US2349870A - Frequency modulation system - Google Patents

Description

Patented May 30, 1944 FREQUENCY MODULATION SYSTEM Winfield R.

noch, nascondere, N. J., annoito Radio Corporation of America, a corporation of Delaware Application May 27, 1942, Serial No. 444,639

12 Claims. (Cl. 250-6) My present invention relates to frequency modulation systems, and more particularly to a novel method of, and means for, securing the benefits of high deviation modulation with a narrow band receiver.

In present frequency modulated carrier wave (FM) transmission and reception there is employed a channel width of 200 kilocycles (kc.). The FM band itself extends from 42 to 50 megacycles (mc.) The carrier is actually deviated over a range up to a maximum of 75 kc. on either side of the carrier, or center, frequency. In other words, the channel width is considerably in excess of the audible modulation range. Various proposals have been made for receiving such high deviation FM waves in a receiver whose pass band width prior to demodulation is relatively narrow. Such prior proposals have involved complex networks, and function solely at the receiver.

My present invention has as its main object the provision of an FM system wherein the benefits of high deviation modulation. are secured, while utilizing a receiver having selector circuits of relatively narrow pass band width prior to the demodulator; the system generally being characterized by its utilization of a.` super-audible control signal in the transmitted FM wave energy.

In the past it has been proposed to utilize superaudible frequencies within the same transmission channel as employs audible signals as modulation. For example, facsimile picture signals are multiplexed in the same channelwith audible signals. Hence, the presently proposed use of a super-audible control frequency which can be transmitted simultaneously without interference, is quite feasible.

Accordingly, it is another important object of my invention to provide a radio transmission system wherein at the transmitter a carrier can be. deviated over a relatively narrow range by modulation signals, but a control tone of super-audible frequency is included in the channel for expanding the dynamic range of the receiver thereby to simulate the action of a receiver of wide pass band characteristics.

.Another object' of my invention is to provide in an FM system a method of transmission wherein the carrier deviation is increased with audio signal amplitude up to a maximum value, which value is relatively smaller than has been employed in the past; there being included in the FM energy a super-audible control tone whose frequency is a function of modulation amplitude; and at the receiver 'the control tone being employed to expand the dynamic range of the audio network f or audio signals of high amplitude.

Still other objects are to improve generally the simplicity and efficiency of FM transmission, and more especially to provide a system of the FM type which is reliable in operation.

The novel features which I believe to be characteristic of my invention are set forth in particularity in the appended claims; the invention itself, however, as to both its organization and method of operation will best be understood by reference to the following description taken in connection with the drawing in which I have indicated diagrammatically several circuit organizations whereby my invention may be carried into effect.

In the drawing:

Fig. 1 schematically shows an FM transmitter embodying the invention,

Fig. 2 shows the modulation characteristic of the transmitter,

Fig. 3 illustrates the control tone characteristic at the transmitter,

Fig. 4 schematically shows a receiver used in my system,

Fig. 5 illustrates the eflect of the control characteristic at the receiver,

Fig. 6 shows the control tone discriminator characteristic at the receiver.v

Referring now to the accompanying drawing, wherein like reference characters are used to designate similar circuit elements in the different figures, Fig. l schematically shows the various networks of a frequency modulation transmitter. This invention is in no way restricted to FM, since it can be readily applied to phase modulation. The expression angular velocity-modulated carrier wave is to be understood as generically denoting either a. frequency or phase modulated carrier wave. The source of audio waves I may be a microphone, phonograph record pick-up, line or any other type of audio modulation source. Further, the modulation need not be audio; since modulation frequencies of a super-audible frequency range can be used. The control tone in that case may be at some frequency above the 4modulation range.

latter is connected across the tank circuit of thev master oscillator l. The tank circuit is tuned so that the oscillations, in the absence of modulation, have a carrier frequency in the 42 to 50 megacycle (mc.) band. The latter is the presently assigned FM broadcast band.

Of course, the invention is not restricted to peration in the 42-50 mc. band. Any higher, or lower, band` may be utilized. What is important is that the carrier frequency (say 49.9 mc. for illustration) be deviated in proportion to the amplitude of the modulation; The rate of carrier, or center, frequency deviation is the modulation frequencies per se. In the present FM band of 42-50 mc., it is the practice to deviate the carrier frequency a maximum of '15 kilocycles (kc.) to each side. The FM channel itself has a width of 200 kc. The FM wave energy is radiated from the radiator; the latter may be a dipole 5.

