US2420518A - Power reduction circuit for radio receivers - Google Patents

Description

y 3, 1947. G. M. BROWN POWER REDUCTION CIRCUIT FOR RADIO RECEIVERS Filed Jan. 22, 1945 Inventor v George M. Brown, by F442 W Hi Attorney.

Patented May 13, 1947 UNITED STATES ?ATENT OFFICE POWER REDUCTION CIRCUIT FOR RADIG RECEIVERS York Application January 22, 1943, Serial No. 473,199

4 Claims.

My invention relates to radio receivers and in particular to receivers of the type which operate under standby or no-signal conditions for a major portion of the time. It is an object of my invention to provide an improved receiver in which the energy consumption is greatly reduced.

Mobile communication receivers of the type employed in police cars, on aircraft, and similar installations usually operate under standby or no-signal conditions for more than ninety per cent of the total time. It is customary in such receivers for most satisfactory operation to use an audio power amplifier as an output tube in which plate current fiowsat all times. Since such receivers are usually operated from a low voltage storage battery, the large amount of energy consumed in maintaining the plate current of the power amplifier at its average operating value during standby periods accomplishes no useful purposes. It is a further object of my invention, therefore, to provide an improved receiver circuit in which the plate current of the power amplifier is reduced to or near its cut-off value during standby periods.

A further object is to provide the aforesaid result by means which are purely electronic.

Another object of my invention is to reduce the energy consumption of a communication receiver during standby periods by means operative, in response to the noise present in the receiver circuits in the absence of a carrier wave in the receiver circuit.

A further object of my invention is to provide a power reduction circuit for a receiver which introduces no distortion in the output of the rece ver.

The features of my invention which I believe to be novel are set forth with particularity in the appended claims. My invention itself, however, both as to its organization and method of operation, together with further objects and advantages thereof, may best be understood by reference to the following description taken in con.- nection with the accompanying drawing, in which the single figure represents one embodiment of my invention.

Referring to the drawing, I. have shown my invention as applied to a frequency modulation receiver. Many of the elements of this receiver may be conventional and their details of construction are not material to my invention. To simplify the drawing, such elements have merely been indicated in block form. Thus, the receiver illustrated is for the most part a conventional superheterodyne receiver. Signals received at an antenna iii are amplified in the usual radio frequency amplifier II and combined in a mixer l2 with locally generated waves supplied from an oscillator I3. Ihe carrier waves of fixed intermediate frequency are then amplified in intermediate frequency amplifier I l. In frequency modulation receivers of the usual design, it is customary to pass intermediate frequency waves through an amplitude limiter. Hence, the output waves of the intermediate frequency amplifiers are limited in the circuit of the limiter I 5 after which they are demodulated in a slope filter or discriminator IS. The output signals are amplified in an audio frequency amplifier ll, the output thereof being supplied to a power amplifier l8. Theoutput of the power amplifier, in turn, is supplied to any suitable signal translatin device, such as the loud speaker [9.

The limiter l5 comprises a pentode amplifier having a control electrode 20 to which signals of the intermediate frequency are supplied from the tuned secondary winding of the transformer 2|. The intermediate frequency signals appearing on the tuned secondary of the transformer 2| are impressed between the control electrode 20 and the cathode 22 in series with a grid resistor 23 having a by-pass capacitor 24 in parallel therewith. An output circuit, including a tuned circuit 25, is connected to the anode 25 of limiter l5, operating potential being supplied in a conventional manner from a suitable source of power which is not shown, but merely indicated by the connection, designated B+ on the drawing, and connected between the usual voltage dropping and decoupling resistor 21 and ground. Operating potential for the screen grid 28 is similarly supplied from the common power supply source through a voltage dropping resistor 29, bypass capacitors 3!! and 3| being provided between the anode and screen grid circuits, respectively, and cathode.

The limiter circuit thus far described is of well known form. It is self-biased by means of the resistor 23 and the grid capacitor 24 and is adjusted to produce anode current variations in response to variations in grid voltage only between the limit at which the grid becomes positive with respect to the cathode and grid current flows and the limit at which the grid becomes sufficiently negative that anode current cutoif takes place.

