US2469218A - Negative feed-back transmission system - Google Patents

Description

H. P. THQMAS .NEGATIVE FEEDBACK TRANSMISSION SYSTEM F1166. April 50, 1947 Uhr;

. high modulating frequencies, is

Patented May 3, 1949 NEGATIVE FEED-BACK TRANSMISSION SYSTEM `Hem-y P. Thomas, Fayetteville, N. Y., assignor to General Electric Company,

New York a. corporation of Application April 30, 1947, Serial No. 744,845

(Cl. S32-.18)

, 9 Claims.

1 My invention relates to a. transmission system employing negative or degenerative feedback, and it has particular utility in a system for improving the stability and fidelity of an angularlymodulated transmitter of the phase-or frequency modulation type.

It is an object of my invention to provide an improved degenerative feedback system for increasing the stability and linearity-of a, signal channel including both an amplifier which is supplied with modulation frequency signals and a modulator which develops a high `frequency modulated output wave.

Another object of 'my invention is to provide a selective feedback network for a high frequency modulation system which is particularly effective to stabilize the operation at lower modulation.

frequencies, thereby realizing the reduction in distortion and hum which is characteristic of a negative feedback system. without adversely affecting the over-all gain-frequency characteristic of the system.

-Still another object of my invention is to provide an improved selective negative feedback system which is particularly adapted to the requirements of a phase or frequency modulation network.

A still further object of my invention is to provide an improved selective degenerative feedback network for a frequency modulation system in which undesirable regeneration, which might otherwise occur due to excessive phase shifts at prevented Without adversely affecting over-all gain-frequency characteristics of the system. k

The features of my invention which I believe to be novel are set forth with particularity in the appended claims. My invention itself, however, together with fur-ther objects and advantages thereof, may best be understood by reference to the following description taken in connection with the accompanying drawings, in which Fig. 1 is a circuit diagram, partly in simplified block form, illustrating one application of my invention to a carrier wave transmitter of the angularly modulated type; and Fig. 2 is a generalized schematic diagram which will be referred to in analyzing certain fundamental principles underlying my invention.

In one embodiment of the present invention there is provided a radio transmission system which may, for example, be of the frequencymodulation type, employing negative feedback in the system. A feedback network, for a translating system receiving a main vapplied siga tendency for .applied to the input nal from a source, is a. network adapted to apply to the input terminal of the system, along with .the main applied signal, a signal derived from the output terminal of the system. If the signal thus `fed back is opposed to the main signal, that is, if one is increasing when the other is decreasing, then the feed :back network is acting as a` negative or degenerative feedback network. Asis well-known to the art, one of the various advantages of negative feedback is that the linearity of the system is improved. That is, there is more of a tendency for the output signal to be a faithful reproduction of the main applied signal, rather than a distorted reproduction thereof.

In the absence of certain additional features to be mentioned, considerable diiculty would be encountered from the fact that under certain conditions, more high frequencies, there is a tendency for the feedback signal to said, rather than oppose the main applied signal, as will lbe understood by those skilled in the art from considerationsof phase shift within the system. When the feedback signal aids the main applied signal, there is a condition of positive feedback or regeneration, Aand self-oscillation of the system, which is undersirableln this system.

One embodiment of the present invention may be viewed as having signal-translating means, a first network through which the main signal is terminal of the signal-translating means, and a second network through which a signal is fed back from the output terminal to theinput terminal. The feedback network of the present invention is adapted t0 feed back a smaller signal at high frequencies than at low frequencies, which is advantageous in avoiding self-oscillation at those high frequencies where the feedback would become aiding or regenerative. If a smaller signal is fed back at one moment than at another moment, however, there would be a tendency for the total am plication of the system to vary, which would be undesirable. As a special feature of one embodiment of the presentinvention, there is provided another network which is interposed between the source of the main signals and the input terminal of the vsignal translating means, and this lastmentioned network is adapted to control the amplitude of the main signal applied to the input terminal of the signal-translating means so that a relatively large frequencies when the negative feedback signal is large, and a, smaller signal is applied at higher particularly at certain relatively main signal is applied at lowr frequencies when the negative feedback signal is small. Since the main applied signal and the feedback signal are opposed to one another, the net applied signal and hence the output signal are determined by their difference. lation of the two networks may be adjusted so that the aplification of the entire system is constant under various conditions of operation. Hence negative feedback is utilized with its attendant advantages, while self-oscillation and variations in the amplification are prevented, as desired. v

