US2169357A - Phase modulation type frequency inverter - Google Patents

Description

Aug. 15; 1939. I

REL/1T) VE AMPUZ/DE 0F CURRENTS OF EACH FREQUENCY Q Q o. W NsELL Filed Aug. 31, 1938 CARR/E R FREQUENCY FIRST UPPER AND LOWER SIDE FREQ/15h: CY

ZERO

I THIRD UPPER AND J LOWER SIDE FREQ CARR/ER ATTORNEY.

- Aug. 15, 1939. c. w. HANSELL PHASE MODULATION TYPE FREQUENCY INVERTER Filed Aug. 31, 1938 4 Sheets-Sheet 2 MODULATION SIDE NOTE: 50.000 F 1/ cy areas CARR/ER R50 EN RANGES is PHASE k MODULATED 70 m AMPLITUDE A V N K/LOCYCLES FREQUENCY COMPONENTS AT OUTPUT 0F FREQUENCY MUL T/PL/ER MODULATION SIDE BAND PASS F/L TEE FREQUENCY RANGE CHAR/JCT ER/S T C I l I l 45 K/LOC YCLES FREQUENCY COMPONENTS A T OUTPUT 0F FIL TER SOURCE OF 5,000 CYCZE 4, .000 CYCLE POWER i 0 POWER SOURCE /5 DOUBLE 5/05 BAND sup esssm CARR/ER ourpz/r f WIT/1' INVERTED MODULAT/ON 8 v BAND PAss WI??? M5212? fl/5%; wa s; uoaumron "cms 2'' 22 AUDIO INPUT L0 (/4 L20 0 -5000 cm ES INVEN TOR. c. w HA NSELL ATTORNEY.

Aug. 15, 1939. c. w. HANSELL PHASE MODULATION TYPE FREQUENCY INVERTER Filed Aug 31, 1938 4 Sheets-Sheet 3 8 8; J V FREQUENCY SOURCE OF Ml/LT/PL/ER 03 ,57 f-6${' Mi /295R saga 557, 50;

ER ADJUSTER 0 L 6 r v 1 'v Z4 v 0 0 BAND PASS WW 3m MOM/L r02 MULTIPLIEI? 0 55,000 r0 i ourpur PHASE y-h-An/l uruos FREQUENCY 0R muse MULTIPL/ER M00z/LAT0R MULT/PL/ERQS MODULATOR somcs 0F saunas 0F 1 W P455 W we? 40, 49,290 1 POWER POWER (3006mm 10 5 w 3 v V -2 I AMPLITUDE PHASE FREQUENCY on PHASE Maoumm ama/512 Mowmm Q0 /g' Q25 INVENTOR. c. m HA NSELL A TTORNEY.

Aug. 15, 1939. c. w. HANSELL PHASE MO DULATION TYPE FREQUENCY INVERTER Filed Aug. 31, 1938 4 Sheets-Sheet 4 INVENTOR. C. W HANSELL ATTORNEY.

Patented Aug. 15, 1939 PHASE MODULATION TYPE FREQUENCY INVE TER Clarence W. Hansell, Port Jefferson, N. Y., as-

signor to Radio Corporation of America, a corporation of Delaware Application August 31, 1938, Serial No. 227,635

1": Claims. (01. 179-15) Signal frequency inverters are now used in radio communication circuits for the purpose of obtaining secrecy from unauthorized reception or listening. The frequency inverters now used consist of a single side band output modulator followed by a heterodyne detector in which the heterodyne oscillator frequency is near the side frequency produced by the highest modulating frequency instead of the lowest modulating frequency. The output of the heterodyne detector then contains a spectrum of frequenciescorresponding to the original modulating frequencies but the relative positions of high and low frequencies in the spectrum have been exchanged or inverted. The inverted spectrum-of modulating frequencies is then utilized to modulate a radio transmitter in any of the several well known ways. That is to say, in these known systems a carrier and a single side band are produced and beat together in a detector to give an output wherein the modulating frequencies characteristic of the signal or modulating frequencies are inverted, and this inverted, unintelligible band of frequencies is transmitted.

