US2456992A

US2456992A - Frequency shift keying plus phase modulation

Info

Publication number: US2456992A
Application number: US794029A
Authority: US
Inventors: Donald W Pugsley
Original assignee: General Electric Co
Current assignee: General Electric Co
Priority date: 1947-12-26
Filing date: 1947-12-26
Publication date: 1948-12-21
Anticipated expiration: 1965-12-21

Description

Dec. 21, 1948. Dhw. PUGSLEY 2,456,992

FREQUENCY SHIFT KEYING PLUS EHASE MODULATION Filed Dec. 26, 1947 I Inventor; Donald Fugsley,

His Attorney.

i atented Dec. 21 1948 UNITED STATES PATENT OFFICE.

FREQUENCY SHIFT KEYING PLUS PHASE MODULATION Donald W. Pugsley, Bridgeport, Conn., assignor' to General Electric Company, a corporation of New York 2 Claims.

This invention relates to wave signaling systems and more particularly to such systems wherein intelligence is transmitted by carrier-shift operation.

In carrier-shift signaling systems it is usual to transmit telegraph mark and telegraph space signals by shifting the carrier frequency a predetermined amount on either side of a mean frequency. A degree of secrecy may be added to such carrier shift signaling systems by changing t e above-mentioned mean frequency throughout a certain frequency band and even into additional frequency bands, in accordance with a predetermined pattern. It is highly desirable, however, that the frequency variation of such systems caused by the mark and space signals be a certain fixed amount regardless of the carrier frequency setting of the system.

It is an object of this invention to provide a carrier-shift signaling system wherein the mean frequency of such a system may be changed at will throughout a frequency band and wherein the frequency variation of such a system caused by mark and space telegraph signals is maintained substantially constant when any such change in the mean frequency of the system is made.

It is a further object of this invention to provide a frequency shift keying system which may be operated at any one of a plurality of carrier frequencies without readjustment of the system other than the adjustment associated with changing the carrier frequency. The novel features which are believed to be characteristic of this invention are set forth with particularity in the appended claims. The invention itself, however, both as to its organization and method of operation, together with further objects and advantages thereof, may best be understood by reference to the following description taken in conjunction with the accompanying drawing in which Fig. 1 shows one embodiment of the invention, and Figs. 2, 3 and 4 show modifications of one of the stages of Fig. 1.

Referring now to Fig. 1, which shows a schematic diagram of a carrier-shift keying system wherein the mean carrier frequency may be changed throughout a wide range of frequencies and through a plurality of frequency bands, there is shown at I an audio frequency oscillator of the resistance-capacitance coupled type, The audio frequency output of oscillator l is coupled to a reactance tube circuit shown at 2 to frequency modulate the oscillations of a radio frequency oscillator 3. A keying circuit 4 functions to change the bias of reactance tube circuit 2 to abruptly shift the frequency of oscillator 3 from one value to another. There is, therefore, provided at the output of oscillator 3 a radio frequency Wave, frequency modulated by an audio tone and having a frequency which shifts from one value to another on either side of a mean frequency in response to the mark or space position of keying circuit 4.

The audio frequency oscillator I comprises an electron discharge device 5. The anode 6 of device 5 is connected to the positive terminal of the source of unidirectional potential 1 through a resistor 8. Cathode 9 of device 5 is connected to the negative terminal of source 1 through a cathode biasing resistor I0 which is bypassed by capacitor H. The unidirectional source 1 is bypassed for audio frequencies by capacitor l 2. The suppressor electrode 13 of device 5 is connected to cathode 9. The screen electrode I4 of this device is connected to the positive terminal of source I through a resistor Ma and this electrode is coupled to ground through a capacitor 15a.

To set up oscillations in device 5, a feedback circuit is provided from anode 6 to the control electrode l5. This feedback circuit comprises a capacitor l6 and a resistance-capacitance network H. The network I! acts as a phase shifting network for the feedback energy, and is designed so that the energy applied to control electrode I5 is of such phase and amplitude that oscillations are sustained in device 5. The resistance-capacitance network I! includes a variable resistor 18 for adjusting the'frequency of the oscillator l.

Output from the anode of device 5 is impressed across a potentiometer load resistor I9 through a capacitor 20. Additional resistors 2t, 22 and 23 may be respectively connected into the load circuit of oscillator l by an adjustable switch 24. The function of the network I 9, 2!, 22 and 23 will be described in detail hereinafter.

A portion of the output appearing across resistor I9 is impressed on the control electrode 25 of the electron discharge device 26 of the reactance tube circuit 2 through a capacitor 21, switch 28 and one of the resistors 29, 30, 3| selected by an adjustable switch 32. The purpose of resistor 29, 30, 3| and contact 32 will be described hereinafter. A common terminal of the above-mentioned resistors and switch 28 is coupled to ground by a radio frequency bypass capacitor 33 to isolate the audio frequency oscillator and the keying circuit from the radio frequency energy generated by oscillator 3.

