US2455450A - Radio receiver noise suppression circuit - Google Patents

Description

Dec. 7, 1948. R. 1-. THOMPSON RADIO RECEIVER NOISE SUFPRESSION CIRCUIT Filed Jan. 15 1944 INVENTOR ROBERT T. THOMPSON Patented Dec. 7, 1948 NiTED STATES PATENT OFFICE.

RADIO RECEIVER NOISE SUPPRESSION CIRCUIT Robert T. Thompson, Oak Park, Ill., assignor to Zenith Radio Corporation, a corporation of Illinois This invention relates to high frequency wave translating apparatus, and more particularly to such apparatus arranged to be responsive to frequency deviation.

Many types of apparatus, such, for example, as frequency modulation receivers, incorporate a device responsive to frequency deviation of a modulated carrier wave, and it is desirable that such devices be as sensitive as possible to such requency deviation. That is, it is desirable that the frequency deviation responsive circuit produce as large a voltage as possible upon deviation of the frequency of the incoming wave from a predetermined frequency. Various arrangements have been utilized for producing such a large voltage upon frequency deviation. Such arrangements commonly incorporate a network whose reactance changes linearly very rapidly over a relatively narrow frequency range, whereby, for a constant input voltage, the current which flows through the network changes very rapidly as the frequency of the incoming wave changes over that relatively small frequency range.

It is generally desirable that such frequency deviation responsive devices operate normally about a mean frequency in the relatively small frequency range over which they are linearly responsive. This is particularly true in receivers arranged to receive and to demodulate waves whose frequencies are modulated in accordance with signals. Such frequency modulation receivers are usually of the superheterodyne type and incorporate carrier wave voltage limiters which impress on the input of a frequency variation responsive device a voltage of fixed maximum amplitude whose frequency is modulated in accordance with a signal. The loudness of the reproduced signal does not change substantially as the mean frequency of the frequency modulated carrier wave is shifted through the relatively small linear response band of the frequency responsive network unless the mean frequency of the wave approaches the limits of linearity of the relatively narrow frequency band. In such case, there is not merely a reduction in the apparent loudness of the received signal, but there is considerable distortion. This distortion is accentuated as the mean frequency of the received or transformed carrier wave assumes magnitudes further and further removed from a magnitude within the linear frequency response band of the frequency deviation responsive circuit. Regions, defined by the so-called back slope of such frequency deviation devices, just outside of the linear response "band of the circuit,

are effective to demodulate the carrier wave, but produce effects not linearly proportional to the frequency of the signal and for that reason it is very desirable that for best reception the mean frequency of the received or modified carrier wave be kept inside the linear band and not in the nonlinear regions corresponding to the back slope which is present on each side of the linear portion of the frequency response characteristic curve of the frequency deviation responsive circuit.

It is a specific object of this invention to provide an improved frequency modulation receiver which does not reproduce signals except when it is tuned correctly for best reception; that is, to arrange such a receiver so that it will not reproduce undesired fuzzy signals due to demodulation on the back slopes of the frequency response characteristic curve of the receiver.

More broadly, it is an object of this invention to provide a new and improved frequency deviation responsive arrangement of high sensitivity in which there is substantially no response except when the frequency of a wave impressed on the frequency variation responsive arrangement lies within the linear portion of its frequency range.

It is another object of the present invention to provide a simple arrangement which suppresses the distorted responses produced when operating in the range of either one of the back slopes of a frequency discriminator so that tuning of a frequency modulation receiver incorporating the discriminator is easier.

A corollary object of the present invention is to provide an improved frequency deviation responsive arrangement in which the frequency deviation sensitivity is high when the frequency of a wave impressed thereon is within its linear range and which is low when the frequency of such a wave is outside of its linear range.

It is frequently the practice in radio transmission to reduce the range of intensity variation of signals before modulating the carrier wave in accordance with such signals, in order that sounds of the smallest intensity may be reproduced at the receiving station and in order that sounds of the largest intensity may not produce frequency deviation to such an extent that the frequency of the carrier wave is increased beyond certain assigned limits.

