US2312374A - High frequency system - Google Patents

Description

March 2, 1943. w. H. UNGER 2,312,374 l HIGH FREQUENCY SYSTEM Filed Aug. l5, 1941 ATTORNEW Patented Mar. 2, i943 HIGH FREQUENCY SYSTEM William H. Unger, New York, N. Y., assignor, by

mesne assignments, to Patents Research Corporation, New York, N. Y., a corporation of New York Appiication August 15, 1941, Serial No. 406,961

(Cl. Z50-20) 6 Claims.

The present invention relates to high frequency systems embodying frequency responsive and/or control devices such as for improving the frcquency stability, for detecting frequency modulated signals and for other uses and purposes.

One object of the invention is to provide an improved frequency variation response circuit or frequency discriminator for converting frequency changes of a signal current or potential into corresponding amplitude changes substantially free from response to spurious amplitude modulation of the signal caused by noise or other interference, thus making it possible to dispense with a special limiter in a frequency modulation receiver.

Another object is the provision of a simplified and highly efficient frequency or tuning control system for use in a transmitter or receiver to maintain the transmitting or receiving frequency at a substantially constant Value.

A further object is to provide a highly efficient and sensitive receiving circuit for frequency modulated radio signals requiring a minimum of parts and circuit elements and especially suited for use in portable equipment.

These and further objects of the invention will become more apparent from the following detailed description taken With reference to the accompanying drawing forming part of this specification and, wherein:

Fig. 1 is a circuit diagram for a frequency modulation receiver embodying the principles of the invention and specially suited for portable sets for communication over shorter distances, though not limited thereto, and Fig. 2 is a transmitter circuit comprising a simplified frequency stabilizing circuit embodying the improvements of the invention. Fig. 3 is a graph explanatory of the function of the frequency discriminator and Fig. 4 shows an alternative way of deriving the discriminating voltage.

Like reference numerals identify like parts in the different views of the drawing.

Referring more particularly to the receiver shown in Fig. 1, frequency modulated radio waves intercepted by an antennae I serve to generate a signal voltage across a tunable input circuit comprised of an induction coil II shunted by a variable condenser I2 and coupled between the antennae and ground I3 in a manner Well understood. The signal voltage developed by the circuit I I, I2 is impressed between the signal input grid I4 and the cathode of a multi-grid electronic mixer tube or frequency converter I5 provided with an additional oscillation control grid I6 excited from a local oscillation generator I'i in any suitable manner to produce signal variations of intermediate frequency in the plate circuit of the tube. The intermediate frequency signals are selectively amplified by means of an amplifier especially designed for the intermediate frequency and comprising, in the example shown, two cascade connected pentode stages I8 and I9 coupled by means of band-pass type coupling networks Z and 2i' of known design.

The amplified intermediate frequency signals supplied by stage I9 are applied to a frequency detector or discriminator 2@ of the general type described in U. S. Patent 2,208,071 which serves to convert the frequency variations of the received signal waves representing the intelligence or message being transmitted into corresponding current variations suitable for energizing an output device such as a loud speaker or a pair of head phones 2I as shown in the drawing.

The receiver shown utilizes tubes provided with directly heated cathodes suitable for operation from a low voltage heating battery 22 marked A-land A-. The anodes and screen grids of the tubes are energized from a high voltage battery marked B+ and B- in a manner Well understood. Suitable anode, screen and cathode bypass condensers as Well as voltage drop or bleeder resistors are provided in the anode and screen grid leads as shown to prevent self -oscillation and other undesirable effects in accordance With the customary design practice of radio receiving circuits.

The discriminator tube 20 comprises a cathode 24 followed in the order named by a first control or signal input grid 25, a second control grid 26 enclosed by a positively biased screen grid 21' and an anode or plate 28. Intermediate frequency signal voltage supplied by the amplifier stage I 9 is impressed between the grid 25 and cathode 24 of the discriminator tube by Way of a band-pass coupling network comprising primary and secondary tuned circuits 29 and Sii, respectively. In this manner the electron space current passing from the cathode 24 to the plate 28 is subjected to initial variations in accordance with the high frequency signals being received. The control grid 26 is connected to the cathode by way of a resonant impedance such as a parallel tuned circuit 3| resonant to the intermediate frequency of the receiver.

