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Патент USA US3076950

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Feb.` 5, 1963
D. c. DAvls ETAL
3,076,940
FREQUENCY DISCRIMINATOR
Filed Jan. 29, 1957
j; f6' - 5-
3 Sheets-Sheet 1
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Feb. 5, 1963
D. c. DAvls ETAL
3,076,940 "
FREQUENCY DIscRIMzNAToR
Filed Jan. 29, 1957
3 Sheets-Sheet 3
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Patented tirolo. fi, i953
l.,
responding characteristic impedance of each. This may
also be effected by using intermediate filters having im
pedances respectively greater than that of the source irn
í’àyíl’ìdßfêiä
iìiäßitiltálihlßsïût‘t
Donovan C. Davis, Pasadena, Louis A. lille, Alhambra,
pedance to each. However, it must be remembered that
and .iohn i?. Stoltz, Los Angeles, Calif., assignors to
Giifiilan ilìros., line., Los Angeles, Edif., a corporation
of California
File-rl dan. Zâ, 1957, Ser. No. 637,645
1li tìlainis. (Cl. 329--l4l-ti)
the ñlter cannot be a bandpass filter of a rather limited
bandwith if the discriminator is to be employed in pulse
operation with pulses having relatively small time widths.
lt is therefore an object of the invention to provide a.
This invention relates to devices for producing direct l0
current voltages havin.o amplitudes and algebraic signs
which are functions of the difference between the fre
quency of an input signal and a predetermined frequency,
and more particularly to discriminators having unam
biguous responses.
frequency discriminator having a single and true designed
crossover.
It is another object of the invention to provide a sensi
tive frequency discriminator capable of producing a re
liable response over an extremely large band of fre
quencies.
15
The word “unambiguous” as used above refers to the
The novel features which are believed. to be character
istie of the invention, both as to its organization and
operating characteristic of a frequency discriminator
method of operation, together with further objects and
which has only one “crossovcr,” i.e. only one frequency at
advantages thereof, will be better understood from the
which its output is exactly equal to zero.
description considered in connection with the
All present day frequency discriminators generally em 20 following
accompanying drawings. lt is to be expressly understood,
ploy two bandpass ñlters and for this reason very often
however, that the drawings are for the purpose of illus
have several Crossovers in continuous wave (CW) opera
tion as a result of any very slight misalignment of the
tration and description only and are not intended as a
overs are produced. Additional Crossovers are produced
native embodiments of the invention;
definition of the limits of the invention.
filters. This effect is due to the fact that diiferencing is
FIG. l is a schematic diagram of a frequency discrim
performed, not of small voltages, but of voltages which 25 inator
of the prior art;
are large and nearly equal at frequencies far removed
FIG. 2 is a graph of a typical output characteristic of
from the principal or “designed” crossover, i.e. the cross
the discriminator shown in FlG. l;
over at which the discriminator is designed to operate.
FIG. 3 is a graph of the power spectrum of a pulse
In pulse operation, these same discriminators have an
modulated RF signal;
even more unreliable response because additional cross 30
FIGS. 4 and 5 are schematic diagrams of two alter
because of the scallop-shaped power spectrum of pulse
modulated radio frequency (RF) signals. When the fre
quency spectrum of the pulse to be passed through a dis
FlG. 6 is a detailed schematic diagram of another em
bodiment of the invention;
FIG. 7 is a graph of the frequency response of the
criminator has a width which is of the same order as that 35 discriminator shown in FIG. 6;
of the pass band of the discriminator, an anomalous be
FIG. 8 is a detailed schematic diagram of still another
havior is presented. Specifically, if the response of the
embodiment of the invention; and
discriminator to pulses of short duration is observed as a
PEG. 9 is a graph of the output characteristic of the dis
function of the center frequency of the pulse, several
criminator of FIG. 8 as compared to a conventional dis
crossover points may be found.
criminator output characteristic.
in the drawing in FIG. l two bandpass filters iii and
Present day frequency discriminators therefore gen
erally have several Crossovers, and for this reason, have
l2 are shown connected respectively to a diode output
circuit ld including a diode 16 connected from filter l@
and a diode l5 connected from filter i2. It is to be noted
that the diodes l5 and 1S are poled to be conducted in
several disadvantages and particularly when employed
in scrvomechanisms.