It is another object of this invention to provide a frequency deviation responsive arrangement which is substantially unresponsive to waves outside of its substantially linear response range and in which the range of intensity variation of demodulated signals may be adjusted as desired.

Another object of this invention is to provide means responsive to an incorrect tuning condition of a frequency modulation receiver for suppressing the unwanted response due to the back slope of the discriminator characteristic.

Another object of this invention is to provide means responsive to a condition of correct tuning for enhancing the deviation sensitivity of a receiver for frequency modulated waves.

Another object of this invention is to provide a new and improved limiter in a receiver for frequency modulated signals in which the limiter transmits signals only when the receiver is correctly tuned.

Still another object of the present invention is to provide improved arrangements for controlling the deviation sensitivity of a frequency responsive network in accordance with the intensity of a signal passing through such network.

Yet another object of this invention is to provide improved arrangements for controlling the frequency deviation sensitivity of a frequency responsive system having a limiter circuit in which an electrical characteristic of such limiter circuit controls the deviation sensitivity.

Yet another object of this invention is toprovide an improved signal amplitude compression system incorporating frequency deivation responsive apparatus in which voltages developed in either an amplifying stage or limiter stage are utilized to control frequency deviation sensitivity.

Yet another object of this invention is to provide an improved signal amplitude expansion 5..

system incorporating frequency responsive apparatus in which voltages developed in either an amplifying stage or limiter stage are utilized to control frequency deviation sensitivity.

The features of the present invention which are believed to be novel are set forth with particularity in the appended claims. This invention itself, both as to its organization and manner of operation, together with further objects and advantages thereof, may best be understood by wave of constant amplitude and output voltage response, plotted as ordinate, in a frequency deviation response network on which such carrier wave is impressed. There is a linear portion of the line 5 in which the output voltagefor response, of the network changes far more rapidly than the frequency of the impressed carrier wave. There are nonlinear back slopes 6 and I at each end of the linear portion of curve 5, over which output voltage changes nonlinearly as the frequency of voltage applied to such network changes.

One phase of the present invention relates to our arrangement for causing a frequency deviation response network having the characteristic illustrated in Fig. 1 to produce an output voltage, or cause a response, substantially only when the frequency of a carrier wave impressed on the network lies Within the limits of the linear part of curve 5. Such an arrangement is useful and highly desirable in a receiver for receiving frequency modulated waves, and when incorporated in such a receiver it is practically impossible to mistune the receiver to the extent of producing demodulation on the back slopes 6 and 1.

The term receiver as used in this specification, is meant to include any device possessing at least a source of frequency modulated carrier waves and a circuit for detecting the modulation signal on the carrier waves. Purely by way of example, a repeater in which the modulation signal is detected and retransmitted without conversion to sound would fall in the classification of a receiver, in the light of this specification.

Figure 2 is the circuit diagram of a particular type of frequency modulation receiver in which frequency modulated waves intercepted by antenna H] are detected and the modulation signal is reproduced on speaker Hi9. This receiver is of the superheterodyne type, including variably tuned circuits in the radio frequency amplifier stages 63 and 73 and tuned circuits in a frequency converter or oscillator modulator stage 14, the tuned circuits being adjustable to adjust the receiver to receive frequency modulated Waves of any desired mean frequency.

Signals received on antenna in are applied to the primary 60 of antenna transformer 6| which has its output winding 62 tuned by condenser 62A and has one terminal thereof connected to the control grid of variable mu discharge device 63, the other terminal of winding 62 being connected either to the cathode of device 63 through conventional bias resistance 61 shunted by conventional by-pass condenser 68 or connected to the cathode of device 63 through serially connected condenser I20 and resistance 61, depending upon the position of switch I0.

Space current for device 63 is supplied from source 64 which has its positive terminal con- 3 nected to the screen grid of device 63 and also to one terminal of tuned output circuit 65, 66 comprising the primary windin 65 of output transformer H and variable tuning condenser 68, the other terminal of the tuned circuit 65, 66 being connected to the anode of device 63. The negative terminal of source 64 is grounded and connected to the cathode of device 63 through serially connected resistance 61 so that space current flowing from the anode and screen grid of device 63 develops a control grid bias voltage across resistance 61 for causing device 63 to operate over a suitable portion of its operating characteristic curve so as to produce suitable amplification of signals applied across variably tuned circuit 62, 62A. Such signals are amplified by device 63 and then appear across tuned output circuit 55, 66 and then are transferred to the secondary winding 12 of output transformer H. Such amplified signals are again amplified in another radio frequency amplifier stage 13 and then applied to the input of the oscillator modulator stage 14.

