US2297931A - Modulated high frequency transmitter - Google Patents

Description

a. L c. vym-rs' 2,297,931 I ODULA TBD HIGH FREQUENCY TRANSMITTER I Filed la'rch 16,1940

Oict. 1942.

INVIENTOR ERIC L. 0. WHITE g (M1.

ATTORNEY I Patented Oct. 6, 1942 MODULATED HIGH FREQUENCY TRANSMITTER Eric Lawrence Casling White, Hillingdon, England, assignor to Electric & Musical Industries Limited, Hayes, Middlesex, England, a company of'Great Britain Application March 16, 1940, Serial No. 324,315

In Great Britain March 16, 1939 3 (ilaims.

The present invention relates to modulated high frequency carrier-wave signal transmitting systems and has particular, but not exclusive, reference to so-called ultra-short wave radio transmitting systems, for example, for transmitting television signals.

In British Patent Specification No. 421,013 a method of reducing distortion in the modulation occurring in modulated carrier-wave transmitters is described which consists in rectifying voltage derived from the modulated output of the transmitter and utilizing the rectified output to act as a distortion compensating voltage in opposition to the modulation voltage proper. That is to say, a modulated carrier-wave transmitter is provided wherein the signals to be transmitted are applied to the modulator stage together with signals in opposite phas derived from a rectifier connected in the output of the transmitter. The transmitter output, therefore, tends to set itself so as to produce from the rectifier connected in the output almost an exact copy of the signals in the input to the transmitter. This copy is not exact owing to the fact that a finite input is requiredto operate the transmitter and owing to the fact that the transmitter requires large inputs at very low signal levels due to its bottom curvature. This is particularly disadvantageous in the case of television signals where synchronizing signals are produced by reducing th signal current substantially to zero, as in this case the relative amplitude of the synchronized signals in the output of the transmitter is reduced, these signals being thereby in effect, crushed.

The object of the present invention is to provide a modulated carrier-wave transmitting system for television signals in which a feed-back path is provided through a rectifier connected in the output in such manner that rectified signals in anti-phase relation to the input signals 'are applied to the input of the modulator in the system and in which signals of low amplitude are not crushed due to the operation of the feedback means and may even be enhanced relative to signals of greater amplitude.

According to the present invention a modulated carrier-wave transmitting system having connected between the output circuit and the input circuit of a modulating stage of said system a feed-back path which comprises a rectifier arrangement associated with said output circuit and which functions so as to improve the linearity of modulation is provided, wherein the operating characteristic of said rectifier arrangement is so curved that feed-back over said path occurs to a smaller extent for small amplitudes of highfrequency signals appearing in said output circuit than for large amplitudes of said signals, whereby said linearity of modulation is still further improved or non-linear distortion arising elsewhere in said system is reduced or substantially eliminated. Conveniently the desired eurvature in the operating characteristic of the rectifier arrangement is obtained by applying a bias in such manner that the arrangement only operates to produce feed-back in respect of output signals of which the amplitudes exceed a certain level. Preferably the feed-back path in the system comprises a pair of rectifiers connected at points separated by a distance corresponding to a quarter of a Wave length of the carrier-wave of the signal in the feeder, and arranged so that the mean distance of the points of connection of the rectifier from the input end of the feeder is small compared with the wave length of the highest modulation frequency of the currents in the feeder whereby the effect of reflection from the remote end of the feeder is eliminated.

The invention also includes the provision of a radio relay arrangement in which signals are demodulated or detected and the modulation products amplified and used to modulate a further transmitter, the relay arrangement having a number of advantageous features as will appear from the following description of the method of carrying the invention into practice, reference being made to the accompanying drawing, of which the single figure represents the circuit diagram of a radio relay arrangement incorporating the invention.

In the arrangement illustrated received carrier wave signals are fed, after amplification with or without frequency change, through the input I into a detector 2 of which the anode is connected through carrier frequency choke 3 and load resistance 4 to the source of potentia1 5 shown as having its negative terminal earthed. The cathode of detector 2 is earthed on conductor 6 through the secondary of the input transformer. A smoothing condenser l is shown connected across the detector 2 for the purpose of by-passing unwanted carrier frequencies.

The anode end of resistance 4 is connected to one end of a potentiometer resistance 8 having an upper half 8a variable by tap to and a lower half comprising two preferably equal branches 8b and 8c of each of which one end is connected through a choke 9a or 9b, as the case may be, to the cathode ends of load resistances Illa and lBb arranged in the circuits of rectifiers Ha and I ll) connected between the two conductors of the feeder l2 through which the output of the transmitter is fed to an aerial (not shown). The arrangement of these rectifiers H is more fully referred to below.

An amplifier valve I3, shown as being of the screen grid pentode type, is arranged with its anode connected through load resistance It and potential dropping resistance I5 to a source of potential 5, resistance l5 being decoupled for signals by condenser I6. The input electrode of valve I 3 is connected to the tap 8e and to the upper ends of branches 8b and 8c of potentiometer 8, and the cathode of valve I3 is connected to the earthed conductor 6.

The output of valve I3 is applied through the resistance-capacity combination I I to the control grid of a valve I8 which is arranged as a socalled cathode follower valveof whichthe output is developed across a load resistance I9 arranged between the cathode lead of the valve and a source of negative potentialZE. A suitable grid resistance for valve I8 is provided as indicated at 20. The anode of valve I8 is connected directly to the positive pole of source of potential 5.