As described in the above-mentioned patent, a displacement current will be induced in the external circuit of the grid 26 by the action of a virtual cathode or concentrated electron space it will oiier a high non-reactive impedance to the displacement current resulting in a potential upon the grid 26 substantially in phase with the displacement current which, in turn, is 90 out of phase with the space charge iiuctuations or the potential upon input grid 25. If the latter is frequency modulated the circuit 5i will oier increasingly capacitative or inductive reactance to the displacement current in proportion to and according to whether the signal frequency deviates in one or the other direction, respectively, from the carrier or center frequency in accordance with the variation of a modulating signal. Accordingly, therefore, the phase of the voltage developed upon grid 2t will be increased beyond or decreased below the normal 90 phase relation to the input signal upon the grid 25.

As a result of the above operation the electron space current passing from the cathode to the plate 28 will be subjected to a double control by indentical alternating potentials upon grids 25 and 26 but having a relative phase varying both in sign and magnitude in accordance with the frequency departure of the impressed signals from the carrier or center frequency. A double control of this type will cause a component to be produced in the output or plate cur-rent proportional to the product of the control potentials upon grids 25 and 25 which component, in turn, will include a direct current term varying substantially in proportion to the relative phase be-r tween the control potentials, i. e., in proportionto the frequency changes to be detected `or converted. In Fig. 3 the average plate current ip is shown plotted as a function of the frequency f. As is seen, the current ip varies in either direction with respect to the normal (quiescent) plate current io corresponding to the lcarrier or center frequency fo to which the circuit 3l `is resonant substantially linearly within a range b between the lower and upper limit frequencies fl and f2, respectively. The steepness of the curve, vthat is, the sensitivity of the discriminator may be increased by increasing the Q or decreasing the damping of the circuit 3l. `In the latter case the range b will be decreased and in practice the circuit 3| is so designed as to provide optimum sensitivity for a given maximum frequency deviation Without exceeding the straight line portion of the curve and introducing distortion. The average screen current z'sg undergoes a variation similar to the plate current but in an opposite direction as shown by the dotted curve. Thus, if the input signals are frequency modulated in accordance with variations of sound waves such Vas speech, a demodulated current will appear in the -plate circuit suitable for operating the headphones 2 I.

There are principally two methods fof operat ing a space charge type discriminator described hereinabove. According to one method, the-signal input grid 25 is so biased and operated that substantially -no grid current is drawn or, in

other words, that the grid acts as a pure control electrode (class A operation). In this case the total emission current from the cathode 24 after being subjected to the control by the grid 25 will pass towards the screen 2'! to form a virtual cathode between the latter and the control grid 25. Since both control grids are operated on the straight line portion of their operating characteristics l(grid voltage vs. plate current) substantially no rectication Will take place and the tube will be substantially non-responsive to spurious amplitude modulation of the impressed input signals.

According to another method, the input grid 25 vi's so biased and operated and the amplitudes of the input signal so chosen that a substantial grid current is drawn during the positive half- `cycles of the input signal (class C operation), whereby grid 25 acts as an output electrode and the space current passing to the screen 21 will be equal to the difference between the total emission from the cathode and the current flowing through the grid circuit in such Va :manner that again the current passing through the screen 2i will be subjected to variations iny -accordance with the impressed input frequency. This mode of operation has been yfound 'to be superior and more efficient than the one above mentioned utilizing grid 25 as a pure `control element, provided a signal of suiiicient amplitude is applied to the input grid 25. In the latter case, the 'output current variations 'obtained from the discriminator with the proper adjust'- rnent of the circuit elements and operating -conditions will be sufficient to directly energize 'a undesirable amplitude 'changes of an impressed frequency modulated potential caused by iiuctuation or impulse noise or other interference will be detected and cause interference inthe head set or other output device. To 'obviate this i defect in accordance withthe present invention,

the direct current impedance of the grid circuit is kept at a minimum by avoiding anygrid bias resistance and by making the oh'mic vresistance of the secondary 30 of the input Vtransformer as low as possible to prevent iany rectified current from producing an appreciable 4voltage drop in the grid circuit. For this reason the .grid is biased by the aid `of a 'C-battery 32 icy-'passed by 'a condenser 53 which also serves to bias the second control grid 26 to a potential negative with respect to the cathode. The same `consid'- eration's apply to the control grid 26, which'inay be either biased so as to draw no electron current or which, if current is drawn, is returned to the cathode through the smallest possible D. C. impedance to prevent any rectified current from building up any appreciable Voltage drop.