In the radio transmitter illustrated in Fig. 1, audio modulation signals are impressed upon the input terminals I0 of audio frequency amplifier II from any suitable signal source. not shown. This source may, for example, comprise a studio microphone and suitable pre-amplifiers and -audio pre-emphasis networks commonly employed in frequency modulation broadcasting practice. The amplified audio signals appearing across the anode load resistor 22 through blocking capacitor I3 upon a frequency selective network serially including resistors I4, potentiometer I5 and coupling capacitor I6, the function and operation of which will be described in detail presently. Potentials appearing across the lower portion of potentiometer I5 and across capacitor IIB, between an adjustable ta-p I1 and-ground, are impressed upon the control grid of a second audioamplifier I2. A direct current grid-return is provided in shunt yto the capacitor I6 by the relatively high resistance I8 and suitable operating v bias for'the amplifier I2 is provided in conventiona1 manner by the cathode bias resistor I9 shunted by the usual audio bypass capacitor 20.

The remaining proper may be entirely conventional and are therefore represented only in simplified block form. The yamplied audio signals at the output of amplifier I2 may be further amplified in an audio power amplifier before they are iml pressed upon a frequency modulator 26. The

modulator 26 may be any one of a number well knownto the art. 'For example, it may comprise a modulation system of the type shown in application Serial No. 632,977, Francis M. Bailey, filed December 5, 1945, now Patent No. 2,461,250, issued\February 8, 1949, and assigned to the same assignee as the present invention. In this type of system, a special tube and circuits is used for modulator 26 and the mean carrier frequency is controlled'by means of a crystal oscillator 21.

The singularly-modulated output wave from modulator 26 is multiplied to a suitable carrier frequency in conventional manner by a plurality of frequency multipliers, indicated by the blocks 28 and 29. f The modulated carrierwave is then `amplified in the radio frequency powerampliners 30 before being 'supplied to the radiating antenna 3l.

In accordance with my invention, a degenerative or negative feedback path is provided between a point in the high frequency channel' following the modulator 26 and the input of the audio amplifier I2. The input to 4the feedback network may be supplied from any suitable point at which the frequency is wave may be readily amplified and-demodulated, and so connected that the polarity is such that the feedback to amplier I2 is degenerative. As shown in Fig. 1, it is supplied from a point 32 between the frequency multipliers. 28 and 29, each of which may have one or more stages.

are impressed circuits of the transmitter y fact, I have found in practice current lament supply for modulator 26, presuch that the carrier i The signals may be first amplified in a conventional high frequency amplifier 35, and are then supplied to a well known form of frequency discriminator network 36. As illustrated, this discriminator is of the type shown in Patent 2,121,103-S. W. Seeley, granted June 21. 1938, although many other suitable types are known to the art.

The demodulation` components are ksupplied through blocking capacitor 31 to a second frequency selective network which comprises resistor 38 and capacitor I6. Capacitor I6 therefore acts as a common coupling impedance for both the audio input signals and the feedback signals supplied to the grid of amplifier l2.