For example, when they are ern
ployed as components of automatic frequency control
(AFC) circuits, this type of electronic servo may “home”
opposite directions. The output electrodes of each of the
on a crossover frequency other than the desired oscillation
frequency of an associated oscillator.
The present invention overcomes these and other dis
advantages of the prior art by providing a frequency dis
criininator which has only one crossover. This discrimina
tor comprises a high-pass filter for attenuating au input
signal substantially only when it has a frequency below
a predetermined frequency, a low-pass filter for attenuat
ing the input signal substantially only when it has a fre
quency above the predetermined frequency, and means for
rectify/ing the outputs of each of the filters to produce a
direct-current voltage having a predetermined polarity
when the input signal has a frequency above a prede
termined frequency, and to produce a direct-current volt
age having a polarity opposite the predetermined polarity
when the input signal has a frequency below the prede
termined frequency. Although the output of each ñlter
diodes lo and te are connected together and to a smooth
50
ins circuit 2t?.
in the operation of the discriminator shown in FlG. l,
several crossovers may be encountered.
The response
of the discriminator might be that indicated at 2?, shown
in HG. 2. Crossovers may occur
frequencies f1, f2,
fc, f3 and f4, where fc is the design crossover of the dis
criminator. This characteristic may be produced by slight
misalignnrent of each of the filters ifi and 1.2,
In
tron, additional Crossovers may be produced in
pulse operation because during pulse operation the fre
quency spectrum of a pulse may be that as is indicated in
60 FlG. 3 where fo is the fundamental carrier frequency of
a pulse modulated wave.
Y
The present invention overcomes both of these disad
vantagcs of the prior art by providing a discriminator har
ing only one crossover. An e :rbodiment of the invention
may be substantially constant over a wide range of fre 65 ‘which may be employed to eliminate any crossovers ex
quencies, the discriminator has only one true crossover
cept in the “designed” crossover is shown in FlÍG. 4. This
existing at its “designed” crossover.
discriminator comprises high and low pass ñlters 24 and
By using a special feature of the invention, the sensi.2o which are in turn connected to the same diode net
tivity of this discriminator at its crossover may be in
work ld and smoothing circuit 2i) shown in FIG. 4.
creased to a relatively high value. This is done in ac 70 As shown in PEG. 5, if the phases of input signal to
cordance with the invention by terminating each of the
each of the filters 24 and 2a are reversed by transformer
filters in an impedance substantially greater than the cor
means 2S thereshown, the output may be passed through
se 75,9%@
E
diíierent rectiñer and smoothing circuit means ffii and
32, the rectiiier means 3d» including two i'ectitiers 34 and
is
left as viewed in HG. 8.
he right hand terminals of
each of the diodes lid and ißt? are connected to an out
put terminal E32 from which a capacitor lîsfl is con
nected to ground.
poled in the same direction.
Discriminator
is provided with a íii'st input terminal
The discriminator shown in FiG. 6 is provided 'with out~
ddii’ and a secc-nd grounded input terminal i353. Dis
put terminals 33 and du, t‘ne voltages obtained at each
crirninator 4Z is also provided not only with iirst output
being positive with respect to ground. When the dis
i3?. but also a grounded output terminal i3d.
criminator is unbalanced in one direction, the voltage ob
in the circuit `ot PEG. 6, all values are given micro
tained at one of the terminals 38, do will be su stantially
farads, n‘iillihenries and ohms unless otherwise indicated.
greater than he voltage obtained at the other. This dis 10 The circuit shown in FlG. 8 proved non-critical with a 50
36 connected respectively from filters 24 and 26, but
criminator was designed for a 130 megacycle crossover.
