US2429683A - Modulation - Google Patents

Description

Patented Oct. 28, 1947 i MODULATION Johan 'Haantjes, Eindhoven, Netherlands, as-

signor to Hartford National Bank and Trust Company, Hartford, Conn., as trustee Application March 23, 1943, Serial No. 480,184 In the Netherlands June 12, 1940 Section 1, Public Law 690, August 8, 1946. Patent expires June 12, 1960 The invention relates to a system for a modulating high-frequency oscillations wherein one of the side-bands is suppressed, at least partially.

The use of the known transmitting systems of this kind entails the drawback that the modulation envelope of the oscillations transmitted by the transmitter does not correspond to the lowfrequency signal supplied to the modulator, with the result that after reception and detection of such a signal by an ordinary receiver wherein use is made of linear detection there occur distortions of such character that the transmitter cannot be utilized for broadcasting purposes.

Fig. 1 represents a carrier oscillation modulated in amplitude by a single sine-shaped oscillation.

Fig. 2 represents the carrier oscillation modulated by the same sine-shaped oscillation, one side-band being suppressed. Upon comparing these two figures, it is clearly visible that in both cases the modulation envelopes of the transmitted oscillations are quite different. Upon reception of the oscillation represented in Fig. 2 by means of an ordinary receiver in which useis made of linear detection, the voltage obtained after detection does consequently not correspond to the sinusoidal voltage obtained upon reception of the oscillations represented in Fig. 1.

It is furthermore known to suppress a single side-band only partially and to transmit the low modulation frequencies normally with two side bands whereas the higher frequencies of the modulation are transmitted with a single sideband (and a larger depth of modulation). The separation preferably lies in this case in the middle of the modulation frequency band to be transmitted owing to which the harmonics ofthe modulation frequencies to be transmitted with a single side-band are located outside the transmitted frequency band. Upon reception of such a signal by an ordinary receiver the distortion is smaller than upon reception of a signal which only consists of a carrier wave and a single sideband. The frequency band transmitted is, however, only 25% narrower than in the ordinary transmission with two side bands.

It has previously been proposed to suppress the distortion occurring upon reception of a transmitter with single side-band modulation by feeding part of the rectified output voltage of the transmitter in counterphase with the modulating voltage back to the input of the modulator. This transmitting system, has, however, the drawback 3 Claims. (01. 179-4715) ing the correct phase of the degenerative feedback voltage.

The invention has for its object to provide a modulation system wherein a, single side-band is suppressed either wholly or partially and With which it is ensured in a simple manner that with linear detection the modulated signal yields an undistorted modulation voltage.

According to the invention, modulation is effected by supplying the oscillation to be modulated to a network with at least part of which the series connection of a rectifier, a resistance bridged for alternating current and the source of modulating voltage is connected in parallel wherethat complicated devices are necessary for ensuras the modulated oscillation is taken from this series connection and the said network has such a characteristic of the impedance as a function of frequency that for frequencies of the side-band portion to be suppressed substantially no impedance is present.

The invention will be explained more fully with reference to the accompanying drawing.

Figs. 1 and 2 represent already discussed shapes of the modulation envelope in the case of double and single side-band modulation respectively.

Fig. 3 represents an embodiment of a circuit arrangement according to the invention.

Fig. 4 shows the characteristic of the impedance of the resonant circuit of Fig. 3 as a function of frequency.

Upon considering the curve, according to Fig. 2, it appears that the modulation envelope has a shape which points to th presence of a strong harmonic. Moreover, the modulation envelope produced in the case of single side-band transmission of a signal modulated by two frequencies qi and qz has a shape which indicates the presence of combination frequencies q1+q2 and (Ir-Q2.

With the circuit arrangement according to the invention the second harmonics and any existing combination frequencies are suppressed by adding a high-frequency correction voltage whose enveloping curve comprises the second harmonics and any existing combination frequencies of the modulation frequencies in a phase opposite to that of the second harmonics and combination frequencies in the enveloping curve of Fig. 2.

