Carrier frequency signal system

Jan. 7, 1941

w. hAhnle

CARRIER FREQUENCY SIGNAL SYSTEM Filed May 28, 1938

2,227,902

AMPLIFIER DEMODULATOR

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NETWORK PASS LIMITER PASS (COMPENSATOR) FILTER FILTER

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Ph 1/2

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PHASE SHIFTER

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AMPLIFIER

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INVENTOR.

WALTER. HAHNLE

BY

ATTORNEY.

Patented Jan. 7, 1941

2,227,902

UNITED STATES PATENT OFFICE

2,227,902

CARRIER FREQUENCY SIGNAL SYSTEM

Walter Hahnle, Berlin-Siemensstadt, Germany, assignor to Siemens &amp; Halske, Aktiengeseilschaft, Siemensstadt-Berlin, Germany

Application May 28,1938, Serial No. 210,584 In Germany June 18,1937

Claims. (Cl. 250—20)

In the transmission of broadcast programs and the like it is known in the art to transmit one entire sideband and only a part of the other sideband resulting from the modulation of a car5 rier wave with a signal band. Such a method is particularly suited for the transmission of intelligence of carrier frequency in which the lowest modulation frequencies are around zero or but little different therefrom, as, for instance, in 10 telegraphy, picture telegraphy and television. This method of transmission offers practical advantages in both radio and wire transmission. For instance, the requisite frequency band is of less width, while the risk of the sidebands being 15 cut off or extinguished as a result of differences in transit time is obviated.

In the demodulation of a message transmitted with partial suppression of sideband, it is possible to obtain freedom from distortion, theo20 retically, by an ideal rectifier without addition of the carrier when the modulation percentage is very low, and when no appreciable differences in transit time (phase) arise. However, it is uneconomical to operate with low modulation per²⁵ centages, because of the amplifier power which is required. On the other hand, when the modulation percentage is high, the carrier must be added upon demodulation.

If transmission of signals is concerned in 30 which the relation between the carrier frequency and the neighboring sideband “corner” frequency (i. e., its nearest sideband frequency), differs but slightly from unity, in other words, systems in which the lowest modulation frequency is equal 35 to zero or but little different from zero, the carrier to be added, especially where the carrier frequency is high, cannot be obtained by conventional methods. Moreover, the requirements regarding the carrier frequency to be add40 ed are essentially more severe in comparison with normal telephone signal transmissions. For instance, in television transmission, a phase distortion of not over ±10 percent may be regarded as admissible if the lowest modulation frequency 45 contained in the television band is equal to 25 cycles. The assumption is here made that the frequencies from 0 to 4 cycles pertaining to the so-called average picture brightness (background brightness) are sent over a separate channel. 50 Since the carrier frequency itself may fluctuate around ±0.5 percent about its rated value because the stability of operation of oscillator valves as a general rule is not greater, the usual mode of producing the carrier by means of filter55 ing in very narrow bandpass filters has become impossible. Owing to the demands made as regards the phase, moreover, it does not appear possible to make available at the receiving end a carrier presenting the correct phase relation; also, the transmission of carriers of a different 5 frequency fails to lead to the desired success, among other reasons because of the demands respecting transit time.

Now, in order to allow the practical use of carrier-frequency transmission systems in which one 10 sideband is wholly and the other one partially transmitted, while at the same time excluding such drawbacks as inhere in known carrier generation methods, the carrier frequency to be added, according to the invention, is obtained 15 from the transmission or signal band through distortion-corrector networks, bandpass filters, and amplitude limiting means with practically constant amplitude, without additional frequency modulation. If desired, the ensuing car- 20 rier frequency may be fed to a phase shifter of such action that, in the presence of a departure of the carrier frequency from its rated or assigned value, the ensuing phase difference is compensated. The means required for the production 25 of the carrier as a whole are so proportioned that there results no phase rotation for the carrier frequency as long as it has exactly its assigned or rated value. If needed, a further phase rotating (shifting) element must be added. ³⁰

In brief, it may be said that the present invention concerns a demodulation system where an exalted carrier is employed for producing distortionless demodulation. The present invention .. contemplates selecting from the spectrum of sig- 35 nal frequencies a narrow range of frequencies including the carrier and, among other things, limiting the amplitude of energy in said range before it is applied to the demodulator.

