US2482112A - Carrier shift keyer - Google Patents

Description

Sept. 20,

MARK OSCILLATOR H. W. JOHNSON CARRIER SHIFT KEYER Filed Jan. 25, 1946 SPACE OSCILLATOR 1 I 24 I 4 I HETERODYNE 21 1 I OSCILLATOR p J 1 v 22 i l a l g I: I I 1 ALTERNATE a f? OSCILLATOR 7 Aumo COMPARATOR MIXER V 4 L L as 37 fig 32 33 INVENTOR.

HOWARD W. JOHNSON ATTORNEY Patented Sept. 20, 1949 CARRIER SHIFT KEYER Howard W. ohnson, Seattle, Wash.

Application January 25, 1946, Serial No. 643,443

(Granted under the act of March 3, 1883, as amended April 30, 1928; 370 0. G. 757) 3 Claims.

The invention described herein may be manufactured and used by or for the Government'for governmental purposes, without the payment to me of any royalty thereon.

My invention relates to the art of telegraphy in which a carrier is present during both'the marks and spaces of the code, the spaces being differentiated from the marks by a slight shift in carrier frequency, and in particular relates to methods and means for alternating between two carriers of very constant frequencies and constant small frequency difference.

In the prior art of carrier shift systems for telegraphy, teletype, etc., it has been the practice to supply two crystal controlled oscillators on two frequencies which were close enough to pass through the same radio transmitter but distinct enough to be diiierentiated in a superheterodyne radio receiver. However, in low frequency carrier communication low frequency crystal oscillators do not readily lend themselves to carrier shift. Due to the poor activity of the low frequency crystal and the difiiculty in controlling its frequency by circuit constants it has been'found unsatisfactory in carrier shift operation. For example, to achieve a carrier shift of 170 cycles at 100 kc. mean carrier with low frequency crystals it is necessary to have one crystal at 99.915 kc. and one at 100.085 kc. This means that the crystals must be ground practically on location to achieve the accuracy required. The extreme variability of activity in low frequency crystals causes a considerable difference between the strength of the mark frequency control signalsand the space signals which is carried through the entire transmitter unless complex compensation circuits are used. These drawbacks added to the higher cost of low frequency crystals have lators of relatively high frequency, and to securethe two close low frequencies desired for the lowfrequency carriers by separately heterodyning the output of one of these crystal oscillators with the output of each of the other crystal oscillators.

It is an object of my invention to provide a. low frequency carrier shift keying system which shall overcome the drawbacks in the prior art.

It is another object of my invention to provide two close low frequency carriers from three high frequency crystals.

It is an object of my invention to provide means for rapidly alternating between two fixed frequencies having a small fixed difference.

. In the sole figure of the drawings there is disclosed a circuit diagram of a carrier shift keyer embodying my invention.

In the drawing there are disclosed four high- 4 frequency crystal oscillators I, 2, 3, 4 of normal construction and having crystals 5, 6, I, 8 respectively and trimmer tuning capacitors 9, III, II, I2, respectively. Crystals 5, 6, I, 8 are housed in a thermostatically controlled temperature chamber I3. The four oscillators are designated, I mark, 2 space, 3 heterodyne, 4 alternate.

The circuit of mark oscillator I is shown in some detail. The circuits of space oscillator 2, heterodyne oscillator 3 and alternate oscillator 4 are to be understood as similar except for differences in tuning; in the drawing they are shown in block form except for their crystal.

Two similar mixer circuits I 5 and I6 are shown.

Mixer circuits I5 and I6 are of conventional construction and comprise mixer tubes I1 and I8 respectively. Lead I9 connects the output of mark oscillator I' to the grid 2! of mixer tube I'I; lead 20 connects the output of space oscillator 2 to the control grid 22 of mixer tube I8. Lead 25 connects the output of heterodyne oscillator 3, or of alternate oscillator 4, through switch 36 to the injector grids 23 and 24 of mixer tubes I I and I8 respectively. Return circuits here and elsewhere are through ground.

Lead 3| connects one fixed terminal of polar relay 34 to control grid 2I and injector grid 23 of mixer tube 11, and lead 32 connects the other fixed terminal to control grid 22 and injector grid 24. The arm of polar relay 34 is biased negatively to ground by a -C voltage supply. Polarized control voltage for polar relay 34 is supplied through leads 33.

The plates 29 and 36 of tubes I1 and I6 respectively are connected across a common positive tuned plate circuit I4 and to a common positive voltage supply B+. Their combined output is fed to transmitter 35, which is connected across tuned plate circuit I4.

' Leads 21 and 28 connect the outputs of space and mark oscillators'l and 2 respectively to audio mixer 26 which selects the difference of the frequencies of the outputs. The output of mixer 26 is fed to comparator 31 containing a standard tuning fork and head-phones, in which the output of audio mixer 26 and the tone from the tuning fork are heard simultaneously.

By way of example the frequencies of crystal oscillators I, 2, 3, 4 may be taken as 1499.915 kc.

mark frequency, 1500.085 kc. space frequency, 1600 kc. heterodyne frequency, and. 1700 kc. alternate frequency, and voltages of these frequencies are continuously present in the plate output circuits of the respective oscillators.