In my novel method and means, I provide a new and improved frequency inverter, as follows. Assume that a band of audio frequencies ranging from 0 to 5,000 cycles are to be inverted for signaling purposes. I provide a relatively high frequency carrier wave and then phase modulate this wave, at say a 5,000 cycle rate, preferably-with a phase deviation of 2.4 radians. This phase deviation of 2.4 radians has been selected because for this amount of'phase deviation the strength of carrier output from the modulator'is reduced to zero, but there are or tend to be side frequencies above and below the carrier frequency, at intervals of 5,000 cycles. The side frequencies on each side of the original carrier frequency are of relative amplitudes of 52%, 43%, 20%, 6.5%, 1.5%, etc., of the strength of the unmodulated carrier.

If, now, I simultaneously phase or amplitude.

modulate the output of the phase modulator described above by means of the 0 to- 5,000 cycle band of audio frequencies which I wish to invert,

then, each of the side frequencies produced by the 5,000 cycle phase modulation will be modulated,'and have symmetrically grouped around it side frequencies of its own, produced by the 0 to 5,000 cycle modulation. side frequencies adjacent the original carrier, (modulated to zero) wherein the side frequencies produced by the lowest modulating potentials are separated the greatest distance from the car- I now select the rier. In this modulated side band energy the signaling frequencies of both side bands are inverted. In the above illustration the frequencies used are given merely by way of example, and obviously a very wide choice of other carrier frequencies, tone frequencies and bands of signal frequencies may be used. The phase modulation of the carrier at the 5,000 cycle rate and the amplitudemodulation of the output of the phase modulator by means of the 0 to 5,000 cycle band of signal frequencies may be simultaneous, as described above, or the carrier may be phase modulated at the 5,000 cycle rate, then phase or amplitude modulated by the signal frequencies, or these operations may be reversed.

If, now, the original unmodulated carrier is reintroduced with relatively great strength and correct phase relation, and-the energy passed into a linear rectifier or detector, the original 0 to 5,000 cycle modulation is obtained, except that it will be inverted. There will be additional higher frequency beats with the carrier, but these may be eliminated by filtering.

I may modulate a high frequency carrier wave with the inverted signals for transmission from one point to another through any medium exposedto those for which the message is not intended. Of course, persons with specialized equipment may be able to reinvert the modulation and obtain intelligible speechunless other secrecy precautions are taken, but the persons equipped only with the usual broadcast receiver, which number in millions, will not be able to understand the modulation, which-may, for example, be conversations of a private nature.

In an improvement-of the system described above, I use two duplicate phase modulators, both having 5,000 cycle modulated outputs but having their outputs coupled to a common output circuit in phase opposition. Under these circumstances, there will be no carrier or side band output due to the 5,000 cycle phase modulation, assuming there is no phase or amplitude modulation by the band of signals. Now, the 0 to 5,000 cycle modulation is applied to the modulators with opposed polarity or phase, and side frequencies produced by the 0 to 5,000 cycle modulation add in the common output circuit. I next introduce the original carrier wave, ex-

alted in amplitude and correct in phase, to produce a carrier for the inverted side bands. The

rents without the undesired presence of a 5,000 cycle steady modulation tone which would otherwise be present and which in some circumstances might permit unauthorized reception of the inverted signals with a very selective receiver.

When the first described inverter and also with this improved circuit or method, I obtain still further secrecy by phase or frequency modulating or wobbling, at some very low frequency such as 20 cycles, the carrier of transmitters to which the inverted 0 to 5,000 cycle modulating energy is applied in the form of amplitude modulation. This additional carrier wobble prevents the use of heterodyne type of receivers which otherwise could be used to render the inverted modulation intelligible.

The system described may be used at receiving stations to return inverted speech to its normal frequency relation.

In describing the novel features of my method and means more in particular, reference will be made to the attached drawings wherein:

Figure 1 comprises curves showing the relative amplitudes of the carrier and the first five side frequencies obtained when the carrier is phase modulated by oscillations of a single constant frequency.