The invention shall be explained more fully by 40 reference to the accompanying schematic illustrations wherein Fig. 1 illustrates applicant’s system, and Fig. 2 illustrates one form of which the phase shifter Ph of Fig. 1 may take. In referring to Fig. 1, the signal band coming in, for 45 instance, from the line L is fed through an amplifier V to the demodulator DM. By the correcting network or compensator E the aggregate attenuation of the frequencies located in the vicinity of the carrier frequency F, say, in the case 50 of television work a band F±2.10⁵ cycles per second, is to be rendered stable in order that in this range of frequency there may be available only an amplitude modulated carrier without additional frequency modulation. By th? bandpass 5§

2,227,902

ΒΡι, a band symmetric to the carrier F, in the case of television, a band of around 4χ10⁴ cycles per second is filtered out, whence, optionally after amplification by amplifier Vi, the amplitude limiter B and bandpass filter BP₂, a carrier frequency of practically constant amplitude is obtained. The bandpass filter BP2 cuts out a band that is still narrower than the band cut out by the bandpass filter BPi, in fact, it is around one order of magnitude smaller. It could be dispensed with, if no accompanying modulation arose in the carrier frequency through amplitude limitation. The elements from the line end L as far as the bandpass filter BP2 are so chosen that there is occasioned no phase rotation for the carrier frequency if it is exactly of the rated value. However, if the carrier frequency departs from the rated value, then the phase discrepancy is proportional to the frequency drift. According to the design of the bandpass filters there may thus result a departure of ±15 degrees for ±0.5 percent frequency departure. This phase rotation is eliminated by the phase shifter PZi; the latter may be of the simple design shown in Fig. 2.

Referring to Fig. 2, it will be apparent that in the case of an unduly high carrier frequency, the potential at point b lags behind the voltage at point a; similarly, the voltage at a lags behind the voltage at the input end of the distortion compensator. If the angles are small, then, by suitable choice of the number of turns of the differential transformer DU, the no-load voltage at c may be set so as to be in phase with the voltage at the input of the compensating network. Hence, the phase lagger, in the presence of no load or of negligible load, inside a narrow frequency range (for instance, rated frequency ±0.5 percent) causes the transit time between the input to the compensating network and the input to the demodulator to be equal to zero. The pairs of elements I, 2; 3, 4; and 5, 6 are tuned to the carrier frequency so that the voltage at b is in phase with the voltage at a at the carrier. With departure from the carrier frequency, a phase shift is produced between a and b which is similar to that which is produced between the input to E and the input to Ph, and by means of transformer DU this phase shift is reversed. The voltage difference from a to b (due to phase shift) being similar or equal to the voltage difference from line L to a (input of PM, this voltage difference may be doubled by transformer action and reversed and added on to the output of Ph, whereby there is neutralized or cancelled out the voltage produced by phase shift all the way from L to b.

The output of the phase shifter, optionally after amplification in amplifier V2, is fed to the demodulator DM in the form of a carrier frequency; the demodulator output is fed to other receiver stages.

However, the compensator E will only roughly smooth the attenuation inside the channel of the bandpass filter BPi if of standard design. Above this bandpass, even if the compensator is free from faults, a frequency modulation will exist or subsist side by side with amplitude modulation if the carrier frequency departs from the rated value. To be sure, the amplitude limiter

B will almost completely eliminate the amplitude modulation, but the frequency modulation persists. Now, the next bandpass filter BP2 is still narrower than the bandpass filter BPi and it cuts down the frequency modulation by approxi- 5 mately 5 Nepers, occasioned by the band pass filter BPi in case there is a shift in carrier frequency. Still, a residue will remain, in fact, another slight frequency modulation is newly added since the carrier, as will be noted, is slightly dis- 10 symmetric in reference to the bandpass BP2. Hence, under .certain circumstances it may be expedient, especially when a push-pull chopper is not used for demodulation, to insert a further amplitude limiter and another bandpass filter. 15

In order that the method here disclosed for carrier production may be particularly advantageous, the attenuation curve for the aggregate signal band should conveniently be so chosen that the condition where e-^6l+e-^blI=constant, where 20 bi and bn are the attenuations at two* arbitrary points symmetric to the carrier frequency F₂.