Phase-modulated Binary Data Transmission System Employing A Variable Frequency Oscillator

Abstract

A phase-modulated, high-power binary data transmission system includes a variable frequency oscillator klystron generating a carrier signal under the control of a binary information generator. The frequency of the oscillator is periodically varied a small amount sufficient to cause a gradual linear change in the phase of the carrier signal such that the carrier signal at specific clock times is of opposed phase in accordance with the binary information. The oscillator output is continuously sampled and compared in a mixer to the output of a highly stable oscillator operating at the carrier frequency to produce a signal having an average value which is a function of the difference in phase between the mixed signals. This signal is then analyzed and compared to the output of the binary information generator to determine if the klystron is operating at the correct frequency and phase, and if the change in frequency is correct. If an error in any of these parameters is detected an error signal is generated which is then used to shift the value of the klystron input signal to correct the output thereof. The invention described herein may be manufactured, used, and licensed by or for the Government for governmental purposes without the payment to me of any royalty thereon.

Description

The present invention relates to communication systems and more particularly to a binary transmission system for transmitting a phase-reversed carrier signal.

In the field of communications it has been the general practice to transmit binary information by periodically reversing the phase of a carrier signal in accordance with the information being transmitted. If the phase of the carrier signal is reversed instantaneously by, for example, balanced modulation, overtones and beat frequencies will be generated, thereby increasing substantially the bandwidth. However, if phase reversal of the carrier signal is accomplished gradually by instantaneously changing only the frequency of the carrier signal by a small amount, such that a proper phase reversal exists at the end of the clock period, then the bandwidth of the transmitted signal will be relatively narrow. A system for producing such a phase modulated signal is disclosed in my copending patent application, Ser. No. 872,969, filed Oct. 31, 1969, entitled Binary Transmission By Phase Modulation. That system derives from a highly stable oscillator two phase-related signals closely spaced in frequency. A switching circuit switches the output between these frequencies in accordance with the information to be transmitted. Of course, in order to produce high-power signals with this system, high-power amplifiers could be employed at obvious locations. However, to obtain both higher powers and higher frequency carrier signals, it becomes necessary to employ a less stable oscillator such as for example, the klystron oscillator. When using the klystron, minor instabilities in the output frequencies must be constantly detected and error signals must be generated such that the frequency of the klystron oscillator is changed such as to insure the proper phase relationships.

It is, therefore, the primary object of the present invention to provide a relatively narrow-band, high-power, phase-modulated communication system.

The exact nature of this invention as well as other objects and advantages thereof will be readily apparent from consideration of the following specification relating to the annexed drawing in which:

FIG. 1 shows a block diagram of a preferred embodiment of the invention; and

FIG. 2 shows waveforms useful in understanding the operation of the device of FIG. 1.

Referring now to the drawing there is shown in FIG. 1 a transmitter having an antenna 10 for radiating a phase-modulated signal generated by a high-power, variable frequency oscillator 11 such as a reflex klystron. A binary information generator 12 generates, in any well-known fashion, a binary signal in accordance with the information to be transmitted. The generator 12 operates under the influence and timing of a clock 13. The binary signal generated by generator 12 is modified by delays 14 and 15 and half adder 16 and then applied via variable gain amplifier 17 and adder 18 to the repeller of klystron oscillator 11 to vary the frequency thereof. The gain and bias of the delays 14 and 15, the half adder 16, the amplifier 17 and adder 18, are adjusted such that normally the klystron 11 will oscillate at either of two closely spaced frequencies depending on the level of the binary signal applied to the repeller thereof by adder 18. The exact nature and relationship of these two normal frequencies will be described later in greater detail. Generally, the frequency of the klystron 11 is switched such that the instantaneous phase of the output signal will normally reverse according to the information generated by generator 12 and at the rate of the clock signal 13. However, the output frequency of the klystron 11 may be slightly unstable due to excessive heating of the elements or variations in the bias voltages, etc. The instantaneous phase of the output as a result of such instabilities will gradually creep away from the normal or desired value and may, after several clock cycles, become intolerable if not corrected. Therefore, the phase of the output signal from klystron 11 is compared to the phase of an output signal from a stable oscillator 20 by a mixer 21 and a low pass filter 22. The output of filter 22, being an indication of the difference in phase between the oscillators 11 and 20, is then compared in balanced modulator 23 to the information signal generated by generator 12. If the detected difference in phase between oscillators 11 and 20 is not exactly correct at the right times i.e. in accordance with the information to be transmitted and at the clock rate, then feedback signals are generated which adjust the output of amplifier 17 and adder 18 which in turn will vary the bias on the repeller of klystron 11. The feedback signals are generated by first averaging the output of modulator 23 in filter 24 and by comparing in a subtractor 25 the output of filters 24 and 26. The output of subtractor 25 is used to adjust the gain of amplifier 17. Adder 18 adds the outputs of amplifier 17 and filter 26. The output of filter 26 is the average of the output of modulator 27. The output of half adder 16 is applied to a pair of delay elements 30 and 31. The output of delay 31 is subtracted from the output of half adder 16 in subtractor 32. The output of subtractor 32 is applied via normally closed switch 33 to modulator 27. Also applied to modulator 27 is the output of modulator 23 via delay 34.