The balanced frequency discriminator circuit it comprises a transformer having a primary winding 32, which forms part of the tuned output circuit 25 of the limiter I5, and a secondary winding 33, both of these windings being tuned to the intermediate frequency by the capacitors 34 and 35. The primary winding is connected to the mid-point of the secondary winding 33 through a condenser 36. The opposite terminals of the secondary winding 33 are connected to the respective anodes of diodes 31 and 38, the cathodes of these diodes being connected together for alternating currents through capacitor 39 and for direct currents through resistances 4B and M. The cathode of diode 38 is grounded and the midpoint between the resistances 43 and 4| is connected to the mid-point of the secondary winding 33 through a choke coil 42.

In the operation of the discriminator thus described, assuming that the tuning control of the radio receiver is adjusted for accurate resonance with the received carrier waves, the intermediate frequency has the desired value to which the primary and secondary windings 32 and 33 are each tuned. The voltage across the secondary winding of the discriminator transformer, according to well-known theory, is displaced in phase from the voltage across the primary winding by 90. Because of this quadrature relation between the primary and secondary voltages, the voltage on one half of the secondary leads the voltage on the primary by 90 whereas that on the other half of the secondary lags that on the primary by 90. Thus, the voltages applied to the two diodes 31 and 38, when the intermediate frequency is at its desired value, are equal and, accordingly, equal values of unidirectional current flow through each of the diodes and hence through resistances 4|] and 4 I It is observed that these resistances are poled oppositely, that is, the voltages across the two are opposite in polarity in the circuit between the cathode of diode 3'! and ground with the result that the cathode of diode 31 is at ground potential when the intermediate frequency is of the desired value.

The quadrature relation between the primary and secondary voltages exists, however, only when the oscillations supplied thereto have the desired intermediate frequency. If this frequency changes in either direction, the phase of the secondary voltage varies from its 90 relation with the primary voltage in one direction or the other depending upon whether the frequency increases or decreases. For example, if the frequency increases, the phase shift may be in such a direction that the voltage on the upper half of the secondary approaches the aiding relation with the primary voltage, whereas that on the lower half of the secondary winding approaches the opposing relation with the primary voltage. Thus, the voltage applied to diode 31 increases and that applied to diode 38 decreases with the result that the unidirectional potential on resistance Ml increases whereas that on resistance M decreases.

In this manner discriminator I6 functions to demodulate frequency modulation waves which are coupled thereto from the output circuit of limiter I5. The demodulated signals are impressed through capacitor 43 across resistance 43, which has an adjustable tap 44, by means of which they are supplied to the control electrode of the audio frequency amplifier IT through coupling capacitor 45. Operating potential for the anode of signal amplifier I1 is supplied from a suitable source of power, which again is not shown but merely indicated by the connection illustrated as B+ on the drawing, and which is connected between the usual plate load resistor 46 and ground. Connected across the source of potential is a potentiometer comprising the series connected resistors 41-49 connected between the point B+ and ground. The cathode of amplifier H is maintained at a positive potential determined by the potential drop across resistors 48 and i9, while the grid resistance between the control electrode and cathode of amplifier i1 comprises the resistors 53 and 5! connected in series with resistor' iii, the capacitor 52 being connected between the lower terminal of resistor 58 and ground to serve as a by-pass for alternating currents.

The amplified signals appearing in the output of amplifier I! are coupled, by means of capacitor 53, to the control electrode 5 3 of the audio power amplifier IS, the series connected resistors 55, 55, and 57 forming resistance between the control electrode 53 and ground and capacitor 58 forma b'y-pa .cross resistors 56 and 51 for alternating cur. .is. The power amplifier l8 has been shown as a tetrode having its anode connected to the source of potential indicated at B+ through the primary winding of an audio transformer Eli, its screen grid connected directly to the source of operating potential 13+, and its cathode connected to ground through a cathode resistor 6i by-passed for alternating currents by capacitor 52, the capacitor 33 acting as a by-pass for alternating currents in the screen grid and anode circuits. The secondary winding of audio transformer 63 is shown as directly connected to the loud speaker Hi.

In the operation of the receiver circuit thus far described, it has been found that there are always noise voltages present at the limiter even when no voltages are impressed upon the receiver input. It has been demonstrated that the nature of these voltages is that of a spectrum containing an almost infinite number of component frequencies, some of which always coincide with the frequencies of the desired signal. Since it is preferable to provide a very high gain through the radio frequency and intermediate frequency amplifiers, these noise voltages, which may be disturbances received by the antenna H! or disturbances arising because of irregularities in the preceding receiver circuits, have a very large value when impressed upon the input of the limiter l5. When a signal is received by the antenna l0 and amplified, and after amplification in the preceding portions of the receiver is impressed upon the input of the limiter IS, the noise voltages appear as outward peaks of modulation on the envelope of the signal modulated carrier wave. In the limiter circuit, this wave undergoes amplitude limitation so that the noise voltages do not appear in the output circuits of the limiter. Hence, after the signal modulated waves which do appear in the tuned output 25 of the limiter have been demodulated in the slope filter or discriminator IS, in the manner previously outlined, demodulation or signal voltages appear across the volume control resistor 33 and are impressed upon the input of the amplifier I1.