Since a considerable amount of phase shift at the higher moduating frequencies is inevitably introduced by the high frequency circuits of the modulator 26 and following circuits, the feedback may become regenerative at these frequencies'even though the feedback network is supplied from a proper point to provide degenerative phase, at the grid of amplifier I2, at the lower modulating frequencies. This of course cannot be toleratedbecause it will produce circuit instability and parasitics, defeating the purpose of the feedback network. Consequently, the frequency selective nework 38--I6 is employed to provide a gain-frequency characteristic which varies substantially inversely over the audio frequency of the type shown in the aforesaid Bailey application, or an Armstrong phase modulator, the audio frequency distortion is a function of the angular excursion caused bythe applied modulating signal. ulation of the entire frequency modulation system, the angular inversely proportional to the modulating frequency. Hence, the most severe distortion occurs at the lowest modulating this reason, the use of negative feedback at only the lower frequencies is still fully effective in rewhich are commonly encounthe use of alternathum frequencies, tered in transmitters due to ing current lament supplies, is also generally most severe at the lower harmonics of the power supply frequency, such -at cycles and 360 cycles per second', for example. Therefore the negative feedback at lower audio frequencies is also effective in suppressing these effects. In that the direct viously required, canbe eliminated and alternating current supplies used 'throughout the transmitter.

As a practical matter, I have found that It ,is satisfactory to design the frequency selective network 38-I6 to attenuate substantially the higher modulating to 1000 cycles per second but to pass frequencies below this value without' substantial attenuation. Expressed another way, the time constant of this network, that is, the product of its resistanceby its capacity, may correspond to the period of a medium audio frequency of the order of about 400 lto 1000 cycles per second. However, when the feedback is reduced or cut off at the higher modulating frequencies, the relative gain and degree of modulation increases. In accordance with my invention, this is corrected bythe inverse frequency characteristic network in the audio input to the grid of ampliner I2, comprising vthe frequency selective network,

range. In a modulation system For a fixed percentage modexcursion of the modulator is frequency. For

frequencies above about 400- M-IS-i. If the time constant of this net- A work is made equal to that of the network 38-IB, that is, if the resistors I4 and i5 in series have the same resistance as resistor 38, then the tap I'l may be adjusted so that the over-al1 gain 5 i pling impedance Z. Resistance R1 corresponds substantially to resistor i4 in Fig. 1 plus that portion of potentiometer I5 above tap I'I (indicated by point lla in'Fig. 2), assuming that the source Il is a substantially constant voltage feed. Resistance R2 corresponds to that portion of poteniometer I5 below tap I1, and impedance Z corresponds to capacitor I6. The entirersignal translating network between the input to amplifier i2 and the feedback 32 is represented by the block 4| which is indicated as having an equivalent over-al1 gain y. at modulation frequencies. The feedback network, corresponding to the amplifier 35 and discriminator 36 of Fig. l, is represented in Fig. 2 by the block 42 which is indicated as having an equivalent over-all backward gain ,c at modulation frequencies. The second frequency selective network is represented by the resistance R and the common coupling impedance Z in which resistance R corresponds to resistor 38 in Fig. 1. The relatively. high resistance I8 in Fig. 1, which provides a direct current return path between the grid and cathodel of amplifier I2, has substantially no effect upon the operation of the feedback system.

Now if the resistances R and R1 are both made high relative to the resistance of R2, the potential of point Ila can be considered to be essentially the same as that of the Upper terminal 2| of the common coupling impedance simplifying the network analysis.

With the above definitions and assumptions in mind, the gain of the system, with feedback, between points Ila and 32.l is given by the expression:

G1 pl where a is the gain of the system in the forward direction without feedback, as previously described, and 1 is thegain of the total feedback circuit. If the system had a gain independent of frequency before the addition of feedback, ,u in

this expression will be a constant; but with selective feedback the total gain G1 will vary with fregain is then a constant 42) times this expression:

l R J-X The gain G1 of the system between points Ila and 32 in the presence of feedback is then:

The total feedback (the gain of network The gain G2 through the network R1, R2, Z is given by the expression:

RZ-XJL.