Figure 2 is a graphic representation of the relative amplitudes of the carrier and first three side frequencies produced by phase modulating a carrier to zero amplitude.

Figure 3 is a graphic representation of the relative output for various frequencies of a filter, following a phase modulator as described above, wherein the carrierand the first side band are passed.

of the nature of the one shown in Figure 5t with means for transmitting the inverted signals to a utilization point and means for reinverting the signals.

In Figure 1, I have supplied curves representing therelative amplitudes of current or voltage for the carrier and side frequencies of a phase modulated wave when the phase deviation or the modulation by a single modulating frequency ranges from 0 to 6 or 7 radians. (1 radian equals 57.296 degrees) From the curves of Figure 1 it may be seen that starting with a phase deviation of 0 and increasing through deviations of 1 and 2 radians the amplitude of the carrier frequency current decreases steadily until at about 2.4 radians the strength of the carrier wave is reduced to zero. For this amount of phase deviation all the energy of the modulated'wave is represented by energy of side frequency components. In the curves of Figure 1, we may assume for purposes of illustration that a 60,000 cycle carrier current is phase modulated by 5,000 cycles to such a degree, e. g. approximately 2.4 radians, that the carrier amplitude drops to 0. Obviously carriers of other frequencies, phase modulated by constant currents of other frequencies than those given by way of example, may be used.

I have representedthe relative amplitudes and frequency relations of such a phase modu t d wave in Figure 2. I have assumed that the carrier frequency is 60,000 cycles per second and that the carrier is phase modulated by a 5,000 cycle current in such a way as to give a peak phase deviation of about 2.4 radians. Note here that the vector representing the carrier at 60,000 cycles is 0, while the upper and lower side frequencies, three pairs only of which are shown, decrease in amplitude as their spacing from the carrier increases.

The 60,000 cycle carrier may be phase modulated by a constant frequency wave directly or the constant frequency wave may be used to mod ulate a low frequency carrier and the output of frequency multiplied to give the finally desired phase modulated wave. For example, a 15,000 cycle current could be phase modulated plus and minus 0.6 radian and then multiplied 4 to l in frequency to give the 60,000 cycle current with a phase deviation of plus and minus 2.4 radians.

Now I additionally phase, or preferably, amplitude modulate the phase modulated wave described above, and graphically represented in Figures 1 and 2, by a spectrum of audio frequencies representing the signals to be inverted. These audio frequencies have been assumed for purposes of illustration to lie between 0 and 5,000 cycles. This produces a band of side frequencies extending above and below each original com ponent frequency by plus and minus 5,000 cycles.

Referring again to Figure 2, and considering the side frequency components between 55,000 and 65,000 cycles, it will be seen that signal modulation frequencies near 0 frequency produce side frequency components nearly 5,000 cycles removed from the original 60,000 cycle carrier, while signal modulation frequencies near 5,000 cycles produce side frequencies little removed from the original 60,000 cycle carrier. In other words, between 55,000 and-65,000 cycles, I have produced a double set of side frequencies which are reversed in relative position, or inverted, with respect to the 60,000 cycle carrier. By means of a frequency selective band pass filter I reject all frequency components except those between 55,000 and 65,000 cycles per second.

Now, I reintroduce currentat the original 60,- 000 cycle carrier frequency with correct amplitude and phase relation and obtain either an amplitude or a phase modulated wave with inverted modulation. This wave is represented in Figure 3, which also represents the relative output at various frequencies of the band pass filter following the modulator.

Ihe phase or amplitude modulated wave may be used for transmission of inverted speech or sound over a wire or radio circuit in order to prevent unauthorized interception of the speech. In doing this the whole frequency band may be moved up and down in frequency by single side band modulation, on by heterodyning. That is, a fixed frequency may be added to, or a. fixed frequency may be subtracted from, all frequency components of the modulated current by methods well known to persons skilled in the electrical communications art. v

When the 60,000 cycle carrier wave and its spectrum of-inverted modulating frequencies is transmitted as just described it will usually be desirable to reintroduce the carrier with a relative strength equal to or a little greater than the peak value of the algebraic or vector sum of the side frequency components. This will result in a final carrier current modulation equal to, or a little less than, 100%.