If desirable, separate biasingsources 'may be provided for the gridsY 25 and 25. Condenser 3l! serves to by-passthe high frequency components scribed, itis also possible to ldevelop a radio fre# quency Voltage by the insertion of a high frequency impedance such as a tuned circuit in the external plate circuit. This radio frequency component represents an amplitude modulated signal, that is, the tube acts as a converter'of frequency modulation to amplitude modulation. The latter may be detected in any suitable manner such as by the aid of an additional diode embodied in the tube to obtain an output varying substantially according to the curve of Fig. 3 as described in U. S. Patent 2,248,197, issued July, 1941, and entitled Frequency variation response circui to which reference is made for further detail.

A receiver of the type shown in Fig. 1 utilizing the 1R5 type midget tube available on the market as oscillator, mixer and discriminator and two 1T4 types as I. F. amplifiers was found to operate satisfactorily. With a signal strength enough to provide 50 micro-amperes of grid current with 3 volts of bias, the plate current was found to undergo a total variation of nearly .5 milliampere out of an average current of about .'7 milliampere. If a load resistance is used in the plate circuit then the average current and the resulting variations are reduced. Thus, in case of an ohmic load resistance of 50,000 ohms a 300 microvolt signal applied to the rst grid of the mixer tube with a total frequency deviation of 3 k. c. was found to provide a five volt peak-to-peak audio signal in the plate circuit of the discriminator. As pointed out, it is important to provide a low resistance return path for the grids of the discriminator to substantially prevent the tube to respond to variations in amplitude as well as in frequency. In this manner a special amplitude limiter preceding the discriminator as used in the standard frequency modulation receivers may be dispensed with, resulting in substantial saving of parts and simplification of the receiver, which is of special importance for portable sets wherein low weight and bulk are to be given prime consideration.

As pointed out above, the tuned circuit 3|, which may be replaced by any other suitable resonant impedance device, should have the highest possible impedance at the resonant, that is, the I. F. frequency to get the greatest sensitivity compatible with the maximum frequency deviation of the input signals.

In a system of the type described, it is of paramount importance that the center or carrier frequency of the input signal voltage applied to the discriminator should always correspond to the center tuning frequency of the I. F. amplier and the discriminating circuit 3l. If the carrier or center frequency deviates from its assigned or xed value, due to oscillator drift or other causes, the operating point O, Fig. 3, of the discriminator will be displaced, resulting in considerable output signal distortion in addition to other distortions produced in the I. F. amplifier as is well understood. In order to avoid this drawback there is provided in Fig. 1 a most simple and efficient frequency control arrangement to maintain the carrier or center frequency at all times exactly in line with lthe center resonant or tuning frequency of the circuit 3 l, that is the I. F. frequency of the receiver.

For this purpose, there is inserted in the plate circuit of the discriminator a resistance 3B bypassed to ground through an audio frequency condenser 35 to develop a frequency control (AFC) voltage due to slowA carrier. or center frequency variations at a rate belowy the lowest audio frequency being transmitted due to oscillator drift and other causes. This AFC voltage is utilized to directly readjust the frequency of the local oscillator I7 to correct for any initial cen; ter or carrier frequency deviation of the signal potential impressed upon the discriminator input from the fixed or normal value. For this purpose the automatic frequency control (AFC) voltage developed by the resistance 36 in the plate circuit of the discriminator is directly utilized to energize the plate of the triode oscillator tube 3l to determine or control the oscillating frequency.