If R14-R2 is made equal to R so that the two frequency selective networks have the same time constant, then:

G2: R JX v where:

R2 fC-a+ R, (8) Then the total system gain from signal source 40 to the output point 32 is:

K R -jXe G=G1G2=l%- (9) 1 R IYc KR c G* R-J'X. 1+ (1) I: R 1/ K (J'XJ G K[ R- 1 cXi (11) The gain will be independent of frequency if:

' 1 K m73 (l2) and will have a value:

quency. The network R1, Rz, Z is designed to provide a gain-frequency characteristic which is the inverse of that ofthe system with feedback applied, as will appear later. If the gain of this network is represented by Ga, then the complete gain of the system from the signal source 40, to the output is represented by:

In other words, the gain-frequency characteristics of the system will be restored to the value without feedback if: f'

It is possible to use a considerablenumber of variations in these networks and still maintain the desired inverse characteristics. For instance, it can be seen directly from Equation 11 that the simple capacitor used for the. common -coupling impedance may be replaced by a general impedance Z and if the relations of Equations 12 and 14 are still maintained, the gain will still be independent of frequency. For instance, the impedance Z might be a series circuit of inductance and capacity which could be selected to give essentially infinite attenuation in the feedback circuit at any particular desired frequency merely by adjusting the series resonance of L and C so as to occur at that frequency. This may be advantageous in reducing the gain of the feedback circuits to a very low value at the frequency at which the phase shift through the system becomes degrees, thus preventing instabilityvof the sytsem.

Merely by way of illustration, not in any sense by way of limitation, I have found the following values of circuit constants to be satisfactory in Audio band width=5015,000 cycles per sec.

. Resistor 14=200,000 ohms Resistor 15==100,000 ohms Resistor 38=300,000 ohms Capacitor 16:.0082 m-fd.

Resistor 18--1 megohm While I have shown a particular embodiment of my invention, it will of course be understood that I do not wish to be limited thereto since various modifications may b e made, and I contemplate by the appended claims to cover any such modcations as fall within the true spirit l and scope of my invention.

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

1. In a transmitter of angularly-modulated Ihigh frequency waves, a source ci modulation signals extending over a relatively. low-frequency band, a, signal translation channel having an input connected to said source and'having an output, said channel including means for generating said high frequency waves and 'modulating them in accordance with said signals, means for demodulating said angularly modulated waves at said output to develop demodulated signals, a frequency-selective' network having an inverse gain-frequency characteristic over said band, means for feeding back said demodulated signals through said network to said input in degenerative phase, whereby the relativedegree of modulation at said output is increased at higher modulation frequencies, and a second frequency-selective network in the connection between said source and said input including a coupling impedance across said input common to both said networks, said second networkhaving a correlated gain-frequency characteristic which substantially restores the relative degree of modulation at said output to the value without feedback.

2. A modulation system comprising, in combination, a high frequency signal modulator having a predetermined gain-frequency characteris tic and having an input and an output, a first frequency-selective network, means for impressing potentials within a band of modulation frequencies on said network, an impedance in said network coupled to said input, a negative feedback network comprising means for deriving potentials ,at modulation frequencies from said output and a second frequency selective network, said second network also including said input coupling impedance, each of saidnetworks having maximum gain at a relatively low frequency in said band and decreasing gain at higher modulation frequencies, and means to adjust the relative amplitudes of said potentials impressed on said input from said frequency-selective networks, whereby the effect of said feedback network upon said gain-frequency characteristic at said output may be substantially compensated.

3. In a wave transmission system, a signal translating network having an input and an output and a forward gain a over a predetermined operating frequency range, a frequency-selective network serially comprising a relatively high resistance R1, a relatively low resistance Rz and a coupling impedance, said input being connected across said resistance R3 and said impedance in series, means for impressing lsignals within said frequency range across said network. a negative feedback network having input and output'terminals and having backward gain over said range, a second frequency-selective network serially comprising a relatively high resistance R and said impedance, means coupling said output to said input terminals,

and means for impressing signals from said output terminals across said^ second network, the

values of said resistances substantially satisfying the equations R=R1+ Rz and R1+f'R'='1+' fi whereby the frequency response characteristic of said system is .substantially unaffected by said 15 feedback over said range.-

