US2172453A - Radio transmitter - Google Patents

Description

Sept. 12, 1939. c. F. P. ROSE` 2,172,453

RADIO TRANSMITTER Filed April l5, 1938 T Annu nun F /G l-A /NVENTOR lc. E a R055 Patented Sept. 12, 1939 UNITED ,STATESl RADIO TRANSMITTER- Charles F. P. Rose, Asbury Park, N. J., assignor to Bell Telephone Laboratories, Incorporated, New York, N. Y., a corporation of New York Application April 13, 1938, Serial No. 201,735

claims.

This invention relates to the use of reverse feedback in radio transmitters and has for its principal objects to improve the stability against singing of transmitters employing reverse feedback; to facilitate the use of strong reverse feedbacks; and to provide for more effective reduction of signal distortion and noise producing disturbances by means of reverse feedback.

The distortion of speech signals in a radio transmitter occurs principally in those vacuum tubes in the system which are operated at relatively high energy levels and which are therefore likely to be loaded to their maximum capacity. The modulator tubes are frequently the most important source of distortion, particularly where modulation is performed at a high power level. This is as might be expected since the process of modulation requires the use of a device that is non-linear in its characteristics. Another im- 20 portant source of distortion is the output stage of the high frequency power amplifier, that is, the final stage of the transmitter. The distorltion produced here affects both the wave form of the carrier oscillations and the envelope of their amplitudes. Only the latter effect is important since the distortion ofthe carrier wave form does not affect the signal after detection.

The stability of a feedback system against singing is dependent upon the characteristics of the closed loop formed by the amplifiers and the feedback path. When the transmission around this loop is such that the voltage arriving through the feedback path is in phase with the resultant voltage impressed on the input of the amplifiers, singing will occur unless the feedback voltage is sufficiently attenuated. Where a large number of amplifier stages is included in the loop, as is usually the case in a radio telephone transmitter 0f any considerable power, the phase of the feedback voltage changes very rapidly with frequency and at frequencies not far removed from the operating range is likely to reach the coincident value that gives rise to instability. A major problem in the application of feedback to radio transmitters is, therefore, that of so organizing the elements of the feedback path as to provide for strong feedback at the operating frequencies and to diminish the feedback at frequencies removed therefrom at such a rate as to insure ade- 50 quate reduction of its magnitude before the frequency of phase coincidence is reached. The

problem is made more difficult by the fact that the phase shift of the feedback is itself dependent upon .the rate at which its magnitudedimi- 55 nishes. f

In accordance with the present invention two reverse feedback paths are provided in a. radio transmitter, one extending from the output circuit ofthe modulator' or modulating amplifier tasthnesignal input circuit and the other ex- 'frequency of the carrier Wave.

tending from the output circuit of the complete total resultant feedback as hereinafter described.

The nature, of the invention will be more fully understood from the following detailed description together with the accompanying drawing, of which:

Figs. 1 and 1A show the application of the invention to a radio telephone transmitter; and

-Fig. 2 'illustrates a modified form of theinvention.

Referring to Fig.' 1, speech lcurrent source S supplies signal voltage oscillations through transformer I0 and differential bridge II to a three stage signal amplifier comprising vacuum tubes 4I2, I3 and I4 which are coupled in tandem by means 'of series condensers and shunt resistances. A high frequency source C supplies carrier frequency oscillations through an autotransformer I5 to the grids of aduplex modulator comprising vacuum tubes I6 and I'I. The output circuit of the modulator includes a parallel circuit, coil I8 and condenser I9, which is tuned to the is of the well-known constant current type. The plates of signal amplifier tube I4 1nd modulator tubes I6 and Il lare connected together through a midpoint tap in coil I8 and are supplied with direct current through a common high inductance choke coil 20 from a direct current source 2 I.

'I'he input voltages of tubes 22 and 23 are de-.

rived from the potentials across choke coils 26 and 2l. The output circuit of the power amplifier 1s a tuned circuit comprising coils 30 and 3| and The modulator condenser 32. A transmission line 33, 34, which i may lead to an antenna, is connected across cony able grid bias voltages through leads 35 and 36.

