US2298460A - Modulated oscillator - Google Patents

Description

.nected to output coil B. frequency circuit to ground via plates 1, 3 of the Patented oet. 13, 1942 Monm'rsp 'oscnm'ron STATES PATENT OFFICE I,

William H. Boghoslan, Forest Bills, and Foster A. Binshaw, St. `Albans, N. Y., and Henry G. Och, West Englewood. N. J., assignors to Bell Telephone Laboratories, Incorporated, New N. Y., a corporation of New York York,

Application May 3, 1341, Serial No. 391,676

i 3'Clalms. (Cl. 1TB-171.5)

'I'he present invention relates to modulated wave transmission and especially to a circuit for generating a carrier frequency wave and modulating the wave by signals such as speech.

4A particular object is a self-oscillating modulator of: simple construction with provision for vsuppressing the unmodulated carrier component.

Many well-known types of modulatorsdepend upon some form of balance to suppress. the carrier, such as a diierential or bridge arrangement of tubes or impedances whereby the unmodulated carrier component appears across a circuit element that is inv conjugate relation to the outgoing line. I'

One of the objects of the present inventionA is a carrier suppression modulator that depends upona direct shunting of the carrier and that -avoids the necessity. of any bridge or differential arrangement.

The nature and objects of the invention will be more fully disclosed .in the description to follow,v and from the accompanying drawing in which:

Figs. 1 and 2 are schematic circuit sketches of illustrative embodiments and Fig. 3 shows impedance diagrams to be referred to in the de- A scription.

. Fig. 1 shows -a tube I with a crystal 2 which serves the dual purpose of determining the frequency of oscillation and shunting the unmodul lated carrier from the load circuit.

The tube I, has its control grid connected to the secondary winding of input speech coil 3. and thence through resistor l and condenser 5 in parallel, to the cathode. The anode is concrystal 2. Plates l and 9 are in the circuit of a second grid I0. The circuit osclilates by virtue of the coupling within the crystal between the ciicuit 1, l and the circuit l. 3. The circuit The plate has n high oscillates at `the series resonant frequency of Y the crystal. When speech is applied from the voice input circuit through coil 3 to the grid, the carrier waves are modulated byv .thev action of the tube and the two side-bands are transmitted through output coil to the line or load circuit II while the carrier isshunted through the circuit comprising crystal plates 1, 8.

The circuit as shown in Fig. l is provided withnegative feedback for the voice waves in order to reduce distortion and improve stability of operation. A portion of the output side-band is bro .ight back over conductor I2, through loss network I3 to the plate I4 the envelope of tube I. Also, some carrier wave is supplied to the diode plate Il from plate 9 of the crystal via resistors I5 and I6. Demodulated speech i'lows in the diode circuit in series with resistors IB and 4, the carrier being by- .passed through condenser 5. Resistor 4, being' feedback.

In Fig. 2, a generally similar circuit is shown withthe circuit proportioning more fully set forth. The tube 20 has its control grid conf,-v nected to speech input coil 3 and its anode connected to output coil 6 and crystal 2, sirnilafrly.`

to Fig.l. Feedback of the carrier for producing oscillations-,is through condenser 2| to the grid 22, which has a'leak resistor 23 to ground; The cathode is above ground for direct current bythe drop through bias resistor 24 which has the usual shunting condenser 25. The negative feedback path is from the plate through condenser 26 to the diode plate Il,4 this feedback being effective at the frequencies of the lower side-bandand carrier by virtue of impedance network Za as may be seen from Fig. 3 where Z: `is indicated as having high impedance at these frequencies. The demodulated speech ows through the diode and networks Z3 and Z4 in series. Reference to Fig. 3 shows that Z4 has high impedancel over the voice range so that a feedback of the demodulated speech takes place into the grid circuit, the grid being connected to the upper orl ungrounded terminal of the net- 4 work Z4.

The network Z2 is designed to have a high im-v pedance at the, desired output frequencies, in

ymodulated carrier component and transmission of the lower side-band into the outgoing circuit' II. Circuit II contains a band-pass filter 28 for passing the lower side-band and suppressing other frequencies.

A suitable configuration 'of the diode within is given for each o1 lthe networks Z2, Z3 and Z4. Za.for example,

In the case of network Za, the inductive branchy l gives low impedance to low frequencies. the capacity branch gives low impedance to high frequencies while the capacity in commotion with the two other branches provides a major. and a minor antiresonant effect suitably spaced and damped as indicated in Fig. 3. The exact dimensioning will. of course, differ with different frequencies, the Aconfigurations and impedances being illustrative and not limiting.

The circuit details may be varied widely in practice without departing from the invention. For example, while the coupling from plate 9 to the oscillator grid is shown as quite simple in character, it may be advantageous in practice to use other elements in this coupling circuit .in order to secure desired phase shift in particular cases, or to provide for a voltage step-up. These and other modifications will occur to those skilled in the art. The scope of the invention is defined in the claims which fol1ow.`

What is claimed is: l

l. In -a carrier wave modulating circuit, a space discharge device having at least triode elenections for impressing the modulated waves on said transmission circuit 'and .for suppressing transmission of the unmodulated carrier' com ponent, said latter connections comprising a circuit including electrodes of said crystal in shunt across said transmission circuit to provide a substantial short circuit at the carrier frequency.

2. In a modulating circuit. a space discharge tube having at least triode'elements, a crystal having a plurality of plates, a'speech input circuit, an outgoing circuit for modulated waves. a coupling from plate to arid through said crystal causing said tube to generate oscillations of carrier frequency determined by said crystal. connections between said speech inputcircuit and saidl tube-for producing modulation in said tube of said generated frequency oscillations by the ments, a source of modulating waves and a transspeech, means to impressthe modulated waves on said outgoing circuit, and a circuit including a pair of plates of said crystal for shunting the unmodulated carrier component from said outgoing circuit.

3. In combination.. a space discharge device having cathode. grid and anode elements. a crysta'l, circuit means coupling said crystal in a resonant path between plate and grid elements cf said'device, said coupling means causing said device to generate sustained waves of a frequency determined by the crystal, means including a source of signal waves coupled to elements of lsaid device to produce modulation of said waves.

of themodulated waves, said latter means comprlslng a series resonant circuit including said crystal connected in shunt across said outgoing circuit.

WILLIAM H. BOGHOSIAN. FOSTER A. HINSHAW. HENRY G. CCH.

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