The oscillator shown is of the standard regenerative type comprising a frequency determining grid tank circuit comprised of a variable condenser 38 and an induction coil 39, a feedback or tickler coil 40 in the plate circuit and grid bias resistance 4I. Attempts have already been made to apply the discriminating control voltage directly to the oscillator grid for the control of the oscillator frequency and a careful choice of circuit constants has enabled the control of the frequency to be obtained over a restricted range. There are, however, many objections `to this method. A more favorable method of varying the oscillating frequency is to control the plate or -screen voltage of the oscillator. This, however, in the past required a separate tube since the discriminating (AFC) voltage produced by the known discriminators was insufcient to energize either the plate or screen of an oscillating tube.

By the combination of the present invention all the previous diiiiculties are avoided, while greatly simplifying the circuit arrangement by the elimination of an additional control or reactance tube. The discriminator used by the invention supplies a sufficiently high control voltage which is di.- rectly impressed as operating voltage upon the plate of the oscillator for the control of the oscillating frequency. As is well known, the change of the oscillating frequency is due to a reflected reactance fed back from the plate into the grid circuit and varying in proportion to the conductance of the tube which according to the invention is controlled by varying the plate supply voltage. A similar effect may be obtained by varying the screen voltage when using a tetrode for the local oscillator.

In most cases the reflected reactance in the oscillator is of a capacitative nature due to the inherent grid-plate capacity and varies directly in proportion to the trans-conductance of the tube, that is, in the present case the plate operating voltage. Thus, if the oscillator frequency is greater than the signal frequencyv and supposing that the oscillator frequency increases due to drift, temperature, humidity and other influences, then the intermediate frequency will also increase, resulting in a decrease of the average discriminator plate current ip, as seen from Fig. 3. Consequently, the oscillator plate voltage will rise resulting in an increased reflected reactance in the tank circuit 38, 39, thus, in turn, causing a decrease of the oscillator frequency counteracting the initial frequency increase. On the other hand, if the oscillator frequency is below the signal frequency the correction will be in the opposite sense and the oscillator drift increased, thus rendering the system unstable. In the latter case, automatic tuning correction may be obtained by deriving the control (AFC) potential from the screen grid rather than the plate of the discriminator, as shown in Fig. 4. Alternatively, when using an oscillator wherein the-reflected reactance varies inversely with the trans-conductance, automatic'frequency control maybe obtained with the oscillating frequency being lower than the signal frequency by Vusing .the plate voltage of the discriminator in the manner shown inFig. 1.

In order to prevent resi-dual audio frequency variations from being impressed upon the oscillator plate it is important to include a suitable audio ilter in the AFC circuit, such as in the form lof a series choke coil 42 and bypass condenser 43, as shown, otherwise the audio output will be greatly attenuated. This type filter circuit should lbe designed with regard to the phase shift encountered since if the phase shift is more than 90 while the attenuation is not complete, 'the set will oscillate at an audio frequency. A choke coil is used since the oscillator plate current in the above example is approximately 2 maand a resistance of higher value to be effective would reduce the oscillator plate voltage too much. In an arrangement of this type it was found that any changes in the antennae would not appreciably affect the frequency of the oscillator. The set was found to be quite vstable and the AFC effective even fon fairly weak signals.

It will be evident that the frequency discrim inator and automatic tuning or frequency control `arrangement are not limited to a receiver of the specific .type described for illustration but Will have numerous other uses and applications in combination with o-ther circuits, both for transmitting and receiving. Thus, the discriminator vmay be employed without the automatic tuning feature or the automatic frequency control system described may be incorporated in an amplitude modulation receiver without dep-arting from the spirit of the invention. Moreover, the frequency Vcontrol may serve for stabilizing the center `r lcarrierfrequency of an amplitude or frequency modulated transmitter.

An arrangement of the latter type is shown in Fig. V2, wherein, item 50 represents a master oscillator tube provided with an oscillating tank circuit 5I connected tothe tube to form a well known regenerative oscillator system. A quadrature control potential derived from the tank circuit 5| by Way of a phase shift arrangement comprising a resistance 52 and condenser .in series is applied to the control grid 56 of a socalled reactance control tube 55 by way `of a coupling condenser 54. As is well known, the plate cathode path of `such a tube simulates a reactance which is 'connected across the tank circuit 5I by way `of a further coupling condenser 58. The reactance of the tube 55 is controlled in Yany suitable manner in accordance with the modulating :signal currents impressed by way of Aa low frequency transformer upon a suitableV `control element, such as the suppressor grid as shown. The frequency modulated oscillations are passed :in amanner known Vthrough a frequency multiplier 59 to a power amplifier 66 feeding an Aantenna 62 by Way of coupling transformer BI.