'Ihe grids of the speech amplifier tubes are selfbiased by means of resistors included in the cathode leads as shown. The cathode heating circuits of the several tubes havev been omitted for the sake .of clarity. f

The over-all feedback path is connected to the Aoutput .circuit of the power amplifier through `condensers 39 and 4l Awhich are preferably of veryA small capacity, and insulated to withstand the high radio frequency voltages. Full wave rectification ofV the modulated carrier wave is and j! and shunt resistors I3 and Il. The detected signal is impressed on a feedback line 46 effected in a linear rectiner comprising diodes Il feedback can be insured by poling the connection" I of line I6 to the rectifier output.

The internal feedback path vextends from an 'adjustable tap in resistance 48 in the cathode lead of tube Il, through blocking condenser I9 to,

the cathode of tube l2 and cathode lead resistance 50.

yResistance carries not .only the direct current of the plate circuit of tube I4 but also a superimposed alternating current corresponding to the signal frequency variations of the current in this tube and modulator tubes It and I1. It is to be no ted that as Ithe plate current in tube Il increases, the plate current in tubes il' and I1 diminishes and vice versa, the two-plate current circuits together formingv a single series path for the signal frequency variations.` Because of this the current variations in resistance Il correspond very closely to the envelope variations of the modulated wave and reproduce any distortion of the signal that may appear in the envelope variations.: The feedback -voltage is impressed upon vresistoriil which is included also in the grid circuit of tube Hand is thereby transmitted to the vacuum tube grid.

The connection of to the'input of. the signal amplifier through the bridge [I makes this feedback path conjugate to the signal source S. 'Ihe high impedance of the grid circuit oftube I2 effectively separates the two feedback paths and `prevents direct feedback from the transmitter output to the modulator which might otherwise take place through the two)feedback paths in series. yComplete conjugacy of the two feedback paths can be obtained by means of the double bridge input circuit illustrated in'Fig. 1A, which represents an alternative arrangement of the system of Fig. 1 to the left of the line XX'. lIn this alternative, the

.conductor of the internal feedback path is connected to the corner ofthe second bridge I I' instead of to the cathode of the amplifier tube I2' asinvFig. .1.

The cooperative action of the two feedback' paths will now be explained.V

It is'well known that the stability of a simple feedback amplifier and the distortion' reducing effect of the feedback are dependent upon the net amplification experienced bya voltage or current oscillation in the course of a single traverse of the closed loop and upon the variation of this vector quantity with frequency. The net loop amplification is-usually denoted by.. where p defines the vector amplification in the amplifier portion of theloop and p the transfer factorI for the feedback path. The effect of the feedback is to diminish the enective amplification of the system in proportion to thequantlty (1-p) il and to reduce distortion and noise by substanthe over-all feedback path tially the same amount. The output level may be restored to its original' value by an appropriate increase in the amplifier input voltage without losing the benefits-of the distortion and noise reduction.

The action of the two feedbacks in the system lof Fig. 1 ma);r be analyzed as follows:

Let m'denote the voltage amplification of the speech amplifiers from input terminals AA to output terminals BB' and let [i1/3i denote the net voltage amplification in the closed loop including thesev amplifiers and the local feedback path.

Also, let an denote the voltage amplification from terminals BB to terminals CC' in the high frequency system and let denote the voltage transfer factor between CC', and 'AA' for trans'- mission through the over-all feedback path. The quantities pz and refer to the signal variations, that is, pz' denotes the `amplification of the signal modulation or'the envelope variation of the carrier wave and the transfer factor relating the rectified signal voltage delivered to AA' to the signal modulations at CC'. For the over-all loop, in the absence of any feedback in Likewise, it or noise voltage originating in the high frequency ampliiler'and contributing a modulation'voltage AE: at terminals CC in the absence of feedback is reduced by the action of the double feedback to the value Comparing Expressions 2, 3, and 4 it is evident that noise or distortion originating in the speech vamplier is reduced by the feedback action to the same extent as the effective gain of the whole system is reduced, while noise or distortion originating in the high frequency portion of the system is reduced to a somlevhat lesser extent. The

difference corresponds the reduction of gain may be shown that a distortion l increased of the speech amplier effected by its local f back.

If the over-all feedback pinze: is made as large as possible, consistent with stability require-- ments when the inner feedback is absent, a certain reduction of the noise and distortion origiby the use of an increased amount of feedback in' the over-al1 loop. This is made possible by the stability of the system brought about by the use of the local feedback around the speech amplifier and the net result of the use of the two feedbacks is to considerably increase the suppression of the modulator distortion.

