US1997012A - Heterodyne circuits - Google Patents

Description

Patented Apr. 9, 1935 UNITED STATES 1,997,012v HETERQDYNE CIRCUITS Harold 0. Peterson, Riverhead, N. Y., assignor to Radio Corporation of America, a corporation of Delaware Application December 22, 1928, Serial No. 327,802

5 Claims.

This invention relates to the combination of energies having different frequencies, and more particularly to heterodyne circuits.

It is frequently necessary to combine energies in order to obtain resulting energy of different frequency, particularly in modulationA and in heterodyning. In heterodyning, the frequencies involved are usually relatively` closely adjacent, so that the tuned circuits for each of the frequencies, unless reactively insulated in somefashion, are vapt to affect one another during the tuning of one or the other. With short Wave energy this difculty is quite marked, even if the frequencies are not close. It is aprimary object of my invention to prevent coupling between such circuits, so that they may be independently, tuned, and this I do by coupling one of the cir,-y

cuits to the combining means through an untuned or detuned coupling tube.

When dealing with very high frequencies the capacitive coupling between the electrodes of the coupling tube may become appreciable, and in such case I nd that this objection may be overcome by the use of a screen-grid tube as a coupling tube, in order to improve the reactive insulation.

It may prove desirable, in some cases, to utilize a harmonic of the local oscillator frequency for combination With the received energy, and in such case the practice heretofore has been to letthe detector tube which is used to combine the received and locally generated energies also function as a harmonic generator for the locally generated energy. In accordance with my invention, for this purpose I merely bias the cou-l pling tube sufficiently to cause it to generate the desired harmonic which is to be fed to the heterodyne detector.

My invention is described more in detail in the following specification, which is accompanied by a drawing in which Figure 1 is a Wiring diagram for a heterodyne receiver employing my invention; and Figure 2 is a modification in which the receiver circuits are symmetrical pushpull circuits.

Referring to Figure 1 it will be seen that energy is collected on an antenna circuit 2, and coupled through a coil 4, to the resonant input circuit 6 of an amplifier tube 3. y A grounded astatic shield I0 may be provided between the coil 4 and the circuit Ii in order to prevent capacitive coupling therebetween. The anode of the tube 8 is connected to a resonant circuit I2, which, like the circuit 5, is tuned to the frequency of the desired signal. This resonant circuit I2 acts as an output circuit for the ampliiier tube 8, and also as an input circuit for the heterodyne detector or combining tube I4, which is given an asymmetric characteristic by means of a blocking condenser I6, and a source of high negative bias potential I8. The energy with which the received energy is to be heterodyned is generated in a local oscillator 20, the resonant output circuit 22 of which is tuned to the frequency of the locally generated energy. This circuit is coupled to the control electrode of the detector tube I4, preferably by a lead 24 connected to the lower potential side of the resonant circuit I2.

Now, in accordance With my invention, the means connecting the oscillator 20 with the heterodyne detector includes an untuned coupling tube, here numberedV 26. As shown, the resonant cir-V cuit 22 isl connected to the control electrode of. the coupling tube 26, and the anode of the coupling tube is connected tothe lead 24 and thence 'to the resonant circuit I2 andthe control electrode of the detectorl tube I4. The tube 26, may be considered as untuned, or, including circuit I2, as, detuned. When dealing with very high frequencyenergythe tube 26, as Well as the ampliiier tube 8, may preferably be of the screengrid type. Asv is customary, the screen-grid of the tube 26 is polarized atthe proper potential by a battery 28, but is connected directly to ground, with respect to radio frequencies, by a, blocking condenser 30.

The anode of the detector tube I4 may include a high pass filter 32 for byepassing the radio frequency components of the detected energy, and the output, if an audio frequency, is translated in any suitable translating device 34, here shown coupled to the anode circuit of the tube I4 by a transformer 3e. It should be understood that in the case of a superheterodyne receiver the output from the detector I4 may be an intermediate frequency, Which then is further amplified in an intermediate frequency amplier, before the nal detection and translation.

It win be noticed that the various stages of the apparatus are shielded to prevent mutual in- 45 terference, and the necessity for this shielding increases With the frequencies being handled.

If a harmonic of the local oscillator frequency is to be employed in the heterodyne detector the negative bias of the tube 26 is made sufficiently high to give the tube an asymmetric characteristic, in order to provide harmonics in its output.