The fixed frequency output voltage of the oscillator modulator or frequency converter 14 is amplified in a fixed frequency intermediate frequency amplifier l5 and is impressed across a tuned circuit including the primary winding 16 of the high frequency transformer 18, connected in shunt with a tuning condenser TI. The tuned circuit comprising elements 76, T! is adjusted to be resonant at the fixed mean frequency at which the intermediate frequency amplifier I5 is operative.

The secondary winding 19 of the transformer 18 is connected in shunt with a tuning condenser 59 and voltage developed thereacross is amplified, and limited in intensity on both half cycles f a v in a amplifyi g d miti di charge device 8|. The discharge device BI is associated with a second discharge device 99 and with a circuit arrangement which causes the limiting and amplifying device 85 to pass to its output circuit substantially no signal when the signal impressed on tuned circuit 19, Si! is below a predetermined intensity, and which causes the limiting and amplifying device 8! to pass to its output circuit a signal of substantially constant intensity when the signal impressed on tuned circuit E9, 86 is above a predetermined intensity.

The output of discharge device Bl, thus modified, is impressed across. a tuned circuit including. the primary 36 of a transformer, the primary 8 h being connected in shunt with a tuning condenser 81 to form a tuned circuit 85, 81 tuned to the same frequency as is the output circuit of the intermediate frequency amplifier 15. The tuned circuit 86, 81 is associated with a frequency variation response network or frequency discriminator 97 of the well known resonant type, arranged to produce an output voltage whose intensity and polarity varies as the frequency of a constant intensity wave impressed on the tuned circuit 86, 81 varies in amount and direction from a predetermined mean frequency.

The output voltage of the discriminator 9'! is applied between the grid and cathode of amplifying discharge device I 02, amplified thereby, and a first portion of such amplified voltage is applied through a coupling condenser III to the plate of a rectifier such as a discharge device I31, rectified in discharge device 13! and then applied across the resistance 92 for controlling the conductivity of discharge device 90, and a second portion of such amplified voltage is applied to a second amplifying circuit till and then applied to sound reproducer I09.

In particular, the audio voltage from discriminator 9'? is applied across the serially connected condenser l!) and resistance IN, the resistance EM being connected between the grid and cathode of amplifying device I02 through conventional cathode biasing resistance Hi4 having connected in shunt thereto by-pass capacitance H35v for preventing degeneration effects. Space current for discharge device I02 is supplied from voltage source 66 which, has its positiveterminal connected to the anode of device Hi2 through serially connected resistance N13. The cathode of device IE2 is grounded through cathode bias ing resistance I04, and the resultingspace current fiowing through resistance I04 produces a suitable negative control voltage for the control electrode of device Hi2 suchthat. device, H12 op-., erates along a substantially linear portion of its control voltage versus space current characteristic.

Signal voltages appearing across resistance 10.! are amplified by device I02 and, as mentioned previously, such amplified signals have a portion thereof applied to rectifier discharge device I31 for producing" a control voltage and another portion thereof which appears across output coupling resistance m3 applied to the terminals of the input circuit ofamplifying circuit l0! through coupling condenser I08, condenser Hi8 being connected to the junction point; of resistance I03 and anode of device H32; and one terminal ofnetwork lll'l being grounded.