rniilivolt input and a. .5 microsccond pulse. A 7 kilocycle
Thesensitivity of t..e discriminator at the crossover fre
error was suiiicient to operate an AFC servo motor. By
quency is a function of the input signal to noise ratio and
taking appropriate but unnecessary design measures, this
the circuit element. For the ‘130 inegacycle per second
error could have been reduced to one kiiocycle.
input, a sensitivity of l5 miliivolts per megacycie at cross 15
in both the circuits of FEGS. 6 and 8 :rz-derived ter
over was obtained. The output characteristic of the
Arninations are used, but they need not necessarily be used.
discriminator shown in FIG. 6 is shown in FiG. 7.
They may be designed to be Butterworth or rïchebyschev
The discriminator of FlG. 6 includes a high pass filter
types. in any case, one type is not preferred over an
44 including serially connected capacitors d6 and ¿ld and
parallel connected inductors ‘5d and 52. A pair of parallel 20 other.
Other changes and modiiications iu the invention will,
connected capacitors £34 and do are connected from in
ductor 52 to ground. A low pass lilter 53 is similarly
provided with serially connected inductors 65) and 62 with
a parallel connected capacitor 6d and a parallel connected
-inductor da. A pair oi capacit-ors 68 and ’itl are then
connected from inductor 66 to ground. Terminations are
respectively provided for filters ¿id and
by resistors 72»
and 74 respectively. A diode 76 is connected from the
upper end of resistor 72 and a diode '7S is connected from
the‘lower end of resistor 7d. Diode '7d is poled to be
conductive in the direction to the left as viewed in lïlG. 6
and diode ’73 is poled to be conductive in the direction to
the right. Smoothing capacitors titi and 82 are con
nected respectively from the right hand side of diode 76
and the right hand side ot diode ’78 to ground. Addi
Ational smoothing capacitors dd and 36 are respectively
connected in parallel to capacitors dit and SZ. Smoothing
resistors 33 and @d are respectively connected in parallel
with smoothing capacitors dfi and S6.
A central conductor 92 is grounded at the right end
thereof as viewed in FiG. 6. The discriminator of FiG. 6
provided with input terminals
and 94', and output
of course, suggest themselves to those skilled in the art
and the invention is by no means limited to the specific
embodiments shown since they are shown merely for pur
pose of illustration, but the invention and its true scope is
only delined in the appended claims.
What is claimed is:
l. A frequency discriminator comprising: a high-pass
filter for attenuating an input signal substantially only
when it has a frequency below a predetermined fre
quency; a low-pass filter for attenuating said input signal
substantially only when it has a frequency above said
predetermined frequency, both of said ñlters being pro
vided with mismatched terminations connected directly in
parallel with each corresponding filter; and means for
combining the outputs of both ot said lilters to produce a
Vdirect-current voltage having a predetermined polarity
when said input signal has a frequency above said pre
determined frequency, aud a polarity opposite said pre
determined polarity When said input signal has a fre
quency below said predetermined frequency, each of said
mismatched terminations being connected to the input of
said combining means.
_terminals 3% and dil, as stated previously.
2. A frequency discriminatori comprising: a high-pass
A frequency discriminator ¿t2 is shown in FIG. 8,
filter for attentuating an input signal substantially only
which may be employed in an AFC circuit. A graph et 15
when it has a frequency below a predetermined frequency;
a conventional discriminator output characteristic is indi
a termination connected directly in parallel with said high
cated at C in FIG. 9. The output characteristic of the
pass filter, said termination having an impedance substan
discriminator shown in EG. S is indicated at A and B
tialiy greater than its characteristic impedance; a low
in HG. 9. The characteristic A was found with a
50 pass filter for attenuating said input signal substantially
matched termination for each filter of, for example, 1GO
ohm resistors. The characteristic i?. uvas found for mis
_ only when it has a frequency above said predetermined
matched terminations ot', tor example, ldd@ ohms, which
is substantially greater than the 10G ohm characteristic
inipedances of cach filter. it is to be noted that the out
put characteristic B does not create ambiguous crossovers
although the discriminator sensitivity is increased at the
frequency; a termination connected directly in parallel
with said low-pass filter, said termination having an irn
pedance substantially greater than its characteristic irn
pedance; and means for combining the outputs of both of
said iilters to produce a direct-current voltage having a
design crossover fc, sensitivity being the slope of the out
put characteristics at frequency' fzfc.