The output of valve I8 is fed through a high frequency choke 2I to the input of a modulating amplifier 24, to which the carrier currents to'be modulated are fed'at '23 through transformer-"22. This modulating amplifier 21 comprises a pair-of valves operating in push-pull relation, the modulated output of the amplifier being fed through transformer 25 into the feeder I2. The transformer 25 is provided with a secondary winding 25a tuned by a seriescondenser 25b and a high frequency choke 250 provides a D. C. path for rectified currents from rectifiers II. Polarizing voltage for rectifiers II is derived through adjustable tap 21a on a resistance 21 from source 26 of which the positive pole is connected to earth. Condensers 28a and 282) are connected across rectifiers Ma and I Ila-respectively to provide a carrier frequency return path for smoothing purposes.

It will be seen that in the arrangement described the outputs of rectifiers 2 and 'II are coupled in anti-phase to the input of amplifier valve IS. The output "of this valve is repeated through the cathode follower valve I8 which isolates the output of valve I3 from the input of the modulated radio frequency amplifier 24 and also provides a low impedance output for connection to the input of the modulated amplifier.

The signals fed from the resistance I9 will be in phase with the signals fed from the valve I3, so that the carrier wave signals in the output of amplifier 24 will be modulated in phase with the signals occurring in the output ofdetector -2,and further, so that the correct phase relations 'between the outputs of rectifiers 'I I and detector 2 will be obtained.

Thus an increase in signal current-through rectifier 2 will be accompanied by an-increased negative feed-back through rectifier II and vice versa. Hence the arrangement will tend to set itself so that an almost exactcopy, but in opposite phase of therectified signals-from the rectifier 2, appears in the output of the rectifierl I, so that the input to the feeder 'I2-wi1l be likewise corrected. If the parallel'resistance of portions 31) and 8c of potentiometer 8 is-made greater or less than the resistance of portion 8a,-the whole arrangement will have a gain or-loss of amplitude respectively, so that a variable tap-8e on the portion 8a forms a convenient gain control.

In the case of a radio transmitter, however, while the negative feed back arrangement described is very efiective-it is not feasible to obtain an exact copy ofthe input signals where those signals are of low amplitude. This is particularly true in the case of positive television transmission, in which synchronizing signals are required to be transmitted by substantially suppressing the carrier current at the appropriate intervals,

owing to the lowslope of "the modulation char- "back for low carrier amplitudes in the outputis reduced or completely suppressed. This may be done, for example, by setting the bias of the rectifiers II by adjusting the tapping 21a of resistanceZI in such manner that a desired result is obtained.

In this way not only can the defective operation-of the modulation amplifier 24 be allowed for, but also undesirable eflects in the original transmitter, or other remote part of the transmission system, in respect of which the relay arrangement is --required to'operate can be compensated for. This feature-provides, in conjunction with the feature of amplification control by the above described adjustment of the 'tapping'Be a highly advantageous combination in that by varying the tappings 2M and 8c, the transmitter can be adjusted for television signals of the kind indicated herein to give a substantially undistorted output with synchronizin'g'signals of satisfactory amplitude.

The arrangement of rectifiers II shown in the drawing is such'as to overcome the eifectswhich may result from mis-match at'the aerial end of the feeder I2. The connections between the feeder I2 and the rectifiers I I a and III) are effected at points A and B respectively separated by a distance corresponding to a quarter of the electricalwave length of the signals in the feeder, and arranged as close as possible to theinput end of the feeder so that'the mean distance of the points of connection of the rectifiers from the input end issmall compared with the wave length of the highest modulation frequency of the currents in the feeder.

With this arrangement, if the feeder and the aerial are mis-matchedfor exampleydue to an error in the impedance-of the'aeria'l'of an amount 6Z0 from the characteristic impedance Z0 required for perfect matching, it canbe shown that the eifectof the reflection on the combined 'response of the rectifiers is negligible if 620 is not more than 10% or soof Z0. Thus,'assuming that the voltage being fed into the feeder I 2 is V1 and the reflected voltage is V2, then V2 V1. Thus, if, at a given instant, the phase angleof V1 at A is pt, and the phase angle of the reflected voltage at B is (pt) then the voltage'VA appliedto rectifier IIa will be V cos pt+ V cos (ptand the voltage VB applied to rectifier I'Ib' will be aerial end. This, however, is not objectionable as it only causes a small amount of phase modulation, and the object of the arrangement as a whole is to control the amplitude of the total input into the feeder.

I claim:

1. A modulated carrier wave transmitting system having a modulating stage, output and input circuits for said stage, a coaxial cable coupled at one end to said output circuit, a feedback circuit connected between said coaxial cable and said input circuit, said feed-back circuit having two parallel paths and a rectifier in each path, the connection of said paths to said coaxial cable being at points separated by a distance corresponding to a quarter of a Wave length of the carrier wave of the signal in said coaxial cable, and means for so applying feedback energies through said parallel paths to said input circuit that compensation is provided for nonlinear distortion arising elsewhere in said system.

2. A system in accordance with claim 1 and including means for equally biasing each said rectifier arrangement and in such manner that feed-back occurs only in respect to output signals having an amplitude in excess of a predetermined value.

3. A modulated carrier wave transmitting sys- I tem having a modulating stage, output and input circuits for said stage, a coaxial cable coupled at one end to said output circuit, a feed-back circuit connected between said coaxial cable and said input circuit, said feed-back circuit having two parallel paths and a rectifier in each path, the connection of said paths to said coaxial cable being at points separated by a distance corresponding to a quarter of a wave length of the carrier Wave of the signal in said coaxial cable, the mean distance of said points of connection from the input end of said cable being small compared with the wave length of the highest modulation frequency of the currents in said cable, whereby the effect of reflection from the remote end of said cable is eliminated.

ERIC LAWRENCE CASLING WHITE.

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