US2124191A - Demodulation system in superheterodyne receiver - Google Patents

Description

'July 19, 1938. M, GEIGER 2,124,191

DEMODULATION SYSTEM IN SURERHETERODYNE RECEIVER Filed Sept. 25, 1936 'Ml/L T/PL/CA 7/0) FACTOR 0F 15' STAGE 02 z osr. 6 7

E QVMPl/IWDE L/M/TEQ INVENTOR MAX QEIGER BY m ATTORNEY Patented July 19, 1938 DEMODULATION SYSTEM IN SUPERHETER- ODYNE RECEIVER Max Geiger, Berlin, Germany, assignor to Telefunken Gesellschaft fiir vDrahtlose Telegraphic m. b. IL, Berlin, Germany, a corporation of Germany Application September 25, 1936, Serial No. 102,475

In Germany October 23, 1935 3 Claims. (01. 250-20) This invention discloses a novel method of and means for demodulation which may be useful which is obtained from the intermediate fre 1' 9 quency by frequency multiplication.

Figures 1 and 2 of the accompanying drawing show diagrammatically two modifications according to the invention.

Referring to Fig. 1' of the drawing, the frel 5 quency to be demodulated (signal or incoming frequency) E-is impressed in Fig. 1 on the control grid I of. a mixer tube or demodulator 2, say, a hexode, also known as a pentagrid converter. At the plate 3 is made available the intermediate or beat frequency I. F. (which for the time being must be imagined to exist) which is then amplified in the intermediate-frequency amplifier 4. This I. F., on the one hand, is then further amplified and/or demodulated, as Well known in the 25 art, and, on the other hand, is impressed upon the frequency multiplier 5. The frequency from the output of the latter then serves as the heterodyning frequency 0, instead of an oscillation which would have to be produced in a distinct 30 oscillator, and is fed to the other control grid 6 of the mixer tube. The I. F. amplifier 4 and the frequency multiplier 5 may be of the convential types as known in the art, and for that reason are shown in block form. 5 Insuring in the multiplier stage 5 a multiplication 1), then the following relations hold good:

I. F.=E-O

40 whence the division ratio between frequencies E and I. F.

# Numerical examples are indicated in brackets in Fig. l. The intermediate frequency is here assumed to be 2.5 megacycle's (120 m.) and the same is raised fifteen times. The ensuing heterodyne r frequency of 37.5 megacycles forms with the incoming signal frequency of 40 megacycles (7.5 meters) the intermediate or beat frequency of 2.5 megacycles. The dividing ratio is so that the incoming frequency is converte'dinto an intermediate frequency 16 times lower; Inthe case of a heterodyne receiver the ad,-

vantage of frequency division is that a distinct oscillator is made dispensable. This is particularly desirable Where ultra-short wave'receivers are dealt with, seeing that the local oscillator normally required works With a very high frequency the stabilization of which is attended with particularly great difficulties. .A shift in frequency is transferred to the intermediate "frequency, and its effect on the latter in terms of percentage is far greater and moreserious; in

fact, there is a risk of the intermediate frequency,

in case of'a sufiiciently serious drift of the oscil lator Wave-length, falling entirely outside the range of transmission of the I. F. amplifier. However, by resorting to the method here disclosed this risk is wholly precluded seeing that no separate oscillator is provided and that the I. F. bears a fixed relationship to the incoming signal frequency.

Another merit worth mentioning is that frequency fluctuations of the signal frequency will make themselves less seriously felt than in standard receivers comprising a distinct oscillator. In accordance with the relation 1 I. F. Em

above laid down, the intermediate frequency I. F.

is changed to of the shift in the incoming signal frequency.

What may also be mentioned isthat the arrangement here disclosed will operate only if the multiplication is so chosen that the heterodyning frequency is lower than the incoming or signal frequency, lest the arrangement becomes unstable. This condition will be understood most clearly by assuming, for instance, that the I. F. grows a little. In that case also the heterodyning frequency increases and as a result by Way of the mixer tube the I. F. becomes smaller again so that a counter-action to the assumed increase takes place. However, this effect will arise only where the incoming frequency is higher than the heterodyning frequency, otherwise a higher heterodyn- UNITED STATES PATENT O ing frequency would in turn result in a higher I. F., etc.

The building-up of oscillations in the system from its quiescent state to the oscillatory state occurs, in the presence of the incoming frequency E, just as in the case of a regenerative transmitter tube as a result of and in response to small accidental changes in the tube current which are always present. the building up of oscillations such as here referred to is shown and described in the U. S. patent to OBrien No. 2,039,923.

If the arrangement is to be capable of receiving different frequencies E without a change of the I. F., the arrangement may be rendered useful by that the multiplication u of multiplier stage 5, when tuning to another station, is so altered that always together with the prevailing signal frequency E the same or a fixed I. F. is produced. If the stage 5 consists of a tube with tuned plate circuit (Whose rotary condenser may be coupled with the condenser of the receiving circuit so as to obtain uni-control) and if the same is operated with a markedly negatively biased grid, then the quantity 1? will always be a whole number. In the light of the last equation above indicated, I. F. will then be always an integral fraction of frequency E. This relation prevents any desired signal frequency E from being received. However, it is possible to pick up those frequencies which are a multiple of the I. F. This fact is particularly valuable and important in connection with television reception. Television stations are to operate with frequencies of 40, 42.5, 45, etc., megacycles. Inasmuch as these frequencies are multiples of 2.5

A system wherein occurs' megacycles, this frequency is chosen in this case as the I. F.

Fig. 2 differs from Fig. 1 by the insertion of an amplitude limiter l. The purpose of the same is to prevent frequency 0 from being modulated in case of reception of a modulated oscillation,

for a modulation would occasion distortions in the mixing process.

It may finally be mentioned that the invention is useful not only for receiving sets, but generally for frequency division work. 1

What I claim is:

1. In the reception of signals by the superheterodyne principle, the method of demodulation which consists in collecting the received signal frequency, mixing the received signal frequency with a second frequency which results in an intermediate frequency, and deriving said second frequency by frequency multiplication of a portion of said intermediate frequency.

2 A demodulation circuit arrangement comprising a vacuum tube provided with a cathode, anode, and a plurality of interposed grills, a signal input circuit connected to one of the grids, an intermediate frequency amplifier having an output circuit fed from the anode circuit of said tube, and a frequency multiplier coupled to the output circuit of the intermediate frequency amplifier, and a connection from the output circuit of the frequency multiplier to one of the interposed grids other than the signal grid.

3. A circuit according to claim 2 wherein an amplitude limiter is interposed between the intermediate frequency amplifier and the frequency multiplier.

MAX GEIGER.

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