US2312080A

US2312080A - Inverse feedback circuits

Info

Publication number: US2312080A
Application number: US423162A
Authority: US
Inventors: Murray G Crosby
Original assignee: RCA Corp
Current assignee: RCA Corp
Priority date: 1941-12-16
Filing date: 1941-12-16
Publication date: 1943-02-23
Anticipated expiration: 1960-02-23

Description

Feb. 23, 1943.

M. CROSBY INVERSE FEEDBACK CIRCUIT File d Dec. 16, 194; 2 Sheets-Sheet l TORY $301733 ATTOR EY INVVEN Q 5 "WWW ORE} k N Feb. 23, 1943. M. G; CROSBY INVERSE FEEDBACK CIRCUIT Filed De c. 16, 1941 2 Sheets-Sheet 2 M A MN ab m2; W

INVENTO A T1:ORNEY Patented Feb. 23, 1943 IN VERSE FEEDBACK CIRCUITS Murray G. Crosby, Riverhead, N. Y., assignor to Radio Corporation of America, a corporation of Delaware Application December 16, 1941, Serial No. 423,162

'7 Claims.

My present invention relates to inverse audio feedback circuits adapted for use in amplitude modulated carrier Wave receivers.

In the past it has been the usual practice to apply audio feedback potentials to an audio amplifier input, the sense of feedback being degenerative thereby to reduce distortion arising within the amplifier. For example, H. S. Black has described such inverse feedback circuits in an article entitled Stabilized feedback amplifiers published in the January 1934 issue of the Bell System Technical Journal. I have found that inverse feedback of audio potentials into the detector circuit of a radio receiver serves to reduce distortion encountered in the detector circuit. By

"the process disclosed herein an audio voltage is introduced into the detector circuit which opposes the audio output voltage in a manner to reduce the effective percentage of modulation of the received carrier waves. Since the percentage of modulation in the detector is reduced maximum modulation capabilities of the detector are not attained, and distortion encountered by virtue of high percentage modulation is reduced.

Accordingly, it may be stated that it is one of the main objects of my present invention to provide an amplitude modulated carrier wave receiving system in which inverse audio feedback is utilized in such a manner as to reduce distortion encountered in the detector circuit.

Another important object of the invention is to reduce the percentage of modulation of the carrier waves applied to the detector by applying the audio feedback potentials as modulation on one of the amplifiers preceding the detector whereby the feedback voltage modulates, or it can equally be said unmodulates, the incoming modulated wave energy so that the percentage modulation of the carrier waves fed to the detector is reduced.

Another object of my invention is to provide inverse audio feedback to a plurality of demodulator devices used in a diversity receiving system thereby efiectively to minimize distortion arising by virtue of selective fading of the type wherein the carrier fades with respect to the modulation side bands.

Still other objects of my invention are generally to improve inverse audio feedback circuits, and more particularly to provide inverse feedback circuits which are reliable in operation and economically assembled.

The novel features which I believe to be characteristic of my invention are set forth in particularity in the appended claims; the invention itself, however, as to both its organization and method of operation will best be understood by reference to the following description taken in connection with the drawing in which I have indicated diagrammatically several circuit organizations whereby my invention may be carried into effect.

n the drawings:

Fig. 1 shows one embodiment of the invention,

Fig. 2 shows a modification of the invention,

Fig. 3 shows a diversity receiving system embodying the invention.

Referring now to the accompanying drawings, wherein like reference characters in the different figures designate similar circuit elements, there is shown in Fig. 1 that portion of an amplitude modulated carrier wave receiving system which is involved. in the present invention. Merely by way of illustration, let it be assumed that the demodulator diode 4 is the second detector of a superheterodyne receiver. In such case the intermediate frequency (I. F.) transformer feeding the diode 4 has its primary and secondary resonant circuits l and 2 each tuned to the operating I. F. value. The primary circuit I would, of course, be coupled to the output electrodes of the last I. F. amplifier tube.

Those skilled in the art are fully aware of the construction of the typical superheterodyne circuits preceding the primary circuit I, and, therefore, they need not be shown in detail. In general, the signal collector may collect signals in the broadcast range of 550 to 1700 kilocycles (kc.), or it may collect signals in the short wave ranges if such be the case. One or more stages of radio frequency amplification may be utilized to feed the usual first detector, or converter. The resulting I. F. energy, which may have any suitable frequency value, is fed to one or more stages of I. F. amplification. The demodulator, or second detector, circuit consists of the diode 4 whose anode is connected to the high potential side of the input circuit 2, while the low potential side of the latter is connected to ground through the load resistor 5 which is by-passed for I. F. carrier currents by condenser 6. a

The radio frequency by-pass condenser 9 connects the cathode of diode 4 to ground. It will be recognized that the demodulator circuit is of the usual and well known type. Audio, or modulation frequency, voltage developed across resistor 5 is transmitted to the control grid of the first audio frequency amplifier ll through a path which includes a direct current blocking condenser l. The latter is arranged in series with the potentiometer resistor 8 Whose lower end is audio amplifier II.