The reception of FM wave energy having an overall deviation width of 150 kc. requires specially designed transformers prior to the demodulator. It is, in fact, highly desirable to beable to radiate the high deviation band energy, but to receive the same with a receiver Whose selector circuits need not have excessively wide response characteristics. My solution of this problem depends upon the inclusion of a super-audible coritrol tone (assuming the modulation is at audio frequency) in the FM wave energy and in the concurrent compression of the carrier frequency deviation. At the transmitter this is easily andi effectively accomplished in the manner now to be described.

A pilot oscillator 6 is provided. The frequency thereof may be varied over a range of 20 to 21 kc. A reactance tube 1 is employed to vary the frequency of the pilot oscillator tank circuit. It will be understood that the circuit of reactance tube 1 may be constructed and arranged as was described in connection with circuit 3. The reactive effect due to network l is controlled in accordance withv the amplitude of the modulation energy. Thus, amplifier 2 feeds a portion of its modulation energy to a rectifier circuit 8. The latter may include a diode ,9 whose cathode may be positively biased to provide delayed rectification of modulation currents.

The rectified modulation current flows through output resistor I0, which is properly bypassed for alternating current components, and develops `a direct current control voltage. The voltage may be applied, for example,v with an increasingly negative polarity as the modulation amplitude increases. Thus, the gain of reactance tube 'l decreases, and the circuits of network l will be chosen to provide a reactive effect which acts to increase the pilot oscillator frequency. For ex ample, if network 1 simulated a capacity effect, smaller values of capacity effect would act to increase the pilot frequency. Hence, up to a predetermined value of modulation input level to rectifier 8 no effect on pilot frequency takes place. Thereafter, the pilot frequency increases up to 21 kc. as the modulation amplitude rises. pilot oscillator output energy is applied to modulator 3, so that the current variation of the latter will include the pilot -frequency as a component. Fig. 3 shows the control tone frequency variation as afunction of modulation amplitude.

The modulator output is concurrently varied in the manner suggested in Fig. 2. In the prior art, and as indicated by the dotted line in Fig. 2, the frequency deviation of the carrier increased linearly with the modulation amplitude. Accord- The a function of not permitted to rise above a certain maximum frequency value. Thus, in Fig. 2 the full line curve shows how the deviation is maintained substantially at 25 kc., to each side of the carrier, or center, frequency value above -a predetermined. modulatipn amplitude. This is accomplished by having the rectifier 8 reduce the gain of modulation amplifier 2 as the voltage applied to diode 9` overcomes the delay bias. The voltage across resistor 9 is used to decrease the gain of amplifier 2.

It' will be understood that the fiat portion of the curve of Fig. 3 corresponds to the rising linear portion of the full line curve of Fig. 2. This means that over the horizontal portion of the modulation characteristic of 4the transmitter the control frequency will have a rising course. The negative bias applied to amplifier 2 may be impressed on the control grid over lead II. As the modulation amplitude level rises, the gain of amplifier 2 is caused to decrease thereby to prevent any increase in the reactive effect due to network 3 aecting the master oscillator frequency. However, as pointed out previously, simultaneously there is embodied in the FM energy a component for determining themode and extent of dynamic expansion to be employed at the receiver. The magnitude of the delay bias applied to rectifier I predetermines the extent of the linear rise in Fig. 2, and the horizontal section length in Fig. 3.

Considering, now, the receiving system shown in Fig. 4 the collector I2 feeds the collected FM energy is fed to the FM detector I4 to provide the modulation signals applied at I tothe transmitter.

Each of the tuned selector circuits up to the detector I4 need have a pass band of only 50 kc. This is due to the fact that the oscillator frequency at the transmitter has been deviated up to a maximum of 25 kc. to each side of center frequency. Hence, above the network I3 there has been depicted a response curve representing the pass band of 50 kc., as well as the fact that the midband frequency is Fc. The latter will'have a value depending upon whether it is in the ultrahigh frequency range, or whether it is of intermediate frequency, for example 4.3 mc. The detected FM Wave energy is passed through succesing to my invention, the frequency deviation is sive audio amplifiers I5 and I6 which may be of conventional construction. Any desired type of reproducer may be employed following the last audio amplifier I6. The FM detector may be of any desired construction. For example, the type of detector shown by S. W. Seeley in U. S. Patent 2,121,103, granted June 21, 1938, may be used.

In order to properly reproduce the audio signals applied from source I, the dynamic range of the receiving system is expanded. Since there has been embodied in the radiated FM energy a component whose super-audible frequency varies from 20 to 21 kc. depending upon the audio signal amplitude, there is provided a simple and l put terminals of the filter I1.

point in the demodulated signal network the component corresponding to the control tone. The numeral I'I represents such a control tone filter capable of passing energy of 20 kc. and higher. That is, filter I I is a high-pass filter having a cut-oil below 20 kc.