Alternatively it may be preferred to obtain a band of audio frequencies, with frequency components inverted, in the 0 to 5,000 cycle band to facilitate transmission at audio frequencies. To do this I simply apply the 60,000 cycle current and its spectrum of inverted side frequencies to phase or amplitude modulation detectors, as the case may be, and so derive the inverted modulation. If the 60,000 cycle current is amplitude modulated, for example, I simply applythe current, or potential derived from it, to a linear or proportional rectifier in which case the output from the rectifier will contain a direct current component plus all the inverted audio components.

When the inverted modulation is to be brought back immediately to an audio frequency band, before transmission, it is desirable to give the reintroduced 60,000 cycle carrier a strength considerably higher than the peak vector sum of the side frequency components in order to keep the percentage modulation considerably less than 100%. This adjustment tends to reduce distortion in the detector.

To practice my invention one skilled in the art may do so by suitably combining electrical equipment already well known in the art for performing indicated functions. Since these items of equipment are well known to any communications engineer, it is believed that it would only add confusion and unnecessary complication to describe them in detail.

Referring now to Figure 4, I have indicated at I0 a source of 5,000 cycle power which is supplied to a modulator l2. The 5,000 cycle power may be modulated in phase or amplitude, but preferably in amplitude, in accordance with the audio frequency input at 8. This produces in the out.- put of the modulator 12 a 5,000 cycle current or potential which is modulated by the audio frequency input.

This modulated 5,000 cycle current is supplied to a phase modulator M. The phase modulator I4 is also supplied 15,000 cycle power from a source IS. The output of the phase modulator I4 is supplied to a frequency multiplier I 8. The frequency multiplier l8 may also act as an amplifier to increase the power of. the modulated energy. The use of frequency multiplication in I8 increases the phase deviation of the phase modulated carrier from l8 and makes it easier to obtain the desired amount of final phase deviation, starting with a smaller initial phase deviation.

If the frequency multiplication in I B is, say, 4 to 1, then the final output corresponding to 15,000 cycles will be at 60,000 cycles.

The output of I8 is supplied to a band pass filter 20, which passes the carrier and two side bands. If the carrier is multiplied to 60,000

cycles the output of 20 will comprise the 60,000

cycle carrier and inverted side band frequencies extending therefrom in both directions up to plus and minus 5,000 cycles. In other words, this band pass filter will pass a band of frequencies 7 between 55,000 and 65,000 cycles.

modulated with a peak phase deviation of. plus and minus about 2.4 radians. For this condition the 15,000 cycle current must be modulated plus and minus 0.6 radian or about 34.4 degrees.

For this condition the output from the band pass filter 20 may be substantially zero until audioinput is applied at 8 to the system, in which case a double'set of side bands appears in the output with frequency relations corresponding to inverted modulation. These double side bands may be transmitted to a distant receiver and there demodulated or they may be combined with a 60,000 cycle carrier current to provide an inverted amplitude or phase modulated wave. In the latter case I transmit the modulated wave, usually after transferring the band of frequencies to a higher frequency range by adding a fixed frequency to all component frequencies in a single side band modulator, to a distant receiver over a wire or radio circuit or a combination of both types of circuit and there demodulate the wave. Of course in the demodulation process the modulation will be'reinverted to obtain the original audio modulation.

One possible method of using the arrangement of Figure 4 is to allow the side frequencies due to 5,000 cycles to pass through the band pass filter. These two side frequencies, differing in frequency by 10,000 cycles may be beat together at the receiver to produce a 10,000 cycle current by means of which I produce by frequency multiplication, or control the frequency and phase of a carrier or beating current for use in the demodulation process. This principle of frequency control is partly disclosed in my U. S, patent application No. 230,438 filed November 2, 1927 (RCA docket #1264).

