US2147486A - Radio transmitter - Google Patents

Description

Feb. 14, 1939. y G, w. FYLER 2,147,486

\ I RADIO TRANSMITTER Original Filed Dec. 6, 1935 SIG/VAL AMPL/F/ER IS Attorney.

5- PA 0,40 GGF e Wjjlla, `o l mol/rc v V bg 8 Patented F eb. 14, 1939 PATENT OFFICE RADIO TRANSMITTER George Fyler, Stratford, Conn., assignor to General Electric Company, a corporation of New York Application' December 6, 1935, serial No. 53,172

Renewed January 11, 1938` 9 Claims. (o1. 178-44) My invention relates to radio transmitting apparatus, and more particularly to apparatus of the short wave high power type.

In short wave transmitters, as for example, television transmitters, it is necessary to modulate the radio frequency carrier Wave with signal Waves having frequencies which vary over an exceedingly wide range. Thus, in television transmission the signal currents may contain fre- 10 quencies extending from a low audio frequency of cycles to an exceedingly high radio frequency of 2,000,000 cycles. If satisfactory operation of a transmitter of this type is to be obtained, it is necessary that the carrier wave be modulated to the correct degree by all of the signal current frequencies within this broad range.

In other words, the frequency-modulation characteristic should be flat. Such modulation can only be obtained if a coupling impedance be pro- 20 vided between the modulator amplifier stage, and the output circuit of the carrier power amplifier stage having an impedance characteristic such that the impedance to currents of all frequencies within the range is above a predetermined value.

5 It has been found that the impedance of an iron core coupling reactor ordinarily employed in high power work drops off to a value considerably lower than the predetermined value necessary to preserve the'desired degree of modulation at so the high frequencies in the upper portion of the Wide range specified. This is due to the fact that the type of reactor specified has a high distributed capacity which, in the upper frequency ranges, causes the reactor to function as a condenser having a low capacity reactance. The effect of the low impedance to the high frequency components of the signal current is to reduce the degreel of modulation of the carrier at such high frequenciesand introduce undesired distortion into the modulated carrier transmitted.

It is an object of my invention to obviate the above difficulties by providing an improved modulator stage output circuit coupling impedance network suitable for use in a high power radio transmitter which offers an impedance above a predetermined value to currents of all frequencies within an exceedingly wide range.

In accordance with my invention the above stated object is realized by providing a modulator coupling impedance network which includes an air core reactor having a very low distributed capacity shunted by a resistance and connected in series with an iron core reactor having a high distributed capacity; the impedance values of the component elements of the network being so selected that the over-all impedance between the terminals of the network is maintained at a value above that necessary to produce satisfactory modulation of the carrier wave by signal currents having component frequencies extending throughout the wide frequency range specifled above.

Accordingly it is a further object of my invention to provide an improved coupling impedance network for coupling the output circuit of a transmitter modulator amplifier stage to the output circuit of the carrier wave power amplifier stage which comprises the above-described arrangement of impedance elements.

The novel features which I believe to be characteristic of my invention are set forth with particularity in the appended claims. My invention itself, however, both as to its organization and method of operation, together with further objects and advantages thereof will best be understood by reference to the following description taken in connection with the accompanying drawing in which Fig. l illustrates my invention embodied in a short Wave high power radio transmitter adapted to transmit television signals; Fig. 2 illustrates the impedance characteristic of one of the elements of the coupling impedance network illustrated in Fig. l, and Fig. 3 represents the impedance characteristic curve of my improved modulator coupling impedance network.

Referring to Fig. l of the drawing, I have illustrated my invention as applied to a television transmitter of conventional desi-gn. As shown, the transmitter comprises a radio frequency carrier wave generator I connected to supply its output through a driver stage 2, a push-pull class C connected power amplifier system 3 and the windings 4 and 5 of a coupling transformer 6 to the conductors 'I and 8 of an antenna transmission line system. The carrier wave output from the power amplifier is modulated in accordance with the signal oscillations produced by a signal current source 9. These signal oscillations are amplified by the cascade class A connected amplifiers I0, II and I2 and the constant current modulator amplifier stage I3 and are impressed on the output circuit of the power amplifier 3 through a modulator coupling impedance network I4. 'I'his network comprises an air core inductor I5 having a very low distributed capacity shunted by a resistance I6 and connected in series with an iron core reactor I'I having an inherently high distributed capacity. One terminal of the network I4 is tapped to the midpoint of the winding 4 and the other terminal is connected to the positive side of a high voltage source (not shown) which is provided to supply energy to the respective anode circuits of the transmitter system.