The theory of operation will now be described with the aid of the waveforms shown in FIG. 2. Signal a represents the clock impulses generated by clock 13 that are applied to generator 12 which in turn generates, in any well known manner, a binary information signal such as signal b which represents a 1011101001. It is the purpose of the present transmitter to transmit a carrier signal c which is frequency modulated to cause phase differences of "0" and ".pi." radians at the clock time and in accordance with the information signal b. Therefore, ideally the output of klystron 11 may be represented by signal d, a relatively narrow band, frequency modulated wave having phase reversals at the clock rate and in accordance with the information signal b. The instantaneous phase of the waveform d with respect to waveform c at the clock rate a is shown just above line e and the corresponding binary information is shown below the phase. Waveform d is obtained by periodically changing the bias on the repeller of klystron 11, which is oscillating at a frequency f.sub.1 (waveform c). As a result of the change in bias, the frequency of oscillation is slightly reduced to a frequency f.sub.2. As can be seen from waveform d, f.sub.2 is such that after one clock period or baud the phase of the modulated signal will be shifted .pi. radians with respect to the carrier or reference signal f.sub.1. In the example shown, f.sub.1, is exactly twice the clock rate while f.sub.2 is one and a half times the clock rate or three quarters of f.sub.1. Of course, the closer f.sub.1 and f.sub.2 are to each other the narrower will be the bandwidth of waveform d, the transmitted signal. In general, f.sub.1 and f.sub.2 should differ in frequency by an odd multiple of one-half the clock rate.

Generation of signal d is accomplished by delaying in delays 14 and 15, the output of generator 12 (waveform b) one full clock period to obtain signal g and then applying signals g and b to half adder 16 to obtain signal h. Half adder 16 has a positive output when only one of the inputs b or g are positive. The output h of half adder 16 is then applied via variable gain amplifier 17 and adder 18 to the repeller of klystron 11 to vary the frequency thereof. For present, assume that the gain of amplifier 17 is unity and that the output of filter 26 is zero, so that the signal h passes through amplifier 17 and adder 18 unchanged. By comparing waveforms h and d it is seen that klystron 11 will normally oscillate at f.sub.1 when h is equal to zero and will oscillate at f.sub.2 when h is positive and equal to one unit. The instantaneous phase of the output signal d with respect to signal c will therefore advance linearly as shown by the solid line in waveforms j. However, because the klystron is subject to frequency and phase instabilities due to overheating or slight variations in the supply voltages, etc. the relative phase advancement might follow a path which is some combination of one of the paths j.sub.1, j.sub.2, and j.sub.3. For example, if the frequencies f.sub.1 and f.sub.2 are correct but are out of phase slightly with respect to the reference signal c, then the relative phase advancement will follow the path shown by the long dashed line j.sub.1. The short dashed line j.sub.2 represents the phase advancement, if f.sub.1 is proper but f.sub.2 is off slightly. Dashed line j.sub.3 represents the phase advancement when f.sub.2 is proper but f.sub.1 is wrong. In order to correct these errors in phase, the klystron 11 output is constantly compared to the output k of the highly stable oscillator 20 which oscillates at f.sub.1 and is displaced .pi./2 radians with respect to the carrier signal c.