In Figure 5 I have shown another arrangement suitable for carrying out my invention. In the arrangement of Figure 5 the audio modulation is introduced after the initial carrier wave of 15,000 cycles from I6 is phase modulated at 5,000 cyclesv in i4 and frequently multiplied in H3. The band of modulating potentials are supplied at 8, amplified at 8', and impressed on the amplitude or phase modulator l2. In this case also the inverted double side band output from the band pass filter 20 is combined with 60,000 cycle carrier current to provide a 60,000 cycle amplitude or phase modulated resultant current output at 22. The 60,000 cycle carrier may be supplied in the proper amount and phase from a separate source or may be obtained by multiplying 15,000 cycle current obtained from source Hi. The multiplying and phase regulating means has been indicated at 24.

In Figure 6 I have shown a double or balanced system in which 15,000- cycle carrier currents are modulated by 5,000 cycle currents in two phase modulators l4 and I4 and multiplied in frequency in two frequency multipliers l8 and I8, after which the resultant currents are modulated in two modulators l2 and I 2 and then combined and passed through a band pass filter 20, after which a carrier current is reintroduced to provide a resultant amplitude or phase modulated current with inverted modulation. It is noted here I.

that the outputs of the systems lfl, l8, l2 and ld',

l8, l2 are arranged to oppose on the input of 20' (disregarding the audio modulation supplied by in phase opposition so that their modulation components add on the input of 2B.

In other words, in the arrangement of Figure 6 all frequency components except the side band currents produced by the audio modulation tend to be balanced out. This assists us in suppressing undesired frequency components and makes it more practical to obtain the desired degree of suppression of undesired frequency components.

In Figure 7 I'have shown a complete telephone communication system, with both transmitting and receiving terminals, in which inverted modulation is used between the terminals. It will be noted that inverter equipment used at both the transmitter and receiver terminals may, if desired, be identical in design. In the system of Figure 7 the modulation speech inverter means is the same as that shown in-Figure 5, except as otherwise noted below. The carrier to be modulated is assumed to be 25,000 cycles. The carrier and inverted side bands at 22 are rectified in 28 and the inverted speech or other signal potentials transmitted over means 30 to the desired point where the received inverted modulation is amplified', if desired, in an amplifier 40 and supplied to an amplitude or phase modulator 42. The amplitude or phase modulator 42 is also supplied with energy comprising a carrier, preferably modulated to zero, with side bands resulting from modulating the carrier in phase by voltages of constant frequency'. Since, in Figure 7, it is assumed that identical inverter equipment will be used at both ends of the wire and/or radio circuits, it will be noted that the source 46 supplies the same frequency as unit l6 andthe output of 42 has the same frequency as the output of Ill. The multiplier 48 increases the frequency of the output of 44 by the same factor used in It, in this case assumed to be 3. The input to the rectifier or detector 54 comprises a carrier supplied by 52 and side bands in which the signal components are in proper order. This energy is demolulated in 54 and the original modulating signals derived at 56.

As previously stated, if the intervening circuits 30 comprise a radio transmitter this transmitter may have its carrier frequency frequency modulated or wobbled at some low frequency of, say, 10 to 30 cycles per second, to spoil intelligibility otherwise obtainable with a heterodyne receiver.

What is claimed is:

1. Means for inverting the frequencies of a band of frequencies comprising in combination, a source of carrier current, means for phase modulating the carrier current by constant frequency current and for modulating one of the said currents by the band of frequencies to be inverted.

2. Means for inverting the frequencies of a band of frequencies comprising in combination, a source of carrier current, means for phase modulating the carrier current by constant frequency current, means for modulating one of said currents by the band of frequencies to be inverted and means forselecting a band of frequencies comprising the carrier frequency and the first order side band frequencies.

3. Means for inverting the frequencies of a .band of frequencies comprising in combination, a

source of carrier wave frequencies, means for phase modulating the carrier wave frequencies by constant frequency current and for modulating the constant frequency current by the band of frequencies to be inverted.

a source of carrier wave current, means for modulating the carrier wave current in accordance with said band of frequencies and phase modulating the carrier wave current in accordance with constant frequency current.