In order to neutralize the effect of stray capacity in the carrier power amplifier on the modulator stage an inductance coil I8 is provided which is connected in the modulator stage plate circuit in the manner illustrated. It will be understood that such stray capacity is caused by the distributed capacity of the coils 4 and 5 and-also by the effect of the neutralizing condensers included in the power amplifier-circuit. This stray j capacity has been found to be small as compared to the distributed capacity of the reactor Il and accordingly the value of inductance of coil i8 may be small.

In television transmitters wherein thesource of signal oscillations 9 comprises a photo-electrical cell system, a sourceof light, anda-scanning disk, all suitably arranged `to scan the. object Vto be televised .with a spot of light, the signal current frequencies which are produced by the source and which are amplified and impressed on the plate circuit ofthe power amplifier 3 cover an exceedingly wide frequency band which may extend from a low frequency of 20 cycles to a very high frequency of 2,000,000 cycles. If satisfactory operation of the transmitter is Yto be obtained, all of the component frequencies within this range must be amplified by the same relative amount and impressed on the output circuit of the power amplifier 3 to produce the same relative degree of modulation. If certain of the frequencies are discrimin'ated against, as for example, the frequencies in the high portion of the range, frequency distortion results which produces a blurred image or lack of definition when the signals are reproduced in a receiving system.

One problem involved in the construction of a signal current circuit capable of meeting the above requirement is that of providing a coupling impedance for impressing the output from the modulatorstage I3 on the output circuit of the power amplifier 3 which offers a high impedance having a value above a predetermined minimum value to currents of all frequencies within the band. Thus, it is known that the value of the modulator coupling impedance must be at least two times the impedance-of the load into which the coupling impedance operates at allsignal frequencies within the range, if undistorted modulation is to be obtained.

t has been found that when an iron core reactor-of the type usually employed in high power installations asa modulator coupling impedance is used, the above relation vbetween the coupling impedance and the modulated load impedance is not preserved at the high signal frequencies due tothe relatively high distributed capacity of the reactor. This distributed capacity causes the reactor to act as a low impedance capacitive reactance at high frequencies above a predetermined value. It has further beenfound that as the frequency is raised'above this predetermined value the capacity reactance gradually `approaches zero, which of course means that the desired relation between the coupling impedance and the modulated load impedance is destroyed and unequalized modulation of the amplified signal frequencies results.

The above noted phenomena is clearly illustrated in Fig. 2 wherein the curves A and B, which were plotted from test data taken on an installed iron core coupling reactor, show that the over-all impedance of the reactor decreased gradually to- Ward zero as the frequency of the applied current was raised above 20 kilocycles. In the installation tested the minimum tolerable over-all impedance of the reactor for satisfactory modulation was 10,000 ohms, corresponding to a load impedance of 5,000 ohms. Curve B, which is an extension of curve A on a smaller scale, shows that above 100,000 cycles the impedance of the reactor approached and became less than the load impedance-ofthe load into which the reactor operated. The tests determined the capacitance of the reactor as being 160 micromicrofarads at a frequency of kilocycles which value gradually decreased to approximately 100 micromicrofarads at a'frequency of 1000 kilocycles.

'In accordance with my invention these difficultiesare avoided by providing in series with reactor I'I the air core inductor I5 having a very low distributed capacity -and shunted by the resistance I6. The impedance network thus formed may be resolved into lan equivalent series network, as indicated by the dotted lines, which network is an approximaterepresentation of the circuit inthe high frequency portion of the frequency range if the capacitive reactance of the coil I5 be neglected. The impedance 'values of the respective elements I5 and I6 are of course determined bythe value of the modulated load impedance into which vthe coupling impedance operates. Thus, if the value of the modulated load impedance, and the impedance characteristic of the reactor II,be known,'the impedance'values of the elements I5 and i6 may be readily determined. The arbitrary lower impedance value for the coupling impedance network may be placed at two times the load impedance in accordance with the requirements described above. At this arbitrarily determined minimum the impedance of the reactor is capacitive and from the frequency-impedance characteristic curve ofthe reactor the particular frequency at which'this lower limit is reached maybe determined. Using the frequency value as thus determined, the value of inductance of the element I5 necessary to introduce an inductive reactance I5 into the equivalent series circuit which exactly neutralizes the capacitive reactance I'I of the reactor I'I, andthe value of resistance l5 which produces an equivalent resistance IB equal to the required value of atleast two times the load impedance may becalculated in a well known manner. It can be shown mathematically that the inductor I5 should have at the particular determined frel quency a reactance valueequal to the value of the resistance I6 and that the impedance value of each of the elements i5 and I6 should be at least two times the lower arbitrary impedance limit necessary to preserve equalized modulation over the entire frequency band.