The control tone energy is transmitted to a rectiiier I8, of the diode type, whose anode is connected to ground by a properly bypassed load resistor I9. T'he cathode of the diode is connected back to ground by the secondary winding 20 of discriminator-transformer 2|. The primary of transformer 2| is denoted as 22:4 the transformer primary is connected across the out- The transformer 2l is resonant to a mid-frequency of 19 kc. Fig. 6 shows the resonance curve of the transformer. It will be seen from the resonance curve that maximum control energy is applied toithe rectifier I8 when the filter output is 20 kc. As the filter output energy rises in frequency to 21 kc.,l

the input to the rectifier I8 decreases.

When the control tone frequency is at 20 kc., which is the value corresponding to the horizontal section of the curve of Fig. 3, there will be supplied maximum energy to rectifier I8. As a result the drop across resistor I9 will be a maximum, and there will be supplied a large negative bias to audio amplifier I6. The negative bias across resistor I9 would be supplied over lead 30 through the lter network 3|. The bias may be supplied to a control grid of the audio amplifier tube. As a result, and as shown in Fig. 5, the receiver audio amplifier gain will not rise above the level corresponding to the lower end of the curve in Fig. 5.l However, should the control tone frequency rise towards 21 kc., as it will for audio amplitude above the horizontal section of the Fig. 3 curve, then the input energy to the rectifier I8 will decrease. This causes the direct current voltage'across the resistor I9 to decrease. Hence, the gainof the controlled audio amplifier I6 will rise, as indicated in Fig. 5.

It will now be seen that the receiver output energy level is expanded in accordance with the increase in frequency of a super-audible control component in the received modulated wave energy. Furthermore, the frequency increase of the control component is dependent upon the modulation amplitude value above a predetermlned level. In this way the transmitter frequency may be deviated over a, smaller range than is usual, while the benefit of wide deviation systems is secured. It will be understood that the super-audible tone can be added to channels of the present width, if desired, of 200 kc. It is desirable to keep the control tone at low amplitude relative to the audio voltage.

While I have indicated and described several systems for carrying my invention into effect, it will be apparent to one skilled in the art that my invention is by no means limited to the particular organizations shown and described, but that many modifications may be Imade without departing from the scope of my invention, as set forth in the appended claims.

What I claim is:

1. A method of signalling which includes gen'- erating oscillations, varying the frequency of the oscillations in accordance with the amplitude of modulation signals, preventing variation of said frequency in response to modulation signals above a predetermined modulation amplitude, concurrently producing a control tone whose frequency is varied in an increasing sense for modulation signal amplitudes above said predetermined `amplitude, varying the frequency-variable oscillations with said control tone, and transmitting the resultant frequency-variable oscillations.

2. A method of signalling which includes gen` erating oscillations, varying the frequency oi' the oscillations in accordance with the amplitude of modulation signals, preventing variation of said frequency in response to modulation signals above a predetermined modulation amplitude, concurrently producing a control tone whose frequency is varied in an increasing sense for modulation signal amplitudes above said predetermined amplitude, varying the frequency-variable oscillations with said control tone, ltransmitting the resultant frequency-variable oscillations, detecting the transmitted oscillations, amplifying the detected energy, deriving from the detected energy said control tone energy, and controlling the said amplification in an increasing sense with increase of control tone energy.

3. A method of receiving frequency modulated waves whose mean frequency has a maximum deviation range corresponding to a predetermined modulation signal amplitude, and which waves include as a component a super-audible control tone whose frequency is a function of modulation amplitude above said predetermined amplitude; said method including demodulating the received waves to produce modulation signal voltage and also voltage of said super-audible control tone, and controlling utilization efciency of the modulation signal voltage in accordance with the frequency of the control tone.

4. A system of frequency modulation communication wherein the following steps are effected:

modulating a carrier frequency by varying the value of the latter in accordance with modulation signal amplitude, limiting the frequency varlation to correspond to a predetermined amplitude value of signal, concurrently varying said carrier frequency in accordance with a. superaudible tone and in response to modulation thereonby signal amplitudes above said predetermined value, radiating the modulated carrier energy, collecting the radiated energy at a receiver,

demodulating the received energy to produce voltage of the original modulation signals. rectifying the component of the demodulated energy corresponding to said super-audible tone, amplifying the demodulated signal energy, and ex-` panding the amplification of the latter energy in accordance with the rectified super-audible tone energy.

