US2358382A

US2358382A - Frequency modulation signal system

Info

Publication number: US2358382A
Application number: US408867A
Authority: US
Inventors: Wendell L Carlson
Original assignee: RCA Corp
Current assignee: RCA Corp
Priority date: 1941-08-29
Filing date: 1941-08-29
Publication date: 1944-09-19
Anticipated expiration: 1961-09-19

Description

Filed Aug. 29,' 1941 2 sheets-sheet 1 Zmventor Cttomeg Sept 19, 1944. w. l.. CARLSON 2,358,382

FREQUENCY MODULATION SIGNAL SYSTEM l Filed Aug. 29, 1941 2 sheets-sheet z Mm/m/f Jaws.

'maentor 621ML bm Patented sept. 19, 1,944-

Y FREQUENCY MonULA'noN SIGNAL SYSTEM Wendell L. Carlson, Haddoneld, N. J., assignor to Radio Corporation of America, a.- corporation of Delaware Application August 29, 1941, Serial No. 408,867

3 Claims. (Cl. Z50- 6) This invention relates to frequency modulation signal systems, and has for its primary ob- `iect to provide an improved system of the character referred to, for wide frequency band operation at ultra high frequencies.

It is a further object of this invention, to provide an improved frequency modulation signal system which operates with compound modulation, including a sub-carrier frequency signal for frequency modulating a main carrier frequency signal, and a'second modulating signal, such as sound in the audio frequency band, for frequency modulating the sub-carrier frequency signal, thereby to provide an improved signal to noise ratio at the receiving end of the system.

It is an object of this invention, furthermore, to provide an improved multiple or compound frequency modulation signal system providing at least two modulating signals, one of which is a sub-carrier frequency signal, and a simplified and effective receiving system for said signals including a narrow .band i-f amplifier for the su-bcarrier frequency signals.

In a system embodying the invention, a subcarrier frequency signal is'frequencymodulated with an audio frequency or other modulating signal, and a main carrier frequency signal is frequency modulated with the modulated subcarrier frequency signal, and the deviation frequency swing for either or both of the modulation signals is made of the order of or greater than the modulation frequencies employed.

Further in accordance with the invention. a receiving system 'for multiple or compound frequency modulation may include conventional radio frequency, or ultra high frequency input circuits including a heterodyne converter and local oscillator, and wide band first intermediate frequency amplifier circuits, followed by a subcarrier detector with or without a preceding limiter.

The output circuit from the sub-carrier detector is coupled to a second or sub-carrier intermediate frequency amplifier of reduced band width and an audio frequency' detector which may or may not be preceded by a limiter. Both detectors are for frequency modulated signals.

The invention will be further understood from the following description, when cosidered in connection with the accompanying drawings, and its scope is pointed out in the appended claims.

In the drawings, Figure l is a block circuit diagram of a transmitter for producing multiple or compound frequency modulation signals for reception and selection in accordance with the invention;

Figure 2 is a similar schematic I-clock circuit diagram of a multiple frequency modulation signal receiving system arranged in accordance with the invention; and

Figures 3 to 6, inclusive, are graphic diagrams illustrating the operation of the system of Figs.

1 and 2 with different sub-carrier frequency signals and deviation frequency bands for the main and suit-carrier` frequency signals.

Referring to Fig. 1, a sub-carrier oscillator 51 which may operate at any suitable sub-carrier frequency, for example, between 25 and 100 kc., is coupled tov a rst frequency modulator 6 to which is applied a modulating signal from a source comprising a microphone l and an audio frequency amplier d, in the present example.`

The sub-carrier oscillations are modulated by sound from the microphone amplifiedv through the amplifier ii, and applied to the modulator 6 with any desired total frequency deviation or swing, for example, 'between 20 kc., and 150 kc. This may represent a relatively wide deviation frequency band orswing with respect to the frequency of the modulating signal, which may be in the normal audio frequency band between 50 and 10,000 cycles.

The frequency modulated sub-carrier signal is amplified in a suitable amplifier indicated at 9, and is applied to a second frequency modulator i@ along with a carrier signal which is derived from a main carrier oscillator, indicated at i i. The multiple frequency modulated signal is then appli-ed to suitable radiating means i2 after amplification in a power amplifier or other amplifying indicated at i3.