Following the determination of the impedance values vof the elements I' 5 and I6, the over-all impedance of the `network I4 may be .determined at any frequency and a curve vplotted illustrating this impedance as a function of frequency.

Referring to Fig. 3 of thedrawing I have shown a curve obtained in the above described manner illustrating, by way of example, the characteristic impedance curve .determined from data taken on the test circuit referred to above. In this circuit the impedance of the modulated load was 5,00G ohms and the reactor Il ,presented an impedance of approximately 10,000 ohms capacitive reactance to signal currents having a frequency of Cil 100,000 cycles. From this data the necessary values of elements I5 and I5 were determined at .032 henries and 20,000 ohms respectively. With the impedance values of the elements I5, I0 and I'I thus determined the curve C was plotted.

It will be observed from curve C that the overall impedance of the network is very high in the low frequency portion of the signal frequency range due to the effect of the reactor I'I, and that it decreases to a minimum value of 10,000 resistive ohms at approximately 100 kilocycles due to the effect of the series inductive reactance I5 in the equivalent series circuit on the capacitive reactance of the reactor II. As the frequency is increased above 100 kilocycles the inductive reactance of 15 increases with the frequency to an exceedingly high value and the capacitive reactance of the reactor Iii approaches a zero value thereby leaving the resistance I6 as the effective coupling impedance. This effect is clearly illustrated by curve C which shows that the over-all impedance of the test circuit considered approached 20,000 ohms, or a value approximately equal to the value of the resistance.

In practice it was found that the distributed capacity of the coil I5 affected slightly the overall impe-dance frequency characteristic curve of the coupling network. The test values obtained indicated that the actual impedance values over certain portions of the frequency range of the network were slightly above the calculated values. The effect of this capacity on the modulator stage is effectively cancelled by the inductance of the coil i8 in the same manner that the inductance effectively equalizes the stray capacity of the carrier power amplifier circuit.

From the foregoing analysis it will be observed that at the lower frequencies the impedance of the iron core reactor I1 predominates and that in the upper portion of the frequency range the parallel-connected resistance I6 and inductance I5 are effective to supply the necessary impedance, It will further be observed that in the upper portion of the frequency range the impedance is largely determined by the resistance i0 and that, therefore, this resistance should bear a predetermined relation to the impedance of the modulated load. For best operating results the value of the resistance I6 should be approximately four times that of the load impedance and the impedance I 5 should have a reactance equal in magnitude to that of the resistance at the lower arbitrary limit of impedance of the reactor I'I. In order to main the effective series impedance I5', Iii high for a considerable range above the natural resonant frequency of the inductance I5, the inductance I5 should have a very low distributed capacity. This may be secured by constructing the inductance I5 of a plurality of series-connected universal wound thin disc coils each having a very low distributed capacity.

From the foregoing description it will be observed that by employing the elements I1 and It connected in the manner illustrated and proportioned in value with respect to the modulated load impedance in accordance with the principles outlined above, an over-all impedance is obtained which insures satisfactory modulation over an extremely wide band' of frequencies. It will further be seen that my improved coupling impedance network is particularly useful in a high power transmitter installation wherein relatively high direct currents flow thru the coupling impedance network elements and extremely high voltages exist between the terminals of the network. Thus, it will be seen that my improved coupling arrangement is particularly useful in those applications where it is not economically practical to use a large non-inductive resistance as a coupling element.

Although I have described my invention as being particularly useful in connection with television transmitters, it will be understood that it is susceptible of being employed in other high frequency applications as well. Thus, for example, my improved coupling network may be employed in any installation wherein it is desired to couple signal currents having frequencies varying over an extremely wide range to a utilizing circuit of any type.

While I have described what I consider to be the preferred embodiment of my invention, it will of course be understood that I do not wish to be limited thereto since many modications in the circuit may be made,V and I contemplate by the appended claims to cover all such modifications as fall within the true spirit and scope of my invention.

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

1. In combination, a source of signal currents having desired frequencies extending over a'wide range, a utilizing circuit, a coupling impedance network for impressing said signal currents on said utilizing circuit, said network including an inductance coil having a high inductive reactance at low frequencies within said range and a low impedance below a predetermined value at high frequencies within said range, an inductance coil in series with said first-named inductance coil having high reactance at said high frequencies, and a resistance in shunt with said second-named inductance coil of such value that the impedance of said coupling impedance network is increased by reason of said shunt resistance to a value above said predetermined value of said Erst-named inductance coil at said high frequencies.