The limiting amplifier device 8| mentioned previously, is arranged to limit the; maximum amplitude of both half cycles; of" an; impressed signal Wave to less than a predetermined intensity, and to that end its cathode is grounded and its first or control electrode 106 is connected to one terminal of the tuned circuit l9, 8!], the other terminal of that tuned circuit being connected to ground through a serially connected resistance 82 connected in shunt with a by passing condenser 83 for by-passing high frequency currents around the resistance 32. The anode H3 of device Si is connected to one terminal of the tuned circuit 863, 81, the other terminal of that tuned circuit being connected through serially connected resistances 8t and 85 to the positive terminal of a voltage source 6 of operating potential, the negative terminal of the source 64 being grounded. A voltage dividing resistance 88 is connected in shunt with the source 54, and its adjustable tap is connected to the second or screen electrode lid of device 8!, which screen electrode connected throu h a condenser 89 to the cathode of device 8|, the reactance of condenser 89 being sufliciently low at high frequencies to prevent substantial potential variation of electrode H 2.

Neglecting for a moment the fact that the discharge device 90 is connected in circuit with discharge device SI, and assuming that the potentials of the various electrodes of discharge device 81 are as they would be adjusted by device 80 in the presence of a signal impressed on the tuned circuit F9, 88 above a predetermined intensity such that device 8! limits and transmits a signal wave to the tuned circuit 86, 8?; limiting action in the discharge device 8| take place on both half cycles of the wave in known fashion. Rectification of the wave on the control electrode IE5 is efiective to develop a bias potential across resistance 82, whose polarity is as indicated, and, assuming the intensity of the impressed wave is sufficient, the control electrode ldli is driven far more negative during a part of each half cycle of the impressed wave than the negative potential required to cut off anode current in the device 8i. By this process negative half cycles of the impressed wave are limited in intensity and appear on the anode I l3 only with such limited intensity.

The potential of the screen electrode ll-l adjusted by the voltage dividing resistance 88 to a low value, such that positive half cycles of the impressed signal wave on control electrode H36 are by certain well known mechanisms also transmitted to the anode H3 in limited intensity.

In order to accomplish some of the purposes of this inventiomrthe discharge device is inter connected with the device 8! to cause the device 8i to cease transfer of the wave to the tuned circuit 86, 8! when the intensity of the wave on the tuned circuit 79', 80 drops below a predetermined intensity. The control electrode H22 of device 99 is connected to that terminal of tuned circuit it, 80 which is connected to resistance The anode 523 of discharge device Bil is connected to a point P between the serially connected resistances and 8.5, and the cathode of device $53 is connected to the negative terminal of a source 9i of biasing potential, the positive terminal of which is connected to ground through a coupling resistance 82. A suitable condenser 59 is connected in shunt to the serially connected source and resistance 9?: for-bypassing high frequency currents around such serially connected circuit elements.

Source SI maintains the cathode of device 98 negative with respect to ground and also with respect to the control electrode 122, so that in the absence of asignal voltage current tends to flow from source 9| through resistance 92 to ground, then through resistance 82, through the resistance 95, to the control electrode E22, and through the cathode of device 90 and back to the negative terminal of source 9|. Such current flow maintains continuously a bias potential across resistance 82 of such polarity as to maintain control electrode I06 of device 8| negative with respect to ground.

When the signal appearing across tuned circuit 79, 80 is of greater than a predetermined amplitude the conductivity of device 96 changes not only as the result of the increased voltage drop across resistance 82 but also as the result of the increased voltage drop across resistance 92. Enhanced signal rectification between control electrode I66 and the cathode of the device BI produces a sufficiently large negative bias across resistance 82 to cut off the anode current of device 90 so that ultimately the voltage drop across resistance 8 is only that produced by a relatively small s ace current flowing to device BI. A correspondingly high direct current potential appears on anode H3 of the limiting amplifier device 8| and its transconductance is correspondingly greatly increased and the signals are efficiently transferred from the tuned circuit '19, 86 to the tuned circuit 86, 81 with an effective limiting of the maximum amplitude of both the positive and negative half cycles of the signals.

