US2171678A - Superheterodyne receiver - Google Patents

Description

Patented Sept. 5, 1939 UNITED STATES PATENT OFFICE SUPERHETERODYNE RECEIVER Application February 5,

1938, Serial No. 188,842

In Germany March 30, 1937 1 Claim.

The present invention relates to homodyne circuits used in receivers operating on the superheterodyne principle.

It is known that in the case of homodyne reception the ampitude of the carrier oscillation of the received signal is increased in the receiver relatively to its side band, or bands, by an auxiliary oscillation, generated in the receiver, of the same frequency as the carrier oscillation of the received signal being added to the latter. If the signal contains the carrier oscillation and a single side band, care should only be taken that the auxiliary oscillation has the same frequency as the received carrier oscillation. Generally, however, the signal contains two side bands symmetrically arranged relatively to the carrier oscillation, and in this case the auxiliary oscillation should have not only the same frequency but also the same phase as the received carrier oscillation. Both oscillations must, therefore, be in isochronism. In the well known circuits for this purpose the said isochronism is obtained by the generator which produces the auxiliary oscillation being controlled in dependence on the difference between the received carrier oscillation and the auxiliary oscillation by the two oscillations to be rectified and the interference oscillations thus obtained influencing the local generator.

In the well known arrangements satisfactory results are, however, only obtained if the percentage of modulation of the received signal is less than 100%. If the percentage of modulation exceeds 100%, which amounts to sudden phase displacements of the carrier wave by an angle of 180, it is no longer directly possible to maintain the desired isochronism between the received oscillation and the oscillation generated locally. This difficulty is, however, obviated by the frequencies of the two oscillations to be synchronized being multiplied, and an even harmonic of each being brought to mutual interference in a detector so that a beat oscillation is obtained which influences the local generator 45 and brings about the desired synchronism.

In order that the invention may be clearly understood and readily carried into effect it will now be described more fully with reference to the accompanying drawing. In the drawing Fig.

0 1 schematically shows a receiving system embodying the present invention, and Fig. 2 shows a characteristic of the network IU of Fig. 1.

Fig. 1 shows a receiver of the superheterodyne type to which the above-mentioned principles are applied. The receiving channel contains a highfrequency amplifier I capable of being tuned to the signals to be received, a first detector 2 with a local oscillator 3, an intermediate frequency amplifier 4, a second detector 5, a low-frequency amplifier 6; and, in addition, an auxiliary 05- 5 cillator l which is coupled to the second detector 5 and which serves to generate the auxiliary oscillation having the same frequency and phase as the intermediate frequency carrier oscillation of the signal received by the amplifier 4. Due to fading, or to partial suppression of the carrier Wave of the signal, which suppression has occurred in the transmitter, the percentage modulation of the signal supplied to the first detector 2 is momentarily, or constantly, higher than 100%. l From this signal the intermediate frequency carrier wave is separated by a filter 8, and by means I of a frequency multiplier 9 with a subsequent oscillatory circuit l0, an even harmonic, for example the second, is derived from the intermediate frequency carrier wave and supplied to a rectifier II.

This harmonic is brought in the rectifier H to interference with a corresponding even harmonic of the oscillation produced by the auxiliary oscillator I, by the latter oscillation being supplied to a frequency multiplier l2 and the desired harmonic being then selected by means of an oscillatory circuit l3 tuned to it. A beat oscillation is thus set up in the output circuit of the rectifier H, and is used for controlling in well-known manner the frequency of the auxiliary oscillator (for example, by altering the inductance or capacity of the oscillatory circuit of the auxiliary oscillator or by altering the electrode voltages of the discharge tube serving for the generation of the auxiliary oscillation) in such manner that the auxiliary oscillation set up is fed to the second detector 5 in isochronism with the intermediate frequency carrier oscillation. It is found that the optimum control is obtained when the two harmonics fed to the rectifier H are 90 out of phase with each other so that it is desirable that one of the branches 8, 9, I!) or I, I2, l3 should include a phase-displacing network bringing about a phase displacement of 90.

The arrangement described above has a disadvantage in that a small detuning of the local oscillator 3, which consequently results in a small 50 variation in frequency of the intermediate frequency carrier oscillation, exercises a great influence on the phase of the harmonic fed to the rectifier I I. It has been found that this undesirable phenomenon is due to the oscillatory circuit 5 I which selects the higher harmonic. The phase displacement P brought about by an oscillatory circuit is even altered considerably, by about 90, for a low difference between the frequency (f) of the superimposed oscillations and the resonance frequency of the circuit. Fig. 2 shows the ratio between the phase P and the frequency (f) for the oscillatory circuit l0 tuned to the second harmonic 2fm of the intermediate frequency carrier oscillation fm. It is obvious that the same undesirable phenomena occur in the branch 1, l2, l3 due to the presence of the oscillatory circuit l3.

According to the invention, the above-mentioned disadvantage is obviated when the desired higher harmonic of the intermediate frequency carrier oscillations, and/or of the auxiliary oscillations is selected by means of a high-pass or a band-pass filter. Fig. 2 shows the phase constant of the most simple kind of high-pass filters indicated by the dotted curve. Below the cut-off frequency T1 the phase constant has the value 2 and above the same, in the transmitting range of the filter, this value approaches to zero With increasing frequency. The path of this curve shows that when a high-pass filter is substituted for the oscillatory circuits l0 and 13 respectively of Fig. 1, low variations of the frequency of the intermediate frequency carrier oscillation, or of the auxiliary oscillation respectively bring about but a small phase rotation of the higher harmonic. The use of high-pass filters exhibiting an attenuation peak in the non-pass range of frequencies is particularly advantageous. In this case, the frequency at which this attenuation peak occurs is chosen to coincide with the frequency of the intermediate frequency carrier oscillation which is fed to the frequency multiplier 9, and which is thussuppressed to a very marked extent. As an alternative, there may be in the non-pass range two attenuation peaks occurring at frequencies slightly higher than, and slightly below, the frequency of the intermediate frequency carrier oscillation so that, also, in the case of variations of this frequency a high attenuationof the intermediate frequency carrier oscillation always occurs in the output circuit of the frequency multiplier. Instead of using highpass filters, use may be made of band-pass filters having one or more attenuation peaks in the nonpass range.

What I claim is:

A method of homodyne reception which includes heterodyning signals with local oscillations to produce signals of intermediate frequency, producing local oscillations of said intermediate frequency, combining the intermediate frequency signals with the intermediate frequency oscillations, deriving higher even harmonics from each of said intermediate frequency signals and intermediate frequency oscillations, combining the derived harmonics to produce energy of a beat frequency, rectifying said energy of beat frequency, and controlling the phase and frequency of said intermediate frequency oscillations with the rectified energy.

THEODORUS JOSEPHUS WEYERS.

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