With the circuit arrangement shown in Fig. 3 the high-frequency oscillation to be modulated is supplied to an oscillatory circuit constituted by an inductance L and a capacity C. In parallel with the circuit LC is connected a series circuit consisting of a diode 2, the parallel connection of a resistance R and a condenser C1 which constitutes a short circuit for the modulation frequency, and the source of modulating voltage 3. The modulated high-frequency voltage may be taken from output terminals 1. The oscillatory circuit LC is tuned to a frequency which differs from the frequency of the oscillation to be modulated. The characteristic of the impedance as a function of frequency is shown in Fig. 4 wherein or represents the frequency of the oscillation ,to be modulated and p represents the highest modulation frequency.

By calculation it is found that with the described circuit arrangement, when the diode '2 operates as a peak detector, there is produced across the terminals 4 a modulated-signal which comprises the carrier wave frequency w, the desired side-band frequencies located between w and +1; and besides correction frequencies w+2q1, w+2q2, w+q1+q2 and w+q1q2 insofar as*these frequen'cieslare located Within the range in which the circuit LC possesses a highimpedance. Owing to the presence of these cor- :rection frequencies the enveloping curve of the modulated signal occurring across :the terminals *4 substantially corresponds to the modulating wave'so that with linear detection distortion does substantially not occur. If, for example, the frequency qis modulated on the carrier wave with :a depth of modulation m the'depth of modulation m2q for the second harmonic 2; which is compensated for thegreater part owing to the presence of the said correction frequency w-2q amounts in the enveloping curve to:

wherein 1' isthe internal resistance of the diode lplus'thato'f the source of voltage 3, Z-is the impedance of the circuit LC for frequencies within the band of frequencies to be transmitted'and R isthe value of the'resistance'R.

With singleside-band transmission the depth :of modulation amounts without correction for the second'harmonic 7712!; to V mq2. The'factor may be or the order of 1/100 so that with 100% tm'odulation (depth of modulation=1) the'olistortion amounts in the first mentioned case to 25% =andinthe last mentioned case to 0.25%.

Of 'amodulation-frequency q locate'd'between i 'p andp thecorrection frequency w-l-2g' is located outside the frequency range to be trans- :mitted where'the circuit LC has alow impedance so that :this frequency does not occur. In this :zcase, however, the second harmonic of the modulating voltage, which occurs due'to the side-band suppression in the low-frequency amplifierof the :receiver, is also located outside the frequency range to be transmitted and may easily be suppressed by an adequate choice of the frequency characteristic of the low-frequency amplifier.

If it is desired to transmit part, for example, the low tones of the modulation frequency band to be transmitted, with two side-bands, this can easily be achieved with the circuit arrangement according to Figs. 3 and 4 by choosing the characteristic of the impedance of the circuit LC as a .-function .of frequency in such manner that for partof'the frequency band from w to wp corresponding to the frequencies to be transmitted with two side-bands the circuit has an appreciable impedance,

It isevident that instead of the circuit LC use may-be made of a more complicated network in order to obtain the desired steep flank of the impedance curve according to Fig. 4.

What "is claimed is:

1. In a modulation system, a source of wave energy to'be mo'dulated, a source of modulating potentials, a circuit tuned to a frequency different than the frequency of wave energy from said source, a rectifier and said source of modulating potential series connected in parallel with said tuned circuit, means for impressing said wave energy to be modulated on said tuned circuit, and means for deriving modulated wave energy from said rectifier.

2. In a system for modulating high frequency oscillations wherein one of the sidebands is suppressed at least partially, a source of oscillation to be modulated, a source of modulating voltage, a network, a rectifier, a resistance bridged for alternating current and said source of modulating voltage series connected in parallel with a part at least of said network,and means for deriving the modulated oscillation from the series connection, said network having such a characteristic of impedance as a function of frequency that for frequencies of the sideband portion to be suppressed substantially no impedance is present;

3. A system as recited in claim 2, wherein the rectifier is a two-electrode rectifier operated as a peak detector.

JOI-IAN HAANTJES.

REFERENCES CITED The following references are of record inthe file of this patent:

UNITED STATES PATENTS Number Name Date 1,592,710 Scott-Taggart July 13, 1926 2,129,820 Chaifee Sept. 13, 1938 2,305,911 Terman Dec. 22, 1942 FOREIGN PATENTS Number Country Date 550,899 Great Britain Jan. 29, 19 13

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