US2208142A - Amplitude and frequency modulation receiver - Google Patents

Description

July 16, 1940. o. TUXEN 2,203,142

AMPLITUDE AND FREQUENCY MODULATION RECEIVER Filed Jan. 51. 1959 IEcAER/ER PHASE 4.1:.

FILTE/Z SHIFTE'R. AMPLIFIER CONVERTER I. F.

DETECTOR AMPLIFIER p j r- F (4 N L I H M z D 25 AMPLIFIER a 5 m OSCILLATOR l ERN/EIZ DEMODULA TOQ TUNING Y MEANS FOR A.F'.C-

INVENTOR orro .EXEN

ATTORNEY Patented July 16, 1940 ALIPLITUDE AND FREQUENCY MODULATION RECEIVER Otto 'Iiixen, Berlin, Germany, assignor to Telefunken Gesellschaft fiir Drahtlose Telegraphic m. b. 11., Berlin, Germany, a corporation of Germany Application January 31, 1939; Serial No. 253,730 In Germany February 21, 1938 2 Claims.

It is already known to provide in a receiver a switching means by which the receiver can be switched at will to receive oscillations modulated in the amplitude, or to receive oscillations modufi lated in the frequency. Furthermore, it has been suggested to provide an automatic switching of this type such that the state is automatically established at which the largest low frequency component appears respectively at the 10 output of the receiving detector.

In accordance with the present invention, a distortion free demodulation of oscillations whether modulated in amplitude or frequency can be obtained without any switching means in that the received oscillation containing the carrier and the side bands is multiplicatively mixed with an oscillation having the same frequency as the arriving carrier and a phase displacement of approximately 45 relative to said carrier.

In the hitherto known methods of multiplicative demodulation it is true that the receiving oscillation has also been mixed multiplicatively with an additional oscillation having the same frequency as the arriving carrier. But in this case in the reception of oscillations with modulated amplitude the phase difference of the additional oscillation relative to the arriving carrier was 0, and in the case of frequency modulated oscillations it was 90. Hence, it was not possible in these known arrangements to receive oscillations with modulated amplitude and oscillations with modulated frequency without a switching performance.

An example of construction according to the present invention is shown in the accompanying drawing. The oscillation received by the antenna A and amplified in the high frequency amplifier H is mixed in the mixer stage M with the oscillation of the local oscillator O. The constant intermediate frequency obtained in the output is amplified by the intermediate frequency amplifier Z. A part of the amplified oscillation is used across the vernier'tuning means S for the automatic frequency control of the oscillator thus serving for maintaining accurately constant the intermediate frequency. Another part of the amplified intermediate frequency is passed across the filter F (quartz filter for instance) which filters out of the arriving band the intermediate frequency carrier without substantially altering its phase. The filtered carrier eventually amplified separately receives in the phase shifting means P a phase displacement of 45 whereafter it is mixed multiplicatively with the unfiltered intermediate frequency in the multiplicative demodulator D.

The demodulator may consist of a. multi-grid tube having two control grids separated by a screen grid. There is applied to the one control grid the filtered-out carrier displaced in phase by 45 and to the other grid the total intermediate frequency oscillation including the side bands. The produced audio frequency is applied to the loudspeaker L across the audio frequency amplifier N. l

Inplace of the filter F the filtering of the carrier may be done by means of an oscillator which oscillates in the intermediate frequency and is actuated by the arriving carrier.

The following deduction shows that in the arrangement according to the invention a distortioh-free demodulation can be obtained in the case of an oscillation with modulated amplitude as well as in the case of an arriving oscillation with modulated frequency. An oscillation with modulated amplitude can be represented by the following equation:

The one or the other one of the two oscillations will now be mixed multiplicatively with the oscillation:

Z=C SlIl(21rHt+zp) If the oscillation Z is to have a phase lead of exactly 45 relative to the carrier of the arriving oscillation, this denotes that in the case of the amplitude modulated oscillation the value c is equal to +45 and in the case of the frequency 5,.

modulated oscillation this value is equal to -45. In each case the multiplication furnishes in addition to a D. C. part and high frequencies an oscillation as sole audio frequency which is represented by the equation:

COS go sin Z-IrNt Hence no upper harmonics of the audio frequency appear no matter what the value of (p may be. However, the amplitude depends on o). For +45, or 45 there is cos =1/ /2 and there fore the amplitude is:

4 about 30 or 60, but in this case the efiectiveness of the demodulation would be greater for amplitude'mo'dulated oscillations than for frequency modulated oscillations.

Instead of having the filtered carrier displaced in the phase relative to the total band received, a side band may also be displaced relative to the carrier.

The receiver is also suited for the reception combination.

of oscillations with modulated phase. When receiving these oscillations it is, however, necessary 'toinsert special distortion reducing means for the compensation of the linear frequency distortions.

Having thus described my invention, what I claim is:

1. In a system for demodulating wave energy "modulated in frequency or in amplitude and comprising a carrier and sidebands, means for pro- 7 'ducing oscillations of a frequency equal to the frequency of said carrier and of a substantially fixed phase having a phase which differs from the carrier phase by a phase displacement which is nearer to an odd multiple of 45 than an even multiple of 45, and means for combining and demodulating said oscillations and sidebands.

2. In a system for demodulating wave energy modulated in frequency or in amplitude and comprising a carrier and side-bands, means for receiving Wave energy comprising a carrier and side-bands resulting from amplitude or frequency modulation, means connected with said first named means and excited by the carrier only of said received wave energy for producing oscillations synchronized as to frequency with said carrier oscillations, means connected with said second named means for adjusting the phase of said produced oscillations to a phase which differs from the carrier phase by a phase displacement which is nearer to an odd multiple of 45 than an even multiple of 45, and means connected with said last named means and with said first named means for combining said oscillations with received carrier and side-band energy and demodulating the energy resulting from said OTTO TiiXEN.

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