predetermined polarity when said input signal has a tre
quency above said predetermined frequency, and a po
parallel
and
connected
a parallel
c e connected
'
ld?. and
capacitor
lid and lio are
filter for attenuating an input signal substantially only
when it has a frequency below a predetermined frequency;
larity opposite said predetermined polarity when said in
in HG. 8 the high pass ñlter includes serially connected
capacitors
and 93 and a parallel connected inductor 60 put signal has a frequency below said predetermined ire
quency, said terminations beine connected to the input
fritti’. The lovv pass filter includes serially connected in
of said combining means.
`ductors §92 and lit/i, parallel connected inductors loo
3. A frequency descriminator comprising: a high-pass
connected from inductor lud to grou nd. Three parallel
connected cap-chers
and ..22 are connected from
inductor lille to ground.
A resistive termination including a resistor 124i is con
nected in parallel with the low pass iilter from a terminal
of a diode E126 to ground. A resistive termination includ
Y
ing a resist-or 12.3 is connected from a terminal of another
diode llÈi’ì to ground. Diode §26 is poled to be con
ductive in a direction to the right as viewed in HG. 8.
Diode i3d is poled to be conductive in a direction to the 75
a restrictive termination connected directly in parallel
with said high-pass tilter, said resistive termination hav
ing an impedance substantially greater than its character~
istie impedance; a low-pass filter for attenuating said
input signal substantially only when it has a frequency
above said predetermined frequency; a resist-ive termina
tion connected directly in parallel with said 10W-pass
litter, sind resistive termination having an impedance sub
stantially greater than its characteristic impedance; and
"A
8,0 'sasso
means for combining the outputs ol’ both of said filters
to produce a direct-current voltage having a predeter
mined polarity when said input signal has a frequency
above said predetermined frequency, and a polarity op~
posits said predetermined polarity when said input signal
has a frequency below said predetermined frequency,
said resistive terminations being connected to the input
of said combining means.
4. A frequency discriminator comprising: a high-pass
lilter for attenuating an input signal substantially only
when -it has a frequency below a predetermined fre~
quency; a low-pass filter for attenuating said input signal
substantially only when it has a frequency above said
predetermined frequency; a resistive termination con
nected directly in parallel with each of said iilters, said
resistive terminations being of an impedance substantially
greater than the characteristic impedances of said iilters;
a iirst rectifier connected from said high-pass älter and
direct-current voltage having a predetermined polarity
when said input signal has a frequency above said pre
determined frequency, and a polarity opposite said prede
termined polarity when said input signal has a frequency
below said predetermined frequenc‘, said terminations
being connected to the input of said combining
3. A frequency discriminator comprising: a liigli~pass
filter for attenuating an input signal substantially only
when it has a frequency below a predetermined frequency;
a low-pass iilter for attenuating said input signal substan~
tially only when it has a frequency above said predeter
mined freouency; a mismatched termination connected di
rectly in parallel with each of said filters, said termina
tions being about ten times as large as the characteristic
impedances of said filters; and means for combining the
outputs of bot‘i of said filters to produce a direct-current
voltage having a predetermined polarity 'when said input
signal has a frequency above said predetermined fre~
quency, and a polarity opposite said predetermined polarity
said low-pass filter and poled in the opposite direction, 20 when said input signal has a frequency below said pre
input electrodes of said rectitiers other than those con
determined frequency, eaeh of said mismatched termina
nected to corresponding ones of said filters being con
tions being connected to the input of said combining
poled in one direction; a second rectifier connected from
nected together; and means connected from the junction
of said rectiiiers for producing a substantially constant
means.
