US2433442A - Radio transmitter - Google Patents

Description

Dec. 30, 1947. J, MI DODDS |51-Y AL 2,433,442

" RADIO TRANSMITTER Filed Deo. 29, 1945 AMPLIFIER alle: uylu- Patented Dec. 30, 1947 anni T OFFICE RADIO TRANSMTTER John Mathieson Dodds, Timperley, and .lohn

v Heywood Ludlow, Bowdon, England Application December 29, 1943, Serial No. 516,106 In Great Britain May 13, 1938 (Cl. Z50-17) 3 Claims.

This invention relates to radio transmission systems.

Systems of radio communication have inherent limitations as to the speed and/or quality of the transmission of intelligence by reason of their inability beyond a limit to produce and radiate high frequency signal-s, which, when received, can be resolved into satisfactory reproduction of the desired original signals. One limitation arises by reason of the uneven frequency response of the tuned circuits in the transmitter. Means such as overcoupling may be employed whereby improvements in the above respects can be achieved when the wavelength is a few hundred metres. The defect however is difficult to eliminate or even reduce on the shorter wavelengths of say below 50 metres, especially in transmitters which are ar ranged for operation on several different wavelengths.

The present invention provides a method by which the defect above referred to may be considerably reduced in a transmitter so that the speed or quality or both of the transmission may be improved to a degree where it is substantially limited only by the receiving means which are employed for the transmitted signals. Since the performance of the receiver can be improved relatively cheaply, the present invention provides, inter alia, a saving in the cost of the system.

The present invention is concerned not so much with improving the performance of transmitters modulated with relatively complex signals such as sound or vision modulation proper, but relates more particularly, although not exclusively to systems using instantaneous changes of carrier level, such as the marking and spacing of code and the impulses used in television for synchronisation.

The problem with which the present invention deals is as follows. When an attempt is made to excite a resonant circuit in conformation with a signal element of rectangular wave form such as is required in the systems mentioned in the preceding paragraph, the resulting response in the circuit is modified in a manner dependent on the resistance and capacity in the circuit. It is convenient to consider the case where a circuit having resistance R and capacity C is required to respond to a rapid succession of dots, that is square topped envelopes recurring at relatively high frequency. As is well known, the level of the rst part of the dot will cause the radio frequency current to build up exponentially in the circuit asymptoting to the mark level whilst the end of the signal will initiate an exponential fall to the space level which is usually zero. The time taken for, the current to rise or fall to a reasonable percentage of the nal value is dependent on the time constant RC of the circuit.

If this time is comparable With the duration of either mark or space required in say high speed signalling, considerable distortion of the wave form results.

In short wave transmission the value of C is frequently irreducible below the internal capacity of the power valves employed. Similarly these valves for efcient operation require a certain minimum value of R determined by the electrical characteristics of the valves. Thus the time constant RC is more or less xedin value on those transmitters which are the best suited for high speed operation.

The case under consideration is similar to that involved in picture modulation in television transmission, Indeed the latter, which is normally considered as a complex wave form containing all frequencies from zero (direct current) up to the limit .frequency of the system (a few megacycles) might in the ideal embodiment cornprise va succession of horizontal steps occurring at various levels in accordance with the brightness of successive picture elements joined by verticals, the length of the steps corresponding with the size of the picture element as is predetermined in the system. In practice this condition is not fulfilled nor with the existing limitations is it necessary, for since by analysis the stepped wave must cover an infinite frequency spectrum, the method would not be practicable. At present the upper limit of frequencies transmitted is between two and three megacycles. In order to' achieve even this performance however, it is found necessary to reduce the effective value of R in the tuned circuits of the transmitter to a value far below the optimum from a point of view of the valve characteristics, with consequent poor eiciency. Thus it is found that in a transmitter requiring a frequency response at within three decibels over a band of 1.5 megacycles the con- Version efficiency is less than 15%: 60 kw. input being necessary to obtain 7.5 kw. output at 50 megacycles carrier frequency: see Conklin and Gihring, Television transmitters, R. C. A. Re- View, July 1937.

The rectangular wave forms now under consideration constitute, as mentioned above, a parallel or similar case with the foregoing and the object of the presentI invention is to provide a satisfactory output wave form with relatively great efficiency.

g ing in the aerial the tailing portion 3 (Fig. 2) as hereinbefore described. This 'anti-phase burst must be so controlled that its energy is substantially wholly and solely utilised in augmenting the rate of decrease of the in-ph'a'se signal. It "is of course undesirable that any of such anti-phase modulation should actually appear in the aerial.

It will be appreciated that the important parts of the additional impulses inserted by the device -29 (Fig. 3) are rstly the steepness of the impulse fronts and secondly the effective amplitudes thereof. In the example given theimpulses are narrow rectangular ones, but apart from the two characteristics mentioned above the impulses need not conform to this shape. In fact any impulses having steeper wave fronts and higher initial amplitude than that produced by the signal alone will clearly eiect an improvement. The particular circuit combinations used to provide the distortion may thus be of simpler form than that required for generating the narrow wave shape shown.