V4. In a wave translating system, a signal translating path having a gain a between input and output terminals, a signal frequency source having an output, a first frequency-selective network composed of a relatively high resistance R1. a krelatively low resistance Ra and a coupling reactance in series, said input terminals-being coupled 'acrosssaid resistance Rz and said reactance in series, means for impressing signals from said source across said first network, and a negative feedback patnhaving a gain between its input and output,.said input being coupled to said output terminals and saidr output being coupled across a second frequency-selective network composed of a relatively high resistance R in series with said coupling reactance, in which both said networks have substantially the same time constant and in which-the relationship 1a. l. RVi-R, l-p

is substantially satisfi 5. In a modulation and comprising mean s to demodulate said waves and to develop demodulated signals lyingl within said band, a second frequency selective network, means to supply said demodulated signals back to said input through said second network in de generative phase, each of said frequency-selective networks having substantially the same time,

constant and having an inverse gain-frequency characteristic over said band, whereby the relative degree of modulation at said output tends to be increased at higher frequencies within said band duetto said feedback, and means to adjust the relative amplitudes of said signals supplied to said input from said first and second networks,

whereby the effect of said feedback network upon the relative degree o f modulation may besubstantially compensated.

6. In a modulation system, the combination of a source of audio frequency signals, .Ja signal translating channel having an input andan output, said channel comprising an audio amplier supplied from said input and a modulator for producing high frequency waves modulated by said signals at said output, a first frequency sel lective network comprising a potentiometer and a coupling impedance serially connected across said source, said network having a gain which varies inversely with frequency over the audio system, the combination of range, means for impressing audio signals appearing across said impedance and an adjustable portion of said potentiometer upon said input, a negative feedback network supplied from said output, said feedback network including means for demodulating said waves and deriving therefrom the audio modulation components, a second frequency-selectivenetwork serially comprising a resistor and said coupling impedance, and means for impressing said modulation components across said second network, said second network having substantially the same time constant as said rst network. Y

7. In a wave transmission system, a signal translating network having an input and an output, a frequency selective network serially comprising a resistor and a capacitor, means for supplying audio frequency signals across said network, means for impressing upon said input audio signals appearing across said capacitor and a tapped point on said resistor, a negative feedback network comprising means for deriving signals within the audio frequency range from said output, a second frequency-selective network, and means for impressing said derived signals across said second network, said second network serially comprising a second resistor and said capacitor, both said frequency-selective networks having substantially the same time constant.

8. In a wave transmission system, a signal translating network having an input and an output, a frequency selective network serially comprising a resistor and a capacitor, means for supplying audio frequency signals across said network, means for impressing upon said input audio signals appearing across said capacitor and an adjustable tapped point on said resistor, a negative feedback network comprising means for deriving signals within the audio frequency range from said output, a second frequency-selective network, means for impressing said derived signals across said second network, said second network serially comprising a second resistor and said capacitor, both said frequency-selective networks having substantially the same time constant corresponding to the period of a mediumfrequency audio wave, and means for adjusting said tapped point to adjust the frequency transmission characteristics ofsaid system.

9. In a carrier wave transmission system, the combination of a signal channel having input and output terminals, said channel comprising an Aaudio ampliier supplied from said input terminals and a modulator for producing high frequency waves modulated by said signals at said output terminals, a source .of audio frequency signals, a first frequency-selective network serially consisting of a potentiometer and a coupling capacitor, means for impressing signals from said source across said rst network, means for impressing audio signals appearing across said capacitor and a point on said resistor upon said input terminals, a second frequency-selective network serially consisting of a second resistor and said coupling capacitor, a negative feedback network including means for deriving signals within the audio frequency range from said output terminals and impressing them across said' second frequency-selective network, both said frequencyselective networks having substantially the same time constant corresponding to the period of a medium audio frequency, whereby the relative degree of modulation at said output terminals tends to be reduced at lower audio frequencies due to said feedback, and means for adjusting said point on said first resistor to compensate for said tendency. L

HENRY P. THOMAS.

REFERENCES CITED The following references are of record inthe le of this patent:

UNITED STATES PATENTS

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