9. A frequency discriminator comprising: a high-pass
direct-current voltage representing the average value of 25 filter for attenuating an input signal substantially only
the voltage appearing at the junction of said rectiñers.
when it has a frequency below a predetermined frequency;
5. A frequency discriminator comprising: a high-pass
a low-pass filter for attenuating said input signal substan
filter for attenuating an input signal substantially only
tially only when it has a frequency above said predeter~
when it has a frequency below a predetermined frequency;
mined frequency; a mismatched termination connected di
a low-pass iilter for attenuating said input signal sub 30 rectly in parallel With each of said filters, said termina
stantially only when it has a frequency above said pre
tions being of a magnitude sufdciently large to produce a
determined frequency; a resistive termination connected
substantial increase in the rate of change of the output of
directly in parallel with said filters, said resistive termina
said filters with respect to frequency at their crossover
tion being of an impedance substantially greater than
points over that which would be produced by terminating
their characteristic impedances; a rectilier connected from
said filters in their characteristic impedances; and means
the outputs of each of said filters, said rectiiiers being
for combining the outputs of both of said ?lters to pro
poled in the same direction and having input electrodes
duce a direct-current voltage having a predetermined
connected both to said filters and to said resistive termi
polarity when said input signal llas a frequency above said
nation; and means connected between output electrodes
predetermined frequency, and a polarity opposite said
of said rectifiers for producing a substantially constant
predetermined polarity when said input signal has a fre
direct-current voltage having a predetermined polarity
quency below said predetermined frequency, each of said
when said input signal has a >frequency above said pre
mismatched terminations being connected to the input of
said combining means.
determined frequency, and a polarity opposite said pre
determined polarity when the input signal has a frequency
below said predetermined frequency.
6. A frequency discriminator comprising: a high-pass
filter for attenuating an input signal substantially only
10. A frequency discriminator comprising: a high-pass
filter for attenuating an input signal substantially only
when it has a frequency below a predetermined frequency;
a low-pass iilter for attenuating said input signal substan
when it has a frequency below a predetermined frequency;
tially only when it has a frequency above said predeter
a low-pass filter for attenuating said input signal substan
mined frequency; a mismatched termination connected di
tially only when it has a frequency above said predeter 50 rectly in parallel with each of said filters, said termina
mined frequency; a mismatched termination connected di
tions being resistors having resistances of a higher order
rectly in parallel With each of said filters, said termina
of magnitude than the characteristic impedances of said
tions bcing of a different order of magnitude than the
filters; and means for combining the outputs of both of
characteristic impedances of said lilters; and means for
said ñlters to produce a direct-current voltage having a
55
combining the outputs of both of said filters to produce
predetermined polarity when said input signal has a fre
a direct-current voltage having a predetermined polarity
quency above said predetermined frequency, and a polarity
when said input signal has a frequency above said prede
opposite said predetermined polarity when said input sig
termined frequency, and a polarity opposite said predeter
nal has a frequency below said predetermined frequency,
mined polarity when said input signal has a frequency
said mismatched terminations being connected to the in~
below said predetermined frequency, said terminations 60 put of said combining means.
being connected to the input of said combining means.
7. A frequency discriminator comprising: a high~pass
References Cited in the tile of this patent
iilter for attenuating an input signal substantially only
UNITED STATES PATENTS
when it has a frequency below a predetermined frequency;
a low-pass íilter for attenuating said input signal substan~
2,138,341
Crosby _____________ ...__ Nov. 29, 1938
tially only when it has a frequency above said predeter
2,243,417
Crosby ______________ __ May 27, 1941
mined frequency; a mismatched termination connected di
2,413,913
1947
rectly in parallel with each of said filters, said termina
2,782,302
1957
tions being of a higher order of magnitude than the char
acteristic impedances of said filters; and means for com 70
bining the outputs of both of said filters to produce a
FGREIGN PATENTS
615,536
Great Britain ______ ..-„_..__ Ian. 7, 1949
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