US2474261A - Modulator - Google Patents

Description

Patented June 28, 1949 MODULATOR,

Frank A. Leibe, Quakertown, Pa., and Benjamin B. Mahler, Newark, N. J asslgnors to- Federal Telephone and Radio Corporation, New York, N. Y., a corporation of Delaware Application May 10, 1947, Serial No. 747,214 12 Claims. (01. 332-29) Our invention relates to modulators and is .particularly directed to means for frequency modulating oscillators of the phase-shift type.

In teletype transmission, via wire lines or radio, the telegraphic signals are preferably applied, by keying, to a low frequency source, the marking and spacing impulses being characterized by different frequencies which are easily distinguished by filters at the receiving end. The marking and spacing frequencies, are preferably inthe audio range and for best results should be constant in amplitude and free of transients or key-clicks.

The desired low frequencies can not be easily generated with electronic oscillators having the usual inductance-capacitance tuning circuits. As the lower reaches of the audio frequency spectrum are entered, thetuning coils and condensers become prohibitively large and expensive. Beat frequency generators are complex and not particularly stable as to amplitude or frequency.

The object of our invention is an improved low frequency generator and means for modulating the frequency of the generator with telegraphic signals, the modulated output being constant in amplitude and free of transient currents.

Another object of our invention is a low frequency oscillator-modulator that is stable in frequency and amplitude and is relatively inexpensive.

Our invention is defined in the appended claims and one embodiment is described in the following specification and shown in the accompanying drawing, in which the single figure of the drawing is a wiring diagram of an oscillator and modulator according to our invention.

The particular oscillator to which our inven tion is applied in this description comprises an amplifier tube l with anode 2, cathode 3 and control grid 4. A tube with screen and suppressor grids may be used if desired. The amplifier I is resistance coupled to amplifier tube 5.

For self sustained; oscillations in amplifier tube I, the output circuit is coupled to the input circuit through a capacity-resistance network 6 with the required number of meshes to shift the anode voltage applied to the grid 180 electrical degrees. The network shown comprises four meshes, each with a shunt resistor I and series capacitor 8. One resistor is tapped to divide the resistor into two portions, Ia and 1b. The operating frequency of such an oscillator is a function of the resistances R and capacities C of the network and may be written F=K/RC, where K is a constant determined by the circuit parameters. decrease in the ohmic value of one or more re- 2 sistors of the phase-shift network will cause an increase in frequency, and an increase in resistance will lower the frequency.

We propose to change the effective value of one of the resistances of the network, such as lb, at keying rate to change the frequency of oscillations between two values for marking and spacing signals. We propose to connect the anodecathode space of an electron discharge device 9 across resistor lb so that the plate resistance of said device may shunt the resistance of resistor lb. By alternately making the electrondischarge device conducting and nonconducting, the effective resistance between the ends of lb is made, I respectively, substantially zero or the full value of vice and a single-pole double-throw switch H is connected in circuit with the source to apply either a high positive or a high negative voltage to the cathode. magnetically controlled by a manual key or a teletype sender. A condenser 12 is connected be tween the cathode and ground so that when the discharge device is made conductive, by applying a high negative voltage to the cathode, resistor, lb iseffectively short circuited for alternating voltages. By changing the cathode voltage from negative to positive all direct and alternating components of space current are interrupted and resistor 1b is restored to the oscillator circuit. It will thus be seen that the two positions of the switch produce two values of resistance in the RC network and two frequencies of oscillation. Either frequency may be called the marking frequency and the other the spacing frequency.

According to a further and important feature of our invention, the transition between marking and space frequencies is made without disturbing transient currents. A resistor l3 in series with the keying voltage and the condenser l2 comprise an RC time-constant circuit that will delay changes in potential of the cathode and will insure gradual changes in frequency. So that the charging and discharging pattern of condenser I! may be the same, to make marking and spacing signals symmetrical in shape, another electron discharge device M is placed across the condenser l2. The polarity of the two space paths The switch armature may be as it does on negative battery thus assuring symmetrical modulating pulses and eliminating bias, and distortion, by the modulator circuit.