A portion ofthe amplified energy is impressed upon the ihputgrid of :discriminato-r S4 through a resonant coupling transformer 63. The discriminatorisxof substantially the saine type `and operated in Va manner similar to that described in connection with Fig. 1. Item 65 represents the vdiscriminating circuit which, in this case, is resonant to the centeror carrier frequency being transmitted. Since the discriminator is not used as a demodulator, the Q of the circuit 65 maybe chosen sufficiently Vhigh to obtain an extreme frequency sensitivity. Alternatively, the circuit 65 may be replaced by a piezo electric crystal to further increase the sensitivity of the tube to frequency changes. Item 66 is the output resistor corresponding to resistor 3B in Fig. 1 which acts a variableV bleeder or drop resistance in the 'anode supply lead for the oscillator tube 50. lIhe frequency stabilization functions in substantially the same manner as in the case of Fig. 1 by controlling the anode potential of the oscillator in a manner to counteract initial deviation of the transmitting frequency from the assigned carrier or center frequency to which the circuit 64 is resonant. This system has the f-urther advantage over known arrangements in that the frequency modulation and stabilization are separated, resulting in a further increase of the operating stability of the transmitter. Depending upon the particular type of `oscillator used, the plate (AFC) voltage is derived from either the plate oir the screen of the dis-criminator to ob-tain a frequency correction in the proper sense. In Fig. 4 there is shown a discriminator wherein the screen grid circuit includes a resistance 68 by-passed to cathode by condensers 69 and 10 and serving to develop a varying (AFC) voltage for energizing the oscillator in Fig. l vor 3. The ,plate of the discriminator in this case is connected to a source of high potential either directly or through a resistance 'il suitably icy-passed for high frequency.

According to a modification, the connection of the grids 25 and 26 of the discriminator may ybe interchanged, that is, the input signal may be applied to grid 25 and the discriminating circuit connected to grid 25 without substantially affecting the operation of the discriminator, except for a reversal of the polarity of the curves shown inV Fig. 3. The filter in .the circuit in Fig. 2 may be so designed as not only to pass slow variations due to oscillator frequency drift, but to Vpass modulating Vfrequency currents as well to apply a. negative feedback voltage to the oscillator plate to improve the operation of the transmitter bly diminishing noise and distortion in a manner Well understood by those skilled in the art. In this case the circuit 55 should be designed With a Q to provide an operating range b to cover the maximum frequency deviation caused by the modulating signals applied to the terminals a-b.

In Fig. rl the grids of the mixer I5 and the oscillator are directly connected but they may vbe coupled in any suitable manner, such as capacitatively, in which case a grid leak should be provided for the grid i6 'of the mixer. The variable condenser i2 of' the .input circuit and the condenser 38 of the oscillator tank circuit are arranged lfor uni-control, as indicated by the dot and dash lines .and the circuits suitably tracked to .produce the same intermediate frequency for all tuning adjustments nin a manner well understood by those skilled i-n the art.

It will be evident from the foregoing that fthe invention is not limited to the specific circuit arrangements and details shown and disclosed therein for illustration, but that the underlying novel principles will be susceptible of numerous variations and modifications coming within a broader Vscope and spirit of the invention as delined Vin `the appended claims. The specification and drawing are accordingly to be regarded in an illustrative rather than a limiting sense.

I claim:

l. A frequency variation response circuit comprising an electron discharge tube provided with a cathode, an anode and a plurality of grids, an input circuit connected to one of said grids and said cathode, an output circuit connected to said anode and said cathode. means including resonant impedance means connected between said cathode and another of said grids for causing variations of the anode current substantially in proportion to the relative frequency departure of an impressed input signal frequency from the resonant frequency of said impedance means, said input circuit being designed to offer a low enough impedance to the modulation frequencies of an undesired amplitude modulation of said input frequency to render the effect thereof on the anode current due to rectification in said input circuit substantially negligible.

