated greater 30 and maximum values, and modulating the timing
of oscillatory energy in accordance with the mod
than linearly for signal amplitudes intermediate
ulating potentials so ampliñed.
minimum and maximum signal amplitudes, a
timing modulated oscillation amplifier circuit ex
WILLIAM A. FITCH.
cited by said modulated oscillatory energy, and
with said source of signal current and output
electrodes associated With said apparatus, and
a non-linear resistance in shunt to said output
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