5. A system .of 'frequency modulation communication wherein the following steps are effected: modulating a carrier frequency by varying the value of the latter in accordance with modulation signal amplitude, limiting the frequency variation to correspond to a'. predetermined amplitude value of signal, concurrently varying said carrier frequency in accordance with a super-audible tone and in response to modulation thereon by signal amplitudes above said predetermined value, radiating the modulated carrier energy, collecting the radiated energy at a receiver, demodulating the received energy to produce voltage of the original modulation signals, rectifying the component of the demodulated energy corresponding to said super-audible tone, amplifying the demodulated signal energy. and expanding the amplification of the latter energy in accordance with the rectified superaudible tone energy.

6. A system of communication with angular velocity-modulated carrier waves wherein the following steps are effected: modulating a carrier frequency by varying the value of the latter in accordance with modulation signal amplitude, I

y ponent of the demodulated energy in accordance with the rectified super-audible tone energy.`

7. A system oflradio communication wherein the following steps are eected: frequency modulatlng a carrier frequency by varying the value of the latter in accordance with modulation signal ampiitude, concurrently varying said carrier frequency in accordance with a control tone and in response to modulation thereon by signal ainplitudes above a predetermined value, transmittingthe modulated carrier energy, demodulating the transmitted energy to produce voltage of the original modulation signals, rectifying the component of the demodulated energy corresponding to said'control tone. and controlling the utilization of the demodulated energy in accordance with' the rectified tone energy.

8. A method of reception wherein waves are received whose mean frequency has a maximum deviation range corresponding to a predetermined modulation signal amplitude and which include as 'a modulation component a. superaudible control tone whose frequency is a function of modulation signal amplitude above said predetermined amplitude; said method including demodulating the received waves to produce modulation signal voltage and energy of said super-audible control tone, and controlling amplification cf the modulation signal voltage in accordance with the frequency of the control tone.

9. A system of radio communication wherein the following steps are effected: varying a carrier frequency in accordance with modulation signal amplitude, limiting the carrier frequency variation to correspond to a predetermined amplitude value of signal, concurrently varying said carrier frequency in accordance with a control tone and in response to modulation thereon by signal Varnplitudes above said predetermined value, transmitting the variable-frequency energy, demodulating the transmitted energy to producevoltage of the original modulation signals, rectifying the component` of the demodulated energy corresponding to said control tone, amplifying the de-, modulated signal energy, and expanding the amplification of the latter energy in accordance with the rectiiled tone energy.

10. A system of frequency modulation communication wherein lthe following steps are effected: modulating a carrier frequency by varying the value of the latter in accordance with audio modulation signal amplitude. limiting the frequency variation fromcenter frequency to about 25 kilocycles, preventing frequency variation in response to modulation signal Yamplitude above a predetermined value, concurrently varying said carried frequency over a range of 20 to 21 kilocycles in response to modulation thereon by signal amplitudes above said predetermined value, radiating the resultant modulated carrier energy, collecting the radiated energy at a receiver, demodulating theV received energy to produce voltage of the original modulation signals, rectifying the component of the demodulated energy corresponding to said super-audible variation, amplifying the demodulated signal energy, and expanding the amplification of the latter energy in accordance with the rectified super-audible energy. v

11. A method of .receiving angular velocitymodulated carrier waves whichare deviated over a relatively narrow range by modulation signals and which include as a component a superaudible control tone whose frequency is a direct function of modulation signal amplitude above a predetermined value, the method including detecting the modulated waves to derive modulation signal voltage, amplifying the modulation voltage, deriving from the detected voltage a control tone corresponding to said component, rectifying the latter control tone energy, and controlling the amplification of the modulation signal voltage in direct accordance w'ith'the rectied-con trol tone energy, thereby to expand the dynamic range and simulate the action of a receiver of wide pass band characteristics.

12. A method of communicating with frequency modulated carrier waves wherein the benefits of high deviation modulation are secured While utilizing a receiver having selector circuits of relatively narrow pass band prior to the demodulator; said method including transmitting said waves with a mean frequency which has a maximum deviation range corresponding to a predetermined modulation signal amplitude and which includes as a modulation component a super-audible control tone whose frequency is a direct function of modulation signal amplitude above said predetermined amplitude, demodulating the waves at'the receiver to produce modulation signal voltage and energy of said superaudible control tone, and controlling the amplication of the modulation signal voltage in direct accordance with the frequency of the control tone.

WINFIELD R. KOCH.

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