In order to provide symmetrical receiving circuits it is often desirable to employ pushpull aniplifier and detector stages. In such case the local to the pushpull input circuit of the heterodyne detector, which provides considerable insulation against mutual reaction between the tuning of the oscillator and the tuning of the receiver circuit. However, at very high frequencies I find that this alone is not suiiiciently preventative, and that my invention may be applied to this type of circuit with equally good results. An arrangement illusrating this is shown in Figure 2, in which the antenna circuit 2 is coupled through a coil 4, provided with a grounded astatic shield I0, to the resonant input circuit 6 of a pushpull' amplifier stage using the amplifier tubes 52 and 54. The anodes of these tubes are connected in series by the resonant circuit I2, which acts as a pushpull output circuit for the amplifier tubes 52 and 54, and as a pushpull input circuit for the heterodyne detector tubes 56 and 58. The anodes of the detector tubes are connected in series to an output coil 69, which is coupled to a translating devicev 34. Condensers 62 may be used to by-'pass the radio frequency components' of the detected energy. Y

The control electrodes of the tubes 52 and 54 are biased by a potential source 62, which is connected to the midpoint of the coil of the input circuit 6, While the detector tubes 56 and 58 are biased for detector action by a source 64, which is coupled to the midpoint of the resistance 66. The anodes of the tubes 52 and 54 are polarized from a potential source 68, which-is connectedy to the midpoint of the coil of the circuit l2,-and the anodes of the detector tubes 56y and 58 are polarized by a source 10, connected to the midpoint of the output coil 60. The oscillator20 is connected through a coupling tube-26 and a lead 24 to the midpoint of the coil of the resonant circuit Vl2. This being a zero potential point with respect to the received frequency, there is an immediate reduction in the tendency-for reactive coupling between the circuit I2 and the oscillator circuit 22, and any remaining tendency is counteracted by the reactive insulation between these circuits which is provided by the coupling tube 26. l

In this case the tube 26 need not` be a screengrid tube, and preferably isy not a screen-grid nary screen-grid tubes, for the reason 'that theA tube is coupled to the mid potential point of the coil of the circuit I2, and must feed that circuit with considerable energy. However, if a low impedance screen-grid tube is available it may well be used;

As before, the control electrode of the tube 26 is biased from a suitable source of biasing potential, `the magnitude of which determines whether or not the coupling tube 26 is to act also as a harmonic generator.

In the claims which follow I shall use the term untuned to mean either untuned or detuned, broadly, that is, an electron emission tube arranged primarily, not for amplification, but rather for reactive insulation.

I claim: v

l. In combination a source of radio frequency energy, shielding means Vfor said source, a second source of radio frequency energy, the frequency of said last named source differing from the `frequency of said first named source, shielding means for said last named source, a thermionic device for combining energies from both of said sources, shielding means for said last named device, an aperiodic screen grid thermionic device for coupling one of said sources to said combining device, and shielding means for said coupling device.

2. Means for demodulating a signal modulated high frequency carrier comprising, thermionic means responsive to said carrier, thermionic rectifying means coupled to said carrier responsive means, a thermionic oscillation generator, and means for coupling said thermionic responsive means and said thermionic oscillation generatorl to said rectifying means and for preventing reaction between said means comprising aA screen grid thermionic tube having its input electrodes coupled through an untuned circuit to said generator and its output Aelectrodes coupled to said rectifier, and separate shielding means for said recti-V fying means, for said generator, and for said coupling tube.

3. In combination, a signal source of energy, another source of energy comprising' an oscillation generator, a device for combining said energies, a path coupling said signal source to said device, and another path including a screen grid l tube for coupling said oscillation generator to said device, and separate individual shielding means for said sources, said screen grid tube and said device.

4. In combination, in a heterodyne receiving circuit, an antenna, an electron discharge device having input and output circuits tuned to the incoming frequency, said input circuit being coupled to said antenna, said output circuit including an inductance, a detector, an oscillation generator, a connection from one end of said inductance to said detector, and a connection from a point intermediate the ends of said inductance to said oscillation generator, said last connection including an untuned screen grid coupling tube for preventing reaction betwn said oscillation generator and said output circuit'.

5. In combination, a signal lsource of energy, an oscillation generator, a circuit tuned to the frcquency of said signal source comprising an inductance, a device for combining the energies from said signal source and said oscillation generator, a connection from said signal source and from said device to the same terminal of said inductance, and a connection including a screen grid tube from a point intermediate the ends of said inductance to said oscillation generator.

HAROLD O. PETERSON.

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