Title:
Carrier frequency signal system
United States Patent 2227902


Abstract:
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 carrier wave with a signal band. Such a method is particularly suited for the transmission of intelligence of...



Inventors:
Walter, Hahnle
Application Number:
US21058438A
Publication Date:
01/07/1941
Filing Date:
05/28/1938
Assignee:
SIEMENS AG
Primary Class:
Other Classes:
333/138, 375/321, 455/148
International Classes:
H04J1/06
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Description:

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 carrier 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 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 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, theoretically, 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 percentages, 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 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 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 added 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 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. 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 filtering 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 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 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 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 carrier 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 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. 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 signal 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 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 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 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 the bandpass r§ BPi, a band symmetric to the carrier F, in the case of television, a band of around 4X104 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 Ph; 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 Ph), 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 approximately 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 dissymmetric 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. 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-bI+e-bnI=constant, where br and bii are the attenuations at two arbitrary points symmetric to the carrier frequency F2.

What is claimed is: 1. In a signal receiving system, a demodulator. means for impressing the received signal on the demodulator, and separate means for selecting from said signal a band of frequencies including the carrier, limiter means for substantially removing amplitude modulation from the ratio frequency energy in said band, a band pass filter for passing the carrier but rejecting side band frequencies produced by said limiter, and means for impressing the energy passed by said band pass filter upon said demodulator with amplitude large compared to the amplitude of and in predetermined phase relation to the carrier impressed on the demodulator by said first named means.

2. The method of obtaining a modulation free carrier from a broad spectrum of frequencies including a carrier, which includes the steps of selecting a narrow range of frequencies including the carrier from said spectrum, limiting the amplitude of energy in said range, and rejecting from said amplitude limited energy side frequencies produced by the limiting action. -14 3. In a demodulation system where an exalted carrier is employed for producing distortionless demodulation, the method of obtaining an exalted carrier which comprises selecting from a spectrum of frequencies a narrow range of frequencies including the carrier, limiting the amplitude of energy in said range, and rejecting from said amplitude limited energy side frequencies produced by the limiting action.

4. In a system for receiving a signal having one side band partially suppressed, an element upon which the received signal is impressed, a demodulator, a connection from said element to said demodulator, and another connection from said element to said demodulator including in the order to be named a network for restoring substantial equality between the upper and lower side frequencies near the carrier, whereby the carrier and the substantially equal upper and lower side frequencies represent a purely amplitude modulated wave, a band pass filter, and an amplitude limiter.

WALTER HAHNLE.