2. A frequency variation response circuit comprising an electron discharge tube provided with a cathode, a rst control grid, a positively biased screen grid, a second control grid and an anode, all arranged substantially in the order named, an input circuit connected to one of said control grids and said cathode, means whereby electron current is drawn by said last control grid during the positive signal half cycles, an output circuit connected to said .anode and said cathode, resonant impedance means connected to said cathode and the other of said control grids for causing variations of the anode current substantially in proportion to the relative frequency departure of an impressed input signal frequency from the resonant frequency of said impedance means, said input circuit being designed to offer a low enough impedance to the modulation frequencies of an undesired amplitude modulation of said input frequency to render the effect thereof on the anode current due to rectification in said input circuit substantially negligible.

3. A frequency variation response circuit comprising an electron discharge tube provided with a cathode, a first control grid, a positively biased screen grid, a second control grid and an anode, all arranged substantially in the order named, an input circuit connected to said first control grid and said cathode, an output circuit connected to said anode and said cathode, resonant impedance means connected to said second control grid and said cathode, whereby said second control grid is excited by electron coupling from an impressed input signal frequency at varying relative phase thereto in dependence upon the relative frequency departure of said input frequency from the resonant frequency of said impedance means to cause anode current variations in accordance with said frequency departure, said input circuit being designed to offer a low enough impedance to the modulation frequencies of an undesired amplitude modulation of said input frequency to render the effect thereof on the anode current due to rectincation in said input circuit substan-A tially negligible.

4. A frequency variation response circuit comprising an electron discharge tube provided with a cathode and anode and a plurality of grids, an input circuit connected to one of said grids and said cathode, means whereby electron current is drawn by said last-mentioned grid during the positive half cycles of an impressed input signal frequency, an output circuit connected to said anode and cathode, means including resonant impedance means connected between said cathode and another of said grids for causing variations of the anode current substantially in proportion to the departure of said input frequency from the resonant frequency of said impedance means, said input circuit being designed to oifer a low enough impedance t0 the modulation frequencies of .an undesired amplitude modulation of said input frequency to render the effect thereof on the anode current due to recticatio-n in said input circuit substantially negligible.

5. In a frequency discriminator, resonant impedance means having a resonating frequency relatively variable with respect to a high frequency signal potential, an electron discharge tube comprising means for producing an electron space current, means including a iirst control electrode for said tube and an input circuit connected thereto for varying said space current in accordance with said signal potential, a second electrostatic control electrode in said tube, means for connecting said impedance means to said second control electrode, further means for producing a concentrated electron space charge adjacent to said second control electrode to excite the same through electron coupling with said space charge by said signal potential at varying phase thereto in accordance with the relative frequency departure of said signal potential from the resonating frequency of said impedance means, an output circuit for said tube, and means operatively associated with said output circuit for developing energy varying in proportion to said frequency departure, said input circuit being designed to offer low impedance to the modulation frequencies of an undesired amplitude modulation of said signal potential to render the effect thereof on said output current due to rectication in said input circuit substantially negligible.

6. In a frequency discriminator, resonant impedance means having a iixed resonating frequency, an electron discharge tube comprising means for producing an electron space current, means including a rst control electrode for said tube for varying said space current `in accordance with a high frequency signal potential having a frequency varying relative to said fixed frequency, a second electrostatic control electrode in said tube, means for connecting said impedance means to said second control electrode, further means for producing a concentrated electron space charge adjacent to said second control electrode to excite the same through electron coupling with said space charge by said signal potential at varying relative phase thereto in accordance with the frequency departure of said signal potential from said fixed frequency, an output circuit for said tube, and means operatively associated with said output circuit for developing energy varying in proportion to said frequenci7 departure, said input circuit being designed to offer low impedance to the modulation frequencies of an undesired amplitude modulation of said signal potential to render the effect thereof on said output circuit due to rectification in said input circuit substantially negligible.

WILLIAM I-I. UNGER.

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