With this arrangement, in which the limiter device SI is made effective or ineffective to pass signals from the tuned circuit '19, 86 to the tuned circuit 86, 81, as such signals in the tuned circuit I9, 86 are above or below a predetermined intensity, the selectivity curves of the tuned circuits I6, 1! and I9, 80 and other tuned circuits in the radio frequency amplifier stages 53 and I3, oscillator modulator 14 and intermediate frequency amplifier 75 are substantially fiat topped over a frequency range corresponding to the frequency range illustrated in l as linear within the parts B and C of the curve 5. The selectivity curve outside of the frequency range is such that a very sharply reduced transfer of energy is produced through the above mentioned tuned circuits outside of that frequency range, that is, in the region corresponding to the back slopes E and I of the curve 5 in Fig. 1 and within the frequency ranges marked A and D. Accordingly, if those tuned circuits are so adjusted in band width that an incoming signal lies within the range A or within the range D shown in Fig. 1 such signal is sufiiciently attenuated at the tuned circuit I9, 86 of Fig. 2 so as to be below the predetermined intensity at which the discharge device 8I transfers signals to the tuned circuit 86, 81. It is only when a signal on the antenna I lies within the linear frequency range of the curve 5 of Fig. 1, that is, within the frequency ranges B and C of Fig. 1, that it appears in the tuned circuit I9, 86 in sufficient intensity to bias device 90 beyond cutoff, with the result that the signal is amplified by device 8|.

As has been mentioned previously, a portion of the audio signal amplified by discharge device I62 is fed to rectifying discharge device I3I through condenser III. The cathode of device I3I is grounded through a tapped resistance I36 which is shunted by an audio frequency bypass condenser I3'I. Rectified voltage appearing between the tap on resistance I36 and ground is filtered by serially connected filter inductance II! and shunt connected filter condenser II4. A portion of the voltage appearing across resistance I36 is filtered and applied across resistance 92 when a serially connected switch H6 is closed by connecting the movable tap on resistance I36 to one terminal of resistance 92, the other terminal of resistance 92 and one terminal of resistance I36 being grounded. This rectified voltage from device I3I causes the ungrounded terminal of resistance 92 to become more positive as the signal voltage applied to discriminator 9'! increases, and the net continuous negative bias voltage appearing on grid I22 of device becomes sumcient to cut off current flow in device 90 with the result that the anode voltage on device BI, and consequently the transconductance of that device, increases. The output signal from device BI correspondingly increases with a resulting increase in the amplitude of the audible signal from the reproducer I69.

In Figure 2, when switch 10 is connected as shown to the grounded terminal. the deviation sensitivity of the receiver is controlled in accordance with the audio voltage output. When switch III; is closed and when switch it is connected to filter inductance II5, the deviation sensitivity of the receiver is controlled not only by the audio voltage output but also by the signals appearing across the output of intermediate frequency transformer 18.

The receiver shown herein may be tuned by conventional means, for example, the tuning means may comprise variable condensers or variable inductances. Such tuning means are represented by the variable capacitance 62A and 66 shown in Fig. 2. The grid input circuit and the plate output circuit of the radio frequency amplifier 13 may also be tuned. Tuning may be connected in conventional fashion.

It is preferable, but not essential, to use a pentode type of discharge device in the main signal translating channel. While the auxiliary control discharge device 90 is shown as a triode type, it may be desirable, under certain conditions, to use a tetrode or pentode in place of the triode.

While the particular embodiments of the present invention have been shown and described. it will be obvious to those skilled in the art that changes and modifications may be made without departing from this invention in its broader as pects, and therefore the aim in the appended claims is to cover all such changes and modifications as fall within the true spirit and scope of this invention.

I claim:

1. A frequency deviation responsive device including a source of frequency modulated carrier waves of varying amplitudes and various mean frequencies, means for selectively amplifying a carrier wave from said source, a frequency modulation signal detector of the resonant type having a principal linear frequency response region and adjacent non-linear frequency response regions, said amplifying means having a frequency sclectivity characteristic with substantially more attenuation in said non-linear regions than in said principal linear region, an electron discharge device having a control electrode, anode and cathode. an input circuit connected between said control electrode and cathode including means for producing self-bias voltage in response to a carrier wave impressed on said control electrode, means for impressing a selected carrier wave from said amplifying means on said input circuit, an output circuit connected between said anode and cathode and coupled to said detector, said device and circuits being arranged to limit to a predetermined maximum amplitude any carrier wave impressed thereon, a second electron discharge device having a control electrode, anode, and cathode, means for impressing self-bias voltage from the control electrode of said first discharge device on the control electrode of said second device negatively with respect to the oathode of said first device, and means for connecting the anode and cathode of said second device to said output circuit effectively in shunt with the anode and cathode of said first device, said second device having sufficient transconductance with said circuits adjusted for satisfactory signal detection in said principal linear region that substantially no signal detection takes place in said adjacent regions.