The reactance tube circuit 2 may be of any conventional design, and in the embodiment shown comprises an electron discharge device 25 3 of the pentode type. Anode 34 of device 25 is coupled to the positive terminal of a unidirectional source 35 through a choke coil 36, the negative terminal of source 35 being connected to ground. Suppressor electrode 38 of device 26 is connected to the cathode of this device which in turn is connected to ground. Screen electrode 39 of device 26 is directly connected to the positive terminal of source 35, and bypassed to ground by capacitor 31. 1

Radio frequency energy from oscillator 3 flows to the anode of device 26 through capacitor 40 and to the control electrode 25 through the voltage divider action of capacitor 4| and the resistor 29, 30 or 3! selected by switch 32. The

capacitors

40, 4| and resistors 29, 30, 3| are given such values that oscillations applied to the control electrode 25 lead the oscillations applied to the anode 34 by substantially 90, and hence circuit 2 simulates a capacitive reactance, the value of this reactance being among other things a function of the potential applied to the control electrode 25.

The oscillator 3 is also of conventional design and in the embodiment shown in Fig. l is of the electron coupled type in order to achieve a maximum degree of frequency stability and freedom of loading effects caused by succeeding stages of the system. This oscillator comprises an electron discharge device 42 and a resonant circuit shown generally at 43. The circuit 43 includes

coils

44, 45, 46 and 41, 48, 49 connected as shown. The common junction of these coils is connected to the positive terminal of a source of unidirectional potential 50, the negative terminal of this source being connected to ground and the source 55 being bypassed by a capacitor i. The other terminals of

coil

44, 45, 46 may be selectively coupled to the control electrode 52 of device 42 through capacitor 53 by means of a selector'switch 55a. Similarly the coils 41, 48 and 49 may be selectively connected to the screen'electrode 54 by means of a selector switch 55. The suppressor electrode 55 is connected to the cathode 51 which in turn is coupled to ground through a choke coil '58. The frequency of oscillator 3 may be varied by variable capacitor 59. The anode '60 .ofidevice 42 is connected to unidirectional 'source 62a through resistor 6i and choke coil 62. The oscillator output is applied to terminals 62c-through capacitor 62b, as shown, and thence to succeeding stages in the system. Q

In the circuit thus far described, the reactance tube circuit 2 is connected across the resonant circuit 43 of radiofrequency oscillator 3 and acts as a variable capacitance, thereby changing the frequency of oscillator 3 in response to variations of the potential of control electrode 25 of device 26. The output of the audio frequency oscillator l is coupled to the above-mentioned control electrode 25, and output of the; radio frequency oscillator 3 is therefore frequency modulated by the audio frequency oscillator i. 1 I

The function of the network 4 is to apply bias potential to the control electrode of device 25, this bias having distinctive values dependent on the position of a telegraph key 63, and thereby shifting the frequency 'of-oscillator 3 on'either side of a mean frequency, responsive to'a mark or space signal from the keying circuit 4.

When key 33 is depressed, the potential of the movable arm of potentiometer -54 is applied to control electrode 25 through a resistor '55 and when the key is in its normal position the potential of the movable armof potentiometer 66 is applied to the above-mentioned control electrode.

The potential of the movable arm of potentiometer B4 is determined by the potential of sources 61 and 68a, and the value of resistors 68, 65, 10, H, 12, the resistance of potentiometer 64 and 6B and the position of the movable arm of potentiometer 54. The value of this potential may be altered by adjusting potentiometer 54, resistor 69 or resistor 10, The potential of the movable arm of potentiometer 66 is established primaril by the potential of source 68, resistor 15, 'H and 12, potentiometer 66 and the position of the movable arm of potentiometer 56. The value of this potential can be changed by adjusting potentiometer 66 or resistor 10.

One of the features of this invention resides in providing means for altering the carrier frequenc of oscillator 3 and concurrently therewith maintaining a predetermined frequency variation of this oscillator due to the operation of keying circuit 4 and modulations by oscillator l.

The oscillator 3 may be switched from one frequency band to another by selector switches 55a and 55 which insert inductances 44, 4501' 46 and 41, 48 or 49, respectively, into the resonant circult of the oscillator. To maintain a predetermined frequency shift when the switches 55a and 55 are operated, switches 32 and 24 are mecham ically connected to 55a and 55 for unicontrol operation. Switch 32 selects one of resistors 29, 30'

and 3| and inserts the selected resistor into the control electrode circuit of device 25, thereby changing the effective reactance of this device. Switch 24 selects a correspondingly positioned one of resistors 2!, 22 and 23 to alter the amplitude of the oscillations applied to the control electrode of device 25 by the audio frequency oscillator I, and hence the frequency deviation of oscillator 3 due to the audio frequency oscillator I. The values of the above-mentioned resistors are chosen so as to provide a substantially constant frequency variation in the oscillations of oscillator 3 despite changes in the mean frequency due to operation of switches 54 and 55.