For example, if signal d is proper and the phase is advancing along the solid line of j, then the output of mixer 21 may be represented by waveform m. Further the output n of filter 22 will then be the average value of m. Waveform b is delayed one-half the clock period by delay 14, the output p of which is balanced modulated with n in modulator 23 to produce waveform q the average value of which is zero. Therefore, the output of filter 24 will be zero, if waveform d is properly phase related to oscillator 20 output k.

Also, signal h is delayed a half clock period in each delay 30 and 31 to produce signals r and s. Signal h is subtracted from s in subtractor 32 to give signal t. Signal t is switched by normally closed switch 33, under the control of signal r, to produce an output signal u. Signal q is delayed a half clock period by delay 34 and then balanced modulated with signal u in modulator 27. The output v of modulator 27 will be zero because signal q, after a delay of half a clock period, will line up with signal u such that u is zero when q is not and vice versa, thereby producing a zero output. Signal v is, of course, not shown in FIG. 2. It should now be clear that the output of modulator 27, filter 26, and subtractor 25 will be equal to zero when signal d is properly phase related with respect to signal k and the phase advancement of signal d is proceeding along the solid line j. In this case, therefore, the klystron 11 is stable and properly phased or, in other words, the signal h is providing the proper voltages to the repeller of klystron 11 to produce the correct output d and need not be altered by either amplifier 17 or adder 18.

The output of klystron 11, however, as explained above, may be in error thereby requiring adjustment of signal h. As indicated earlier, any error in the klystron output may be resolved into some combination of any one of three causes (1) only the phase is off and f.sub.1 and f.sub.2 are proper (path j.sub.1), (2) f.sub.2 is wrong and f.sub.1 is proper (path j.sub.2), and (3) f.sub.1 is wrong and f.sub.2 is proper (path j.sub.3).

To illustrate the first case, assume that the phase of the output of klystron 11 is off by .pi./2 radians (worst case). In this case, the output of klystron 11 will be in phase with signal k and the average value of the output of mixer 21 will be of the form n.sub.1. The output of modulator 23 will be q.sub.1 which has an average value different from zero. Signal Q will be averaged by filter 24 the output of which will be proportional to the difference in phase between the actual output of klystron 11 and the desired output d. It is noted that an error in the phase of the klystron 11 output by .+-..pi./2 radians will produce the greatest outputs from filter 24. The signal q.sub.1 is also applied to modulator 27 via delay 34 producing a signal v.sub.1, the average value of which is zero. Therefore, the output of filter 26 is zero in this case and, no correction to signal h will be made in adder 18. However, the subtractor 25 which subtracts the output of filter 26 from the output of filter 24 will have an output which will vary the gain of amplifier 17, thereby changing only the upper value of signal h. As a result, the repeller voltage of klystron 11 will be such that it will temporarily oscillate at a frequency slightly different then f.sub.2, thereby slowly bringing the klystron 11 into proper phase.

To summarize, if the klystron 11 should be oscillating at the proper frequencies f.sub.1 and f.sub.2 but at the wrong phase the gain of the amplifier 17 will be varied such that only the high value of signal h will be correspondingly shifted thereby causing the klystron 11 to oscillate at a frequency slightly different then f.sub.2 during those periods when it would otherwise be oscillating at f.sub.2. It is during this period that the phase of the klystron 11 output signal will be corrected. When corrected of course, the error signal applied to amplifier 17 will also return to zero causing the gain of amplifier 17 to return to unity and the klystron 11 to again oscillate at f.sub.2.

When f.sub.1 is proper but f.sub.2 is off (the second type of error listed above and illustrated by line j.sub.2) the gain of amplifier 17 is again varied. In this case the output of filter 22 would be n.sub.2. The output of modulator 23 is turn is q.sub.2.