2. A frequency deviation responsive device including a source of frequency modulated carrier Waves of varying amplitudes and various mean frequencies, means for selectively amplifying a carrier wave from said source, a frequency modulation signal detector of the resonant type having a principal linear frequency response region and adjacent non-linear frequency response regions, said amplifying means having a frequency selectivity characteristic with substantially more attenuation in said non-linear regions than in said principal linear region, an electron discharge device having a control electrode, anode and cathode, an input circuit connected between said control electrode and cathode including means for producing self-bias voltage in response to a carrier wave impressed on said control electrode, means for impressing a selected carrier wave from said amplifying means on said input circuit, an output circuit connected between said anode and cathode and coupled to said detector, said device and circuits being arranged to limit to a predetermined maximum amplitude any carrier wave impressed thereon, a second electron discharge device having a, control electrode, anode, and cathode, means for impressing self-bias voltage from the control electrode of said first discharge device on the control electrode of said second discharge device and for biasing the cathode of said second device negatively with respect to the cathode of said first device, means for connecting the anode and cathode of said second device to said output circuit effectively in shunt with the anode and cathode of said first device, said second device having sufiicient transconductance with said circuits adjusted for satisfactory signal detection in said principal linear region that substantially no signal detection takes place in said adjacent regions, means responsive to detected signals for producing a bias voltage, and means for changing the conductivity of said second device in joint response to said last mentioned bias voltage and said self-bias voltage.

3. A frequency deviation responsive device including a source of frequency modulated carrier waves of varying amplitudes and various mean frequencies, means for selectively amplifying a carrier wave from said source, a frequency modulated signal detector of the resonant type having a principal linear frequency response region and adacent non-linear frequency response regions, said amplifying means having a frequency selectivity characteristic with substantially more attenuation in said non-linear regions than in said principal linear region, an electron discharge device having a control electrode, anode and cathode, an input circuit connected between said control electrode and cathode including means for producing self-bias voltage in response to a carrier wave impressed on said control electrode, means for impressing a selected carrier wave from said amplifying means on said input circuit, an output circuit connected between said anode and cathode and coupled to said detector, said device and circuits being arranged to limit to a predetermined maximum amplitude any carrier wave impressed thereon, a second electron discharge device having a control electrode, anode, and cathode, means for impressing self-bias voltage from the control electrode of said first discharge device on the control electrode of said second discharge device and for biasing the cathode of said second device negatively with respect to the cathode of said first device, means for connecting the anode and cathode of said second device to said output circuit effectively in shunt with the anode and cathode of said first device, said second device having suflicient transconductance with said circuits adjusted for satisfactory signal detection in said principal linear region that substantially no signal detection takes place in said adjacent regions, means responsive to detected signals for producing a bias voltage and means responsive to said last mentioned bias voltage for aiding the change in conductivity of said second device produced by said self-bias voltage.

ROBERT T. THOMPSON.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,125,723 Henriquez et a1 Aug. 2, 1938 2,144,882 Van Loon Jan. 24, 1939 2,152,515 Wheeler Mar. 28, 1939 2,199,350 Shofstall Apr. 30, 1940 2,248,197 Rath July 8, 1941 2,263,645 Rath Nov. 25, 1941 2,271,203 Okrent Jan. 27, 1942 2,286,442 Schock June 16, 1942 2,296,101 Foster Sept. 15, 1942 2,301,649 Thompson Nov. 10, 1942 2,305,842 Case Dec. 22, 1942 2,316,317 Curtis Apr. 13, 1943 2,323,880 Carnahan July 6, 1943 2,330,902 McCoy Oct. 5, 1943 2,338,526 Maynard Jan. 4, 1944 FOREIGN PATENTS Number Country Date 410,549 Great Britain May 24, 1942

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