Frequency changes of oscillator 3 within a selected frequency band may be effected by operation of variable capacitor 59 in the resonant circuit of oscillator 3. Corresponding changes in the frequency variations due to oscillator l and keying circuit 4are provided by potentiometer I9 and resistors 69 and 15. The potentiometer l9 and resistors 59 and iii are variable and their movable arms are mechanically connected to ca.- pacitor 59 for unicontrol operation. Therefore, similar to the manner described above, any changes in the frequency of oscillator 3due to operation of capacitor 5!! simultaneously changes the adjustment of the frequency variations of this oscillator due to the keying circuit 4 and the audio frequency oscillator I. The values and tapers of potentiometer l9 and resistors 69 and in are chosen so as to maintain substantially constant frequency variation for changes in the frequency of oscillator 3.

Figs. 2, 3 and 4 are directed to reactance tube stage 2 and show alternate methods of connecting switch 32 toalter the reactance effects of this stage as switches 54 and 55 are operated.

In Fig. 2 the control electrode of device 26 is supplied with potential in accordance with the voltage divider action of resistor 13 and any one of the capacitors 74,,l5and 76 selected by switch 32. The value of resistor 13 is large as compared with the reactance of capacitors 14, 15 and 16 so potential applied to the control electrode of device :26 leads-the potentialapplied to the anode of this device by substantially 90' thereby forming a capacitive reactance. The values of capacitor 14, 15 and 16 are chosen to provide a constant value of frequency shift due to operation of the keying circuit 4 of Fig. 1 when switches 54 and 55 are operated. In Fig. 3 the reactance of device 26 is changed by varying the relative value of reactance of inductance I1 and the values of resistors l8, l9 and 80. This provides a varying potential for application to the control electrode through a capacitor 8|.

Fig. 4 shows an arrangement similar to Fig. 3 except that the position of the inductance and the resistors is reversed. In this modification the reactance of the device 26 is determined by a resistor 82 and the one of inductances 83, 84, 85 selected by switch 32 in accordance with the position of switches 54 and 55.

While a particular type of audio frequency oscillator, reactance tube circuit, radio frequency oscillator and keying circuit has been shown and described in this application, this is given by way of example only and is not to be construed as a limiting feature of this invention as these various components may be replaced by many known circuits of a similar type.

Therefore, while particular embodiments have been shown and described, it will of course be understood that various modifications may be made without departing from the invention, and the appended claims are therefore intended to cover any such modification coming within the true spirit and scope of the invention.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. In a wave signaling system of the carriershift type, a source of radio frequency signals, a source of audio frequency signals, a reactance tube circuit and a keying circuit, means including said reactance tube circuit for modulating the frequency of said radio frequency signals with said audio frequency signals, further means including said reactance tube circuit for causing the mean frequency of said frequency modulated radio frequency signals to shift a predetermined amount on either side of said mean frequency in response to mark and space signals from said keying circuit, means for changing the frequency of said radio frequency signals throughout a wide range of frequencies, means for varying the amplitude of said audio frequency signals, means for varying the apparent reactance of said reactance tube circuit, means for varying the amplitude of said mark and space signals, mechanical means coupling said changing and varying means for unicontrol operation for maintaining a substantially fixed predetermined frequency variation of said carrier wave signal as said carrier wave frequency is changed throughout said wide range of frequency,

2. In a wave signaling system of the carriershift type, a carrier wave oscillation generator for obtaining carrier wave signals, said generator including a resonant circuit for determining the frequency of said carrier wave signals, means comprising a reactance tube circuit coupled to said resonant circuit for varying the resonant frequency thereof, a source of audio frequency signals, means including said reactance tube circuit for causing said audio signals to frequency modulate said carrier wave signals, a keying circuit, means coupling said keying circuit to said reactance tube circuit, means including said reactance tube circuit for causing the mean frequency of said frequency modulated carrier wave signals to shift a predetermined amount on either side of said mean frequency in response to mark and space signals from said keying circuit, means for adjusting the'resonant frequency circuit associated with said carrier wave oscillation generator, and hence the frequency of said carrier wave signals, throughout a wide range of frequencies, means for adjusting the amplitude of said audio frequency signals, means for adjusting the apparent reactance of said reactance tube circuit and means in said keying circuit for adjusting the amplitude of said mark and space signals, mechanical means coupling said adjusting means for unicontrol operation to maintain substantially a fixed predetermined frequency variation of said carrier wave signals for different adjustments of said resonant circuit adjusting means.

DONALD W. PUGSLEY.

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

UNITED STATES PATENTS Number Name Date 2,424,481 McCoy July 22, 1947 2,447,088 Parson Aug. 17, 1948