2. In combination, a source of signal currents having desired frequencies extending over a wide range, a utilizing circuit, a coupling impedance network for impressing said signal currents on said utilizing circuit, said network including an inductance coil having a high inductive reactance at low frequencies within said range and `a low impedance below a predetermined value at high frequencies within said range, an inductance having a low distributed capacity connected in series with said first-named inductance, said secondnamed inductance having high reactance at said high frequencies, and a resistance in shunt with said second-named inductance of such value relative to the impedance thereof that the impedance of said network is increased above said predetermined value at said high frequencies,

3. In combination, a source of signal currents having desired frequencies extending over a wide range, a utilizing circuit, a coupling impedance network for impressing said signal currents on said utilizing circuit, said network including an inductance coil having a high inductive reactance at low frequencies within said range and a low capacitive impedance below a predetermined value at high frequencies within said range, and means including an inductance having a low distributed capacity connected in series with said first-named inductance and a resistance connected in shunt with said last-named inductance for maintaining the impedance of said network above said predetermined value at said high frequencies, the magnitude of the impedance of said last-:named inductance being such as to `neutralize the rcapacitive'impedance of said first-named inductance at that frequency at which theimpedance of fsaid'rst-named inductance equals said predetermined value, and-said resistance `having a ivaluesubstantially equal to said magnitude of the impedance of said second-named inductance at said `last-named frequency.

4. In combination, a source `of signal currents having desired frequencies extending-over a wide range,.azutilizing circuit, a coupling impedance network-*connected toimpress said signal currents on `said :utilizing circuit, said network including an inductance coil having low distributed capacity shunted by a resistance and connected in series With an inductance coil having a high distributed capacity, the impedance of said coils in series being increased at a certain frequency in said range by saidiresistance.

5. In'combination, a source of signal currents having desired ifrequencies-extending over a wide range, a utilizing circuit, a coupling impedance network connected to impress said signal currents on `said utilizingpcircuit, said network including an inductance coil having low distributed capacity yshunted by a resistance and connected in serieszwithan inductance coil having a high distributed capacity which introduces a capacitive reactance into said network at frequencies above aipredetermined'value within said range, said resistance being -of such value asto increase the impedance between `the outer terminals of said coils.

6. VIn combination, -a source of signal currents having desired frequencies extending over a wide range, a utilizing circuit, a coupling impedance network connected to impress said signal currents lon said utilizing circuit, said network including aninductance coil having low distributed capacity shunted by a resistance yand connected in series with an inductance coil having a high distributed lcapacity, said Asecond-named inductance :having :an inductive reactance at low frequencies within said yrange and a capacitive reactance below a predetermined value at high frequencies within said range, 'the magnitude of the impedance of `said iiirst-named inductance being such as to --neutralize the capacitive impedance of said -second-named inductance at that frequency at vwhich the impedance of said lsecondnamed inductance equals said predetermined value, and said resistance having a value substantially greater than the magnitude of the impedance of said second-named inductance at Vsaid last-named frequency, whereby the impedance of said network is vmaintained above said predeterminedminimum value at all frequencies within said range.

J7. In combination, a .source of signal currents having-desired frequencies-extending over a wide range, a utilizing circuit and a coupling impedance network rfor impressing said signal currents on said utilizing circuit, said network including an air corel-reactor `shunted by a resistance and connectedin .series with an iron core reactor, said resistance being -of such value as to increase the impedance between the outer terminals of said network.

v8. In combination, a source of signal currents having desired frequencies extending over a wide range, ya utilizing circuit, a coupling impedance network for impressing said signal currents on said .utilizing circuit, said network including an inductance coil havinga high inductive reactance at low-frequencies Within said range and a low capacitive impedance belowa predetermined value at1high frequencies -within said range, and means for. maintaining .the impedance of said network above ya predetermined value at said high frequencies, said last-named means including a plurality .of series-connected inductance coils each having a low distributed capacity connected in series with said first-named inductance and a resistance connected in parallel with said lastnamed series connected inductance coils, said resistance having Ya value at least two times the magnitude of said predetermined value, and said series-connected Vinductance coils having an impedance value equal to said resistance at that frequency at which the impedance of said rst named inductance equals said predetermined value.

9. Incombination, a source of signal currents having desired frequencies-extending over a wide range, fa .utilizing circuit, a coupling impedance network .for impressing said signal currents on said utilizing circuit, said network including an inductance coil having a high inductive reactance at vlow frequencies within said range and a low impedance below a predetermined value at high frequencies within said range, and means including an inductance havinga low distributed capacity connected in series with said first-named inductance and a resistance connected in shunt .withsaid last-named inductance for maintaining vthe impedance of said network above said predetermined Avalue at said high frequencies, said resistance having a value at least two times the magnitude .of said predetermined value, and said second named inductance having an impedance .value equal vto said resistance at that frequency where the impedance .of -said first named inductance equals .said predetermined value.

VCrEORGE W. FYLER.

Images