Adjustment of the amount of frequency shift is made possible by adjustments of resistors 1a and lb.

set, the polarity of said diode being reversed with respect to said junction.

4. In an oscillator circuit comprising a frequency determining impedance element, an arrangement for angle modulating said oscillator circuit with modulation signals from a source comprising an electron discharge device circuit, said electron discharge device circuit comprising a diode in series with a condenser, means for coupling said impedance element and said electron discharge device circuit in parallel, means for applying modulation signals across said condenser for controlling the conductance of said diode in accordance with the applied signals.

5. An arrangement according to claim 4, wherein said impedance element is a resistor and said oscillator circuit comprises a. resistance-capaci- In one oscillator we constructed, the amplifier tube i was a commercial pentode known as a GSJ'l, and the phase-shift network wascomposed of 200,000 ohm resistors and .0004 microfarad condensers. The mean frequency of oscillations was about 2375 cycles per second, and was made to shift about 100 cycles above and below this value for marking and spacing frequencies, respectively. Discharge devices 9 and II were combined in one envelope, the double-diode known as the "6H6" being found well suited for the purpose. 130 volts positive and 130 volts negative at the terminals of direct current source l0 were found convenient to switch the diodes in normal keying, a conventional power rectifier and filter being used for this source.

Our improved oscillator-modmator is relatively inexpensive to manufacture. The frequency determining circuit of the oscillator is composed of resistors and condensers which, in the low frequency range mentioned, are small andv cost but a small fraction of the cost of the elements of any of the conventional inductance-capacity tuning circuits. Yet the frequency and amplitude stability of our oscillator is remarkably good. The temperature coefilcient of resistance of resistors can be negligible, and fixed condensers can be sealed to be practically independent of temperature and humidity changes. Our novel modulator, with its time delay features, goesfor smooth keying and in combination with the oscillator disclosed may be operated at high speeds without undesirable transients or distortion.

We claim:

l. A signalling system comprising an oscillator having a frequency determining circuit, a resistor in said circuit, a diode and a condenser connected in series across said resistor, a source of positive and negative voltages connected to the junction of said diode and condenser, and a switch operable at signal frequency selectively applying said positive and negative voltages to said junction.

2. The signaling system defined in claim 1 further comprising a resistance element connected in series between said junction and said source of voltage. l

3. The signaling system defined in claim 1 further comprising a resistance element connect- -"ed in series between said junction and said source to voltage, a second diode and a second resistance element connected in series across said condentance across said regenerative circuit including said resistor.

6. An arrangement according to claim 4, wherein said impedance element comprises a resistor.

7. An arrangement according to claim 6, com

prising means for coupling one end of said resistance to the plate electrode of said device. means for coupling the cathode of said device through the condenser to the other end of said resistor. 8. An arrangement according to claim 7, further comprising a second electron discharge device circuit coupled between the junctions of said first device and the condenser, and the other end of said resistor.

9. -An arrangement according to claim 8, wherein said second discharge device circuit comprises a diode, means for coupling the plate electrode of said second device to said junction point and its cathode electrode through a resistance to the other end of said resistor.

10. An arrangement according to claim 9, wherein said source comprises a source of positive and negative potentials, means for applying said positive and negative potentials across said condenser in accordance with the intelligence to be transmitted.

11. An arrangement according to claim 10, wherein said last named apparatus comprises a resistor and switching means for applying said positive and negative potential through said last named resistor across said condenser.

12. An arrangement according to claim 11, wherein the value of the conductance of the diode and resistance of said second discharge device circuit is adjusted to place a charge on the condenser having substantially 'the same but of opposite polarity as the charge placed on the condenser by said first discharge device circuit.

FRANK A. LEIBE. BENJAMIN B. MAHLER.

REFERENCES CITED The following references are of record in the die of this patent:

UNITED STATES PATENTS

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