- The manner in which the inner feedback operates to increase the stability is as follows: From the Expression 2 for the effective amplification of the system, it is seen that the stability is determined by the quantity 1(p11+1;22) (5) and that an unstable condition occurs if the phase angle of the vector (mi--mazz) becomes zero while its magnitude is still greater than unity. Because ofthe larger number of amplier stages and coupling networks included in the over-all feedback loop, the phase shift of the net amplification or feedback in this loop, amarla, varies more rapidly with frequency than does that of the net amplification, mp1 for the inner feedback loop. For any given signal frequency, therefore,the phase shift of the total feedback (msi-i-mzz) from the desired degree value will be less than that of the over-all feedback lnuzz. By the use of resistance networks, as shown in Fig. 1, for coupling the speech amplifier stages, the phase shift of the inner feedback may be maintained at a low value for a range of signal frequencies several times as great as that required for speech transmission. The phase shift of the total feedback is thereby greatly reduced and asubstantial increase in the over-all loop feedback is made1 possible without resulting instability.-

The application of the invention to a radio transmitter employing a grid bias modulator is shown in Fig, 2. This figure shows the manner in which the system of Fig. 1 between the dotted lines XX' and YY may be modied to provide for grid bias modulation. One stage of the speech frequency amplifier is dispensed with and the output of the final stage tube I4 is connected through a resistance network and tuned circuit |8-I9 to the grids of the duplex modulator tubes i6 and I1. Plate current is supplied to the modulator tubes through chokes 53 and 54 from a direct current source connected to lead 55. The carrier source C is connected to the grids in push-pull relation through a transformer coupling to coil I8. The modulated output passes through ltuning condensers 24 and 25 to the power amplier as in Fig. 1.

The feedbackfrom the modulator output extends `from an adjustable contact on resistance 5I infthe cathode lead blocking condenser 49 to the speech input circuit. The ,closed feedback'loop includes three amplification stages, as in Fig. 1, the modulator of tubes I6 and I1 through tube itself functioning as the and providing the desire versals in the loop.

Part of the grid bias on the modulator tube is obtained from the fall of potential in resistance 5|, but additional bias may be supplied by asuitable source', such as battery 52 in the grid leak circuit. If desired the modulator may be operated as a class B amplifier, that is, with the third audio stage grid biased substantially to the cut-oli' point. In

that case, the signal currents appear in resistor 5I as the modulations of amplified half period current pulses ofthe carrier frequency. What is claimed is:

1. In a radio transmitter comprising a signal amplifier, a carrier wave source, a modulator, circuit connections for impresisng on said modulator waves from said source and amplified signal oscillations from said amplifier, and an amplifier for modulated carrier wave coupled to the output circuit of said modulator, a reverse feedback path extending from the output circuit of said carrier Wave amplifier .to the input circuit of said signal amplifier, and a second reverse feedback path extending between the input circuit of said signal amplifier and an intermediate number of phase lrepoint in the system at which amplified signal waves are present and at which the signal amplification is subject to distortion as the result of the action of said modulator.

2.- In a radio transmitter comprising a signal amplifier, a carrier wave source, a space discharge modulator having `a control grid, a cathode, and

an anode, circuit connections `for impressing waves from said source on said modulatorgrid circuit and amplified signal oscillations on said modulator anode circuit, and an amplifier for modulated carrier waves coupled to'said modulator anode circuit, a reverse feedback path extending from the output circuit of said carrier wave amplifier to the input circuit of said signal amplier and including a linear rectifier, and a second reverse feedback path extending from the output circuit of said signal amplifier, to the input circuit thereof.

3. In a radio transmitter comprising in tandem relation a signal amplifier, a grid-bias modulator. and an amplifier for modulated waves, a reverse feedback path extending from the output circuit of said modulated wave amplifier to the input circuit of said signal amplier and including a linear detector, and a second reverse feedback path extending from the output circuit of said modulator to the input circuit of said signal amplifier.

4. A system in accordance with claim 1 including circuit means in the input of said signal amplifier rendering the two feedback paths conjugate to' each other whereby direct traon from the mst-mentioned feedback path to the modulator is prevented. y

5. system in accordance with claim 1 in which the said signal amplier comprises a plurality of vacuum tubes coupled in tandem by resistance networks andin which the variation with frequency of the phase angle of the feedback loop constituted by said signal amplifier and said its - cnginmsnnlosn, n

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