In many cases the correction afforded by the first impulse may be unnecessary whilst on the other hand the spacing of successive signals Will render the second or anti-phase impulse desirable.

It will be appreciated that the advantage of the system above set forth and described is the improved degree of accuracy of transmission obtainable. Moreover, with normal systems of keeping, wherein the spaced condition is obtained by shutting off a valve by applying a large negative potential to its grid, for example, the residual interelectrode capacities of the valve or valves may still permit a small amount of energy to pass through them continuously. A further advantage, therefore, of the system lies in the fact that during the space condition both of the modulators 25 and 26 will pass such energy and their inputs being anti-phase but their outputs in parallel, no energy of lcarrier frequency will pass to the final amplier.

The use of the rst impulse, however, presupposes that the transmitter is capable, at least for a Very short time, of producing considerably more power than is required for the normal mark level. In some cases this may not be considered economical, although it may prove desirable to employ ampliers which will give this condition in all the stages except the output stage. Some improvement will result from this arrangement.

It will be appreciated that the distorting device 29 must be arranged for the generation of impulses. In general, it may comprise a pair of impulse generators of which the output of one is connected between the leads 21 and 24a, whilst the output of the other is connected between the leads,28 and 24a. These generators may for example be of the type utilising switches such as grid-controlled discharge tubes and provided with suitable resistance-capacity circuits to generate when switched or triggeredf impulses of the required form. These two generators are so arranged that the rst is trigger-ed by the rising front of the signal element which is applied to the terminals 33, after which the signal element, with the impulse added to it is passed to the leads 2l and 24a, whilst the wave-front produced by the end of the signal element may be used to trigger the other impulse generator which thus produces a similar impulse on the leads 28 and 2da. Fig. 4 illustrates diagrammatically an arrangement for producing the impulses.

f* Referring te rig. il which is to be considered in eenunetian k'with Fig.. f3, the tendu ers 2c and rildo'f the latter diagram have con-n ated :across fthe resistance Rl which is of low value and Tthe conductor-'s '21 and 24d have #connected across 'er 'the switch blades are adapted to be connected to a source of voltage V through a combination of rsistances and cndensers as follows. The switch terminal :i: is vadapted to be connected to the source V through the resistance R3 which is bridged by a condenser C2, whilst the terminal y of the switch S is adapted to be connected to the source V through the resistance R4 which is high compared with Rl, the latter being bridged by the condenser Cl. It is assumed that the switch S is operated electromagnetically by means of a coil s connected to the terminals 33 (Fig. 3) to which an electro-mechanical keying arrangement is connected. The arrangement shown in Fig. 4 operates as follows.

When no voltage is aplied to the terminals 33,4

the coil s' is de-energised and the switch S occupies the position shown so that in the steady condition the resistances R4 and Rl act as a potential divider for the lead 28 across the source V. Therefore, since the value Rl is low compared with that of R4, the voltage on the lead 28 is near zero whilst the condenser C2 having been discharged by the resistance R3, the potential of the switch terminal .'11 is that of the source V. When the voltage is applied to terminals 33, the switch blades move over to the other position so that the pot-ential of :c appears on the conductor 21. However, owing to the condenser resistance combination C2R3 the voltage on the conductor 21 drops exponentially to a steady level which will be RVi-R2 During this time the condenser CI is no longer bridged by the low resistance Rl and so it charges up to the potential V through the resistance R4. When the Voltage is removed from the terminals 33 the switch S rapidly moves over so that the potential of the conductor 21 falls immediately to zero by reason of the resistance R2, but the potential V on the condenser Cl momentarily `appears on conductor 28, this potential falling exponentially to zero as the condenser CI discharges through the resistance Rl.

We claim:

l. A radio transmission system arranged to transmit rapid changes in carrier level, means to produce an in-phase modulated carrier, means to produce an anti-phase modulated carrier, means controlling the in-phase and anti-phase carriers and arranged upon a rapid decrease in input signal to supply said anti-phase carrier producing means with an initial impulse whereby the anti-phase carrier solely augments the rate of decrease of the in-phase carrier level.

2. A radio transmission system arranged to transmit rapid changes in carrier level, an aerial, means to produce an in-phase modulated carrier, means to produce an anti-phase modulator carrier, means controlling the in-phase and antiphase carriers and responsive to a. rapid increase in input signal to superimpose thereon an initial impulse to augment the rate of increase in carrier -level in the aerial, said last-named means being effective upon a rapid decrease in input signal to supply said anti-phase carrier producing means with an initial impulse to solely auglevel.

3. A radio transmission system as claimed in claim 2, wherein the means controlling the inphase and anti-phase carriers is adjusted to cause the anti-phase carrier producing means to bring down the carrier level in the aerial rapidly to substantially zero.

JOHN MATHIESON DODDS. JOHN HEYWOOD LUDLOW.

REFERENCES CITED The following references are of record in the ment the rate of decrease of the in-phase carrier 5 me 0f thls patfent:

UNITED STATES PATENTS Number

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