During periods when no carrier wave is being received, the noise voltages alone are impressed across the input of limiter I5 and, consequently, these voltages are not removed by the limiter, but appear in the output circuits thereof. Under such conditions, it is undesirable that the noise voltages which appear in the output of the discriminator l6 be amplified and appear in the loud speaker.

In order to silence the audio circuits of the receiver during periods when no carrier waves are being received, the squelch tube is provided having its input circuit coupled to potentiometer 43 in the output of the discriminator it through the filter circuit comprising capacitor H and resistor 69. The cathode of squelch tube 78 is connected to tap 12 on resistor 49 in order to maintain the cathode at a positive potential with respect to the control electrode of the tube. The position of tap 72 is adjusted so that the squelch tube 10 is biased to cutoff, but it passes a small amount of plate current when no signals are being received by the receiver, due to alternating noise voltages present in the receiver which appear across potentiometer 43 and are impressed upon the control electrode of tube I'll through condenser ll. The anode of tube ill is supplied with operating potential from the tap between resistors 4-8 and 49 on the potentiometer 41, 48, d9 through the resistors 5| and 13 connected in series, capacitor 52 serving as a by-pass capacitor for alternating currents, resistor 13 acting as a plate load resistor for noise voltages and resistor 5| serving as a plate load resistor for slow changes in current values in the anode circuit. The potential drop across resistor 5!, due to current flowing in the anode circuit of squelch tube 10, acts as an additional bias in the grid circuit .of amplifier I? and the value of resistor 5! is selected so that this additional bias is sufficient to bias the amplifier I! to cutoff during standby or no-signal conditions of the receiver, thus interrupting the output circuit of the receiver under such conditions.

In order to provide means to reduce the current flow in the power amplifier 63 and thus reduce the energy consumption of the receiver during such periods, the noise voltages appearin in the output circuit of squelch tube Til are coupled, by means of capacitor M, to the control electrode of a noise amplifier which comprises the triode portion of the electron discharge device 15. While the usual type of triode may be used as such a noise amplifier, I have shown device as a diode-triode tube having a diode Within the same envelope with the above-mentioned triode. The anode of the triode portion of device 15 is likewise supplied with operating potential from the common potential source B+ through a load resistor l6 and is coupled by means of capacitor 1! to the anode of the diode portion of the device 15 to impress the amplified noise signals across this said diode. The noise signals thus impressed across the diode are rectified and appear as rectification voltages across the diode load resistor 51. The unidirectional voltages thus developed across resistor 51 by the rectified noise voltages serve as additional bias for the control electrode 54 of power amplifier l8 and, by proper selection of the value of resistor 51, may be utilized to reduce the current flow in power amplifier l8 to either a very low value or to complete cutoff.

When a signal is again received by the antenna Ill and, after amplification, appears across the limiter l5, noise voltages are removed by the action of the limiter so that no noise currents appear in the output of the discriminator. Since the squelch tube H1 is biased to the cutoff point, itscurrent output reduces to zero. Hence, the current output of the triode portion of device 15 and the auxiliary current-reducing bias on device l8 are also reduced to zero. The filter circuit comprising capacitor H and resistance 69 is arranged to block the flow of currents of signal frequency but permit the flow of currents outside of the range of frequencies involved in the desired signal, which may be voice or music. Signal voltages in the output of the discriminator l6, which appear across potentiometer 43, therefore, are not passed by the capacitor H to drive the grid of tube H1 sufficiently positive to initiate current flow in the tube. Therefore, since current flow in tube 10 is stopped, the additional bias across resistor 5| is removed from the input circuit of amplifier l1 simultaneously with the reduction of noise voltages in the input of tube 10 and translation of the audio modulating signals through the circuits of the receiver to the loud speaker I9 is resumed.