It will be seen that with switch 36 thrown to heterodyne oscillator 3, oscillators I and 3 are mixed in mixer I5; so that (in the absence of blocking) frequencies 1499.915 kc. and 1600 kc. are heterodyned to form the difference frequency 100.085 kc., which appears selectively in the out 'quency drift. brought into tune they do not drift apart as 3 put of mixer tube l1, since the plate circuit I4 is tuned to 100 kc. v

Similarly the signals from space oscillator 2 and heterodyne oscillator 3 are mixed in mixer l6; so that (in the absence of blocking) frequencies 1500.085 kc. and 1600 kc. are heterodyned to form the difference frequency 99.915 kc., which appears selectively in the plate output of mixer tube l8, which likewise is circuit It.

By means of the polarized keying signal supplied by leads 33 to polarized relay 34, the relay arm alternately connects negative bias C through contact 3i to grids 2i and 23 of tube I 1 or through contact 32 to grids 22 and 24 of tube [8. Thus, continually one or the other of the two mixers !5, I is blocked by the keying signal .and the output of the other mixer l6, l5, which is unblocked, is passed across tuned plate circuit -I4 to transmitter 35.

' There is thus supplied to the transmitter a low frequency carrier of mean frequency 100 kc. whose frequency shifts by 170 cycles between 99.9.1-5kc. and 100.085 kc. with the code keying.

I In tuning the equipment for the first time the crystals are first brought to the correct temperature in thermostatically controlled chamber 13;. The polarized key signal is then set for mark, which activates mixer l1 and blocks mixer 18. Trimmers 9 and II of mark and heterodyne oscillators l and 3 respectively are then adjusted, one or both, to bring the output of mixer l! to 100 kc. (or more precisely 100.085) within the tolerance permitted for the transmission. This may be done by means of a frequency meter (not shown).

It will be observed that the outputs of mark oscillator I and space oscillator 2 are continually mixed in audio mixer 26. When these oscillators are on their designated frequencies 100.085 kc. and 99.915 kc., the output of. audio mixer 26is an audible tone of 170 cycles. This tone will give a low beat with the tonefrom the standard tuning fork when listened to in the headphones of comparator 31. In the preliminary adjustment of the equipment after mark oscillator i and heterodyne oscillator 3 have been adjusted, space oscillator 2 is detuned by trimmer in so that the frequency is definitely too low and then brought up in frequency until the beat note from mixer 26 matches the 1'70 cycle fork.

It is possible to cover the range of tuning required with simple trimmer capacitors because of the high frequencies of the crystals. The thermostatically controlled chamber minimizes fre- However, when the crystals are might be expected. Even though the actual drift of each crystal may be greater than the 1'70 cycle difference, it is found that the crystals drift together because they are of the same cut and they have been selected to have a similar drift at the operating temperature of the chamber.

When it is desired to change the frequency of the carrier, heterodyne oscillator 3 is thrown out of circuit by switch 36 and alternate oscillator 4 is thrown in in its place. Mark and space oscillators I and 2 are not readjusted. Any adjustment for tuning of .the new carrier is done. by means of adjusting the frequency of alternate oscillator .4 by means of trimmer l2. The result is a new carrier of 1700 minus 1499.915 kc. or 200.085 kc. for mark signal, and one of 1700 1500.085 or 199.915 kc. for space signal. The carrier shiftis still 170 cycles.

Although I have shown and described herein a preferred embodiment of my invention, it is to be definitely understood that I do not wish to limit the application of the invention thereto, ex-

cept as may be required by the scope of the apfrom a mark signal output having a frequency which is the difference between the frequencies of the outputs of said heterodyne oscillator and of said mark oscil1ator,'a space crystal oscillator, a space mixer combining the outputs of said space crystal oscillator and said heterodyne crystal oscillator and deriving therefrom a space signal output having a frequency which is the difference between the frequencies of the outputs -of said heterodyne oscillator and of said space oscillator, an output circuit common to said mark mixer and said space mixer, and keying means alternatively blockingsaid mark and space mixers whereby there is alternatively passed, in code sequence in said output circuit, the mark signal and the space signal.

2. The method of forming a carrier frequency shift signal, comprising the steps of generating a primary mark signal, generating a heterodyning signal, mixing said primary mark signal and said heterodyning signal and deriving a secondary mark-signal having a frequency which is the difference between the frequencies of said prl mary mark signal and said heterodyning sig-.

nal, generating a primary space signal, mixing said primary space signal and said heterodyning signal and deriving a secondary space signal having a frequency which is the difference between the frequencies of said primary space signal and said heterodyning signal, and alternatively selecting said secondary mark signal and said secondary space signal for transmission.

3. A signal system comprising, in combination, meansfor generating a mark frequency, means forgenerating a space frequency, means for gen-.

erating a heterodyning frequency, first mixer means for mixing said mark frequency and said heterodyning frequency and for deriving a difference mark frequency, second mixer means for mixing said space frequency and said heterodyning frequency and for deriving a difference space frequency, means for transmitting said difference mark frequency and said difference space frequency,- and means for alternatively blocking said first and second mixer means whereby the transmissions of said difference mark frequency and said difference space frequency are alternatively blocked and substantially continuous carrier wave signalling is effected.

HOWARD W. JOHNSON.

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

UNITED STATES PATENTS Number Name Date 1,982,340 Forbes Nov. 27, 1934 2,118,917 Finch May 31, 1938 2,167,461 Muth July 25,1939

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