It can be seen by mere inspection that the average value of q.sub.2 which appears at the output of filter 24, will slowly increase as the relative phase of the klystron 11 output deviates more and more from the proper value due to the fact that klystron 11 is oscillating at a frequency different then f.sub.2. Signal q.sub.2 when delayed by delay 34 and modulated with signal u in modulator 27 will result in signal v.sub.2 which has an average value of zero. Therefore, again the output of filter 26 is zero indicating that signal f.sub.1 is correct. The output of filter 24 will be applied to amplifier 17 via subtractor 25. As in the previous case, the gain of amplifier 17 will be correspondingly increased or decreased to raise or lower only the high voltage value of signal h. As before, this change will not alter the f.sub.1 oscillations of klystron 11, but will change the high value of the repeller voltage in a direction to correct the oscillations at f.sub.2.

The third type of error is represented by line j.sub.3 and occurs when f.sub.1 is wrong and f.sub.2 is correct. The output of filter 22 in this case will be n.sub.3 and the output of modulator 23 will be q.sub.3. It can be seen by inspection of q.sub.3 that the output of filter 24 will slowly increase as the phase of the output of klystron 11 deviates from the proper value which is now a result of the error in f.sub.1. Signal q.sub.3 is also modulated with u after a delay in delay 34. The output of modulator 27 will be v.sub.3 and the average value thereof, as obtained from filter 26, will in this case become slowly negative. This can be seen by inspection of v.sub.3 where for every positive pulse there is a succeeding negative pulse which is slightly larger in absolute value. The output of filter 26 is then added to signal h to shift both the upper and lower values. The output of filter 26 is now subtracted from the output of filter 24 in subtractor 25 such that the upper value of signal h is shifted an amount equal and opposite to the amount it will be shifted in adder 18. Therefore, the net result in this case will be to shift only the lower value of h an amount sufficient to correct for the error in only f.sub.1. As a result, spurious fluctuations in the klystron 11 characteristics or in the power supplies and the bias voltages (not shown) will be immediately compensated for in amplifier 17 and adder 18.

After transmission by antenna 10 and subsequent reception by a receiver the phase of signal d may be detected by any one of many well-known methods. For example, the slope of signal d could be measured in the receiver at the clock times as indicated on line e. Another method of detection of signal d is disclosed in my copending patent application, Ser. No. 875,682, filed Nov. 12, 1969, and entitled Binary Information Receivers for Detecting a Phase-Modulated Carrier Signal.

Obviously many modifications and variations of the present invention are possible in the light of the above teachings and the invention may be practiced otherwise than as specifically described.

Claims

What is claimed is:

1. A binary communication system for transmitting a carrier signal which is periodically of equal phase and of opposed phase with respect to a reference signal comprising; a clock means for generating a clock signal; a binary information generator means controlled by said clock means for generating a two-level information signal at the rate of said clock signal; control means connected to the output of said binary information generator means for shifting the levels of said two-level information signal; a variable frequency oscillator means for generating said carrier signal and having the frequency thereof shifted between first and second frequencies by the output of said control means; the difference between said first and second frequencies being equal to an odd multiple of one-half the rate of said clock signal; a stable oscillator means for generating a reference signal at said first frequency and at a fixed phase with respect to said clock signal; mixer means for comparing the outputs of each said oscillators to determine the difference in phases thereof; comparator means for comparing said two-level information signal to the output of said mixer means for generating first and second error signals; said first error signal being a function of the phase differences detected by said mixer means and said second error signal being a function of the change in the difference in phase detected by said mixer means during the periods when said variable frequency oscillator should be operating at said first frequency; and means for applying said error signals to said control means for shifting the levels of said two-level information signal in accordance with said error signals.

2. The device according to claim 1 and wherein said control means includes; a subtractor means for generating a difference signal which is the difference between said first and second error signals; and a variable gain amplifier means being controlled by said difference signal for varying the difference between the levels of said two level information signal.

3. The device according to claim 2 and wherein said control means further includes an adder means for adding said second error signal to said two-level information signal.

4. The device according to claim 1 and wherein said variable frequency oscillator means is a klystron oscillator.