It is, therefore, apparent that my invention as described above combines a highly sensitive background noise suppression circuit with a circuit to reduce energy consumption in the receiver during standby or no-signal conditions. Since in mobile receivers the power supply is usually a 6-volt storage battery and the large amount of energy consumed in maintaining the plate current of the power amplifier It at its average value accomplishes no useful purpose, it is desirable that this current be reduced during such periods. In theusual type of emergency communication receivers, standby or no-signal periods may comprise to 98 per cent of the total time of operation. By reducing the current flow in the amplifier l8 during such periods to its cutoff point, the current drain on the power source may be decreased by approximately one third during such standby operation. Furthermore, by using purely electronic means to efiect such a reduction in current during these periods, successful operation of the circuits under all conditions is assured, since the audio circuits are silenced by noise which is present in the receiver during such periods and are made cperative again upon the appearance of a carrier wave. Also, by using the carrier wave to open up the circuits at the beginning of a period of operation, all clipping of the audio signals is avoided and, hence, distortion in the output circuit caused by operation of the squelch and power economiZer circuits is avoided.

While I have shown my power economizer circuit as applied to a frequency modulation type of receiver, it will be apparent to those skilled in the art that it may likewise be adapted to an amplitude modulation type of receiver. In such a circuit, the discriminator l6 and limiter 15 are replaced by the usual type of detector used for amplitude modulation receivers, which is used to provide audio voltages for the control electrode of amplifier H. An auxiliary limiter and demodulator, energized by the same intermediate frequency signal, may be used to supply the necessary noise signal for the control electrode of tube l0 during standby periods.

While I have shown a particular embodiment of my invention, it will be understood that I do not wish to be limited thereto since various modifications may be made, and I contemplate by the appended claims to cover any such modifications so fall within the true spirit and scope of my invention.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. The combination with a receiver for signal modulated high frequency waves subject to undesired noise voltages, of an output power amplifier in which current normally flows during the operation of said receiver, and means for interrupting the normal current flow in said amplifier in the absence of received carrier Waves, said means comprising means limiting the amplitude of said waves thereby to remove said noise voltages therefrom in the presence of received carrier Waves, said limiting means serving to translate said noise voltages in the absence of a received carrier wave, means for demodulating said received carrier wave and fo reproducing said signals from said Waves, an electron discharge device coupled to said demodulating means, said device being biased to cutofi in the presence of a received carrier wave and being arranged to amplify said noise voltages in the absence of said Wave, and means coupled to the output of said device for applying a negative bias to said power amplifier in the absence of said wave.

2. In a receiver for signal modulated Waves subject to noise voltages, the combination of an output power amplifier, means for reducing the gain of said amplifier in the absence of a received carrier wave, said means comprising, electron discharge means for amplifying said noise voltages in the absence of a wave, means responsive to said amplified voltages for reducing the gain of said power amplifier, and means operative when a signal modulated wave is being received to bias said electron discharge means to cut-off.

3. In a receiver for signal modulated oscillations subject to undesired noise voltages, the combination of a signal translating channel, an output power amplifier, means operative in the absence of received oscillations for simultaneously disabling said channel and for reducing the current flow in said amplifier, said means comprising means for rectifying said noise voltages and deriving a control potential therefrom, means responsive to said potential for interrupting translation by said channel, means for amplifying said potential, means applying said potential to said power amplifier in a gain reducing sense,

and means responsive to said received oscillations for disabling said potential deriving means.

4. In a receiver for frequency modulated carrier waves, the combination of, an audio signal translation channel, an audio power amplifier, and means responsive to the noise voltages present in said receiver and operative in the absence of a received carrier wave for disabling said channel and reducing the current flow in said amplifier comprising, means for limiting the amplitude of said waves to a predetermined value, thereby to remove said noise voltages from the output of said receiver when a wave is present therein, a balanced frequency discriminator, an electron discharge device coupled to said discriminator, said device being rendered non-conductive when a carrier wave is received in said receiver and being operative to amplify said noise voltages in the absence of a received carrier wave, means responsive to said amplified noise voltages for interrupting translation in said channel, means for rectifying said amplified noise voltages, and means responsive to said rectified voltages for reducing the gain of said audio amplifier.

GEORGE M. BROWN.

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

UNITED STATES PATENTS Number Name Date 2,096,625 Brown Oct. 19, 1937 1,919,160 Faraham July 18, 1933 1,925,825 Starrett Sept. 5, 1933 2,028,859 Barton Jan. 28, 1936 2,078,055 Carlson et a1 Apr. 20, 1937 2,152,515 Wheeler Mar. 28, 1939 2,263,633 Koch Nov. 25, 1941 Re. 22,302 Case Apr. 20, 1943

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