The inverse, or degenerative, audio feedback a path comprises lead I 6 connected between the upper end of the damping resistor I 4 and the cathode of diode 4. The feedback path includes the radio frequency choke coil I0. It will, therefore, be seen that the audio voltage is inversely fed back directly into the diode detector circuit. In the operation of this circuit the audio feedback potentials are. degeneratively intro.- duced into the cathode circuit of the detector diode so that the percentage of modulation pres ent on the carrier wave applied to the detector is effectively reduced.

This action would be similar to the type of circuit where the feedback voltage was fed directly to the grid or cathode circuit of tube II, except for the fact that by introducing it as shown the distortion introduced by the diode is reduced as well as that introduced by the It is well known that the ordinary diode circuit introduces distortion when detecting a modulated wave with a high degree of modulation. Consequently, this circuit of Fig. 1, which reduces the efiective percentage of modulation present on the carrier wave in the detector circuit, will also reduce the distortion encountered in that circuit.

In Fig. 2, I have shown a modification of the invention wherein the audio feedback potential is utilized to unmodulate the incoming amplitude modulated carrier wave. In this case the diode demodulator 4 has the primary circuit I of I. F. transformer 3 arranged in the plate circuit of an I. F. amplifier tube I02 which may be of the pentagrid type. Control grid IOI of this tube may be connected to the secondary resonant circuit of the I. F. transformer 3', the primary circuit being arranged in the output circuit of a prior I. F. amplifier. The third grid I9! of tube I02 is connected to the degenerative feedback path I6 which includes the radio frequency choke coil I0. The remaining electrodes of tube I02 are conventional, and their functions are well understood. The cathode of tube I02 may be connected to ground through the usual self-biasing resistor I02 which is by-passed for I. F. currents.

In this circuit arrangement the audio voltage developed across resistor 5 is transmitted to the same type of audio frequency amplifier network as shown in Fi 1. It will be understood that the degenerative feedback lead I6 is connected in the same manner as in Fig. 1; that is'to say, it would be connected to any desired point along the audio amplifier network. Automatic volume control (A. V. C.) may be provided in the usual and well known manner by connecting the A. V. C. bias connection to the anode end of resistor 5. The A. V. C. lead would, of course, include a proper pulsating voltage filter 5. The A. V. C. bias. may be applied to any of the amplifier control grids between the signal collector and tube I02.

The operation of the circuit in Fig. 2 depends upon the fact that the audio feedback voltage fed to grid IEII applies a modulation voltage to that grid in such a phase as to reduce the effective percentage of modulation of the modulated carrier waves fed through transformer 3 to control grid IN. The phase of the modulation voltage on grid IOI is such as to unmodulate the applied modulated carrier wave. It will, of course, be understood that the modulation voltage applied to grid IOI' will not be of sufiicient magnitude completely to remove the modulation on the carrier wave at transformer 3. The magnitude of unmodulated voltage is sufficient so that the modulated carrier wave at transformer 3 has a substantially reduced percentage of modulation. This reduced percentage modulation wave is then fed .to the diode detector 4, and produces less distortion in the detector circuit by virtue of the reduced percentage modulation.

The present invention has a particular advantage in the case of a diversity receiver system in such cases where there is encountered selective fading of the type wherein the carrier fades with respect to the modulation side bands. In Fig. 3 there is shown in highly schematic form a diversity reception system. Since those skilled in the art are fully acquainted with the construction of the diversity reception systems and the functioning thereof, it is believed that the schematic showing is fully warranted. Briefly,

numerals

203, 204 and 205 designate three independent receiving systems, each of which may be of the superheterodyne type. The grounded antenna circuits 200, MI and 202 provide modulated carrier pick-up devices for receivers 203. 204 and 205 respectively.

As is well known, the signal pick-up devices of the respective receivers are geographically so related as to secure the benefits of diversity reception. The diode demodulator 209 is fed with the I. F. energy from receiver I through the I. F. transformer 20%. Diode demodulator H0 is fed from receiver '2 through I. F. transformer 201. Diode ZII is fed with I. F. energy through the I. F. transformer 20%. It will be understood that each of I. F.

transformers

206, 201 and 208 have their respective primary and secondary resonant circuits tuned to substantially the same oper ating I. P. value.

The low potential sides of the secondary circuits of each of the I. F. transformers are connected in common to ground through a common load resistor 213 which is shunted by the I. F. by-pass condenser 2E2. The cathodes of diodes 209, 2H 2H are connected in common, and the common connection is connected to ground through the radio frequency by-pass condenser EN. The audio voltage developed across load resistor H3 is fed through the direct current blocking condenser 2I-I to the control grid of the audio amplifier 2 I 9. The audio potentiometer 2I8 has its resistor in series with blocking condenser 2H. Here, again, there may be utilized one or more audio amplifier stages between the audio amplifier 2I9 and the final audio amplifier 220. The audio transformer 22I may have its secondary winding shunted by the clamping resistor 223, and the jack 222 may be used to feed any audio utilization means from across the sec ondary winding of transformer 22I.