5. In a secret signalling system, means for inverting the frequencies of a band of frequencies comprising, a source of carrier current, a source of constant current of lesser frequency than the frequency of said carrier current, means for modulating said carrier current in phase in accordance with the constant current of lesser frequency, means for modulating one of said currents in accordance with said band of frequencies to be inverted, and means for filtering out the first order side bands resulting from said modulation.

6. In a secret signalling system, means for inverting the frequencies of a band of frequencies comprising, a source of carrier current, a source of constant current of lesser frequency than the frequency of said carrier current, means for modulating said carrier current in phase in accordance with the constant current of lesser frequency, meansfor modulating one of said currents in accordance with said band of frequencies to be inverted, means for filtering out the first order side bands resulting from said modulation, and means for transmitting the resultant energy.

7. In a secret signalling system, means for inverting the frequencies of a band of frequencies comprising; a source of carrier current, a source of constant current of lesser frequency than the frequency of said carrier currents, means for modulating said carrier current in phase in accordance with the constant current of lesser frequency and for modulating said carrier current in accordance with said band of frequencies to be inverted, means for filtering out the carrier and the first order side bands resulting from said modulation, means for transmitting the same over a medium in which secrecy is desired and means for reinverting said band of frequencies.

8. In a system for inverting a band of frequencies, means for phase modulating carrier current substantially 2.4 radians peak deviation in accordance with constant frequency current, and means for modulating said carrier current by said band of frequencies.

9. In a system for inverting 'a band of frequencies, a source of carrier currents, means for phase modulating said carrier current substantially 2.4 radians in accordance with constant frequency, means for modulating said carrier current by said band of frequencies, means for multiplying the frequency of the resultant energy and means for introducing into the resultant energy a new carrier.

10. Ina system for inverting a band of frequencies, a source of carrier current, a source of current of constant frequency, means for phase modulating portions of said carrier current in accordance with said constant frequency current, means for differentially modulating the resultant modulated energy by said band of frequencies, means for combining the resulting energies in phase opposition, and means for reintroducing a carrier in the combined output.

11. Means for inverting the frequencies of a band of frequencies comprising, in combination a source of carrier current, means for phase modulating the carrier current with a substantially 2.4 radian peak deviation by constant frequency current and for modulating one of said currents by the band of frequencies to be inverted,

- 12. The method of inverting the frequencies in a band of frequencies of considerable width which includes the steps of, phase modulating wave energy in accordance with other current of constant frequency and modulating one of said currents in accordance with the band of frequencies to be inverted.

13. A method as recited in claim 12 wherein the phase modulation is accomplished with a peak deviation of substantially 2.4 radians.

14. The method of demodulating carrier and side band energy wherein the side band frequencies are characteristic of a band of signal frequencies which are inverted relative to the carrier which includes, producing carrier current of a frequency substantially equal to the frequency of the said first carrier, beating the produced carrier energy with said carrier and inverted side buds and rectifying the output obtained from I producing carrier wave energy and side band energy wherein the side band energy is characteristic of inverted signals, means for transmitting said carrier andside band through a medium in which secrecy is desired, means for returning the side band's frequencies to their proper relation relative to the carrier comprising, a. source of wave energy of a frequency substantially equal to the said carrier frequencies, means for modulating said wave energy in phase by current of a constant frequency separated from said carrier current by an amount substantially equal to the space between said carrier and side bands, means for beating the resulting energy with said carrier and side bands and means for rectifying the output of said last named means.

16. Means for inverting a band of frequencies comprising, means for phase modulating a carrier wave, means for amplitude or phase modulating the resultant wave by waves of the band of frequencies which is to be inverted, means for adding waves of the carrier frequency and means for rectifying the resultant.

17. A frequency band inverter comprising, means for producing side frequencies of a modulated carrier, means for modulating the amplitude of these side frequencies by means of the band of frequencies which is to be inverted, means for adding the carrier frequency and means for rectifying the resultant.

CLARENCE W. HANSELL.

Images