The degenerative feedback path includes the radio frequency choke coil 2I5. The degeneration voltage is fed from the high potential side of the secondary winding of transformer 22l to the common cathode connection of the various diodes, the feedback path including the radio frequency choke coil 215. Numeral 216 designates the A. V. C. connections to the various receiver amplifier circuits from the anode end of the load resistor 2|3. The manner of making the A. V. C. connections to the various controlled stages of the receivers is so well known that the schematic representation is believed to be sufficient. The function of the A. V. C. circuit, of course, is to maintain the carrier amplitude at each of the demodulator input circuits as constant as possible in spite of the fading occurring at the signal collector devices.

It will be observed in the arrangement of Fig. 3 that the inverse audio feedback is applied to the common cathode circuit of the demodulators 209, 2l0, 2| l. Hence, it can be seen that the inverse feedback tends to reduce the percentage of modulation existing in the demodulators regardless of which receiver is contributing the predominant amount of energy. If a single receiver is predominating, the feedback operates to reduce the effective percentage of modulation so that distortion due to fading of the carrier on that receiver is minimized. If more than one receiver is contributing to the output, the feedback automatically reduces the effective percentage of modulation produced by the resultant of the outputs of the several receivers. It will, therefore, be seen that selective fading occurring in shortwave communication can be effectively reduced by virtue of the diversity reception augmented by the inverse audio feedback to each of the demodulators of the separate reecivers. It is to be understood that the arrangement shown in Fig. 2 could also be utilized in connection with the diversity reception system shown in Fig. 3.

While I have indicated and described several systems for carrying my invention into effect, it will be apparent to one skilled in the art that my invention is by no means limited to the particular organizations shown and described, but that many modifications may be made without departing from the scope of my invention, as set forth in the appended claims.

What I claim is:

1. In a receiving system for modulated carrier waves subject to selective fading, said receiving system comprising a plurality of independent modulated carrier wave transmission networks each including a separate demodulator, a common load element for said demodulators for developing modulation voltage, means for utilizing said modulation voltage, and a degenerative modulation voltage feedback path between said utilization means and said demodulators for minimizing the effects of said selective fading.

2. In a diversity reception system of the type comprising a plurality of separate modulated carrier Wave receivers each provided with an independent signal collector device, each of said receivers having an independent demodulator, a common output circuit for said demodulators for developing modulation voltage, a modulation voltage network coupled to said common output circuit, and an inverse modulation voltage feed back path between said utilization network and said common output circuit for substantially reducing the effects of selective fading.

3. In a diversity reception system of the type comprising a plurality of modulated carrier wave receivers each provided with a signal collector device, each of said receivers having a respective demodulator, a common output circuit for said demodulators for developing modulation voltage, an inverse modulation voltage feedback path connected to said common output circuit for substantially reducing the effects of selective fading, each of said demodulators being of the diode type, and there being a common cathode connection between said common output circuit and the cathodes of said diodes, said feedback path being connected to said common cathode connection.

4. In a diversity system of the type comprising a plurality of separate modulated carrier wave receivers connected to provide a single detected output, an inverse modulation feedback path between said single detected output and each of said separate modulated wave receivers for substantially reducing the effective percentage of modulation of the energy received from said separate receivers.

5. In a reception system of the type comprising separate modulated carrier wave receivers, each of said receivers having an independent demodulator, an output circuit for said demodulators for developing modulation voltage, a modulation voltage utilizing network coupled to said common output circuit, a modulation voltage feedback path between said utilization network and said common output circuit for substantially reducing the effects of selective fading, each of said demodulators being of the diode type, and there being a common cathode connection between said common output circuit and the oathodes of said diodes, said feedback path being connected to said common cathode connection.

6. In a diversity reception system of the type comprising a plurality of separate modulated carrier wave receivers each provided with an independent signal collector device, each of said receivers having a diode demodulator, a common output circuit for said diodes for developing modulation voltage, a modulation voltage amplifier network coupled to said common output circuit, and an inverse modulation voltage feedback path between said amplifier network and said common output circuit for substantially reducing the effects of selective fading.

7. In a diversity reception system of the type comprising a plurality of separate modulated carrier wave receivers each provided with a signal collector device, each of said receivers having an independent diode demodulator, a common output circuit for said demodulators for developing modulation voltage, a modulation voltage amplifier network coupled to said common output circuit, an inverse modulation voltage feedback path between said utilization network and said common output circuit for substantially reducing the effects of selective fading, a common cathode connection between said common output circuit and the cathodes of said diodes, said feedback path being connected to said common cathode connection.

MURRAY G. CROSBY.