@The Afrequency modulation of the maincarrier signal by the sub-carrier may have a. deviation frequency band or swing of the ordery of or greater than the frequency of the sub-'carrier signal. This total deviation, excluding the modulation deviation on the sub-carrier, may be of the order of from 100 kc. to 600 kc., for example, as will hereinafter. be pointed out. Thus the deviation frequency swing for either or both of the modulating signals may lbe greater than the modulating frequency employed and greatly improves the signal-noise ratio at the receiving end of the ried out at ultra high frequencies of the'order of 100 mc. or higher, a wider pass band may be lutilized in the receiving system for the main carrier with much less critical tuning for best noise reduction. The line-up of the circuits is facilitated and is less critical, because the band width for the main carrier, and its modulation frequencies, is relatively large compared with that for the sub-carrier frequency, and its modulation frequencies.

'Ihe correct tuning of the signal to the subcarrier amplifier and its discriminator is a function of the transmitter sub-carrier frequency and the initial adjustments of the receiver sub-carrier circuits, and is independent of inaccuracies in the input tuning adjustment and the frequency drift of the local oscillator In the present example, it may be assumed that the main carrier or the transmitted signal is of the order of 500 mc., this being the center frequency.

At the receiver, one example of which is shown in Fig. 2, the ultra high frequency signals are collected by a suitable antenna, suchl as a dipole I5, and received in suitable conventional input or r-f circuits represented by an r-f ampliner I6, a heterodyne converter I1, and a local oscillator I8, coupled as schematically indicated, to produce the usual intermediate frequency signal for amplification in an intermediate frequency amplifier I9. Q

The amplifier I8 is of the wide band type having a pass band of from 200 to 800 kc., that is, so as to pass a frequency band equal to the transmitted frequency band.

The main carrier intermediate frequency amplifer is followed by a suitable discriminator and detector 20, with a limiter 2i interposed between the carrier i--f amplifier and discriminator and detector, if desired.

The detector 2li is followed by an amplifier 22 responsive to the sub-carrier intermediate frequency and having a relatively narrow frequency band or pass band characteristic with respect to that of the carrier i-f amplier, and in any case under the present assumed values, this band may be of the order from 30 to 200 kc., with a mean or center frequency for the sub-carrier of from to 100 kc.

The remainder of the receiving system may be conventional in that it may include a second discriminator and audio frequency detector 23 preceded by. a limiter 24 and followed by a suitable audio frequency amplier 25 and output device such as a loudspeaker 26 for reproducing the initial modulation frequencies or signal.

Referring to Figs. 3 to 6, along with Figs. 1 and 2, in the frequency arrangement illustrated in Fig. 3, the pass band of the first or carrier intermediate frequency amplier is of the order of 200 kc. and the main carrier is frequency modulated by the sub-carrier frequency with a maximum swing or frequency deviation of 50 kc. on either side of the mean or center frequency of quency amplifier provides a pass band of the order of 200 kc.

In the example shown in Fig. 5, the main carrier is widely modulated with a maximum frequency deviation or swing of the order of 600 kc., by the sub-carrier, while the sub-carrier of 100 kc. is frequency modulated in the same manner as for Fig. 4 with a maximum frequency swing of 150 kc. in response to the audio frequency modulation, and the guard band is the same. The total frequency band, including the modulation and guard bands is 800 kc. in that case.

It is preferable that one of the deviation frequencies exceeds while the other is less than the modulating frequency as in the example of Fig. 4. For exampleywith the same wide swing frequency modulation of the main carrier as in the example of Fig. 5. the sub-carrier frequency may be reduced to 25 kc., and it may be frequency modulated by thev audio frequency signal within the narrow range of 20 kc. and with a pass band of only 30 kc. for the secondi-f amplifier, as indicated in Fig. 6. With this arrangement, the first intermediate frequency amplifier may have a pass band of the order of '700 kc. with wide range modulation of the main carrier and with a high degree of selectivity for the narrow band sub-carrier i-f amplier, resulting in an improved signal-to-noise ratio.

'I'he different frequency combinations illustrated in the graphs comprising Figs. 3 to 6 are only given by way of example of the various combinations which may be employed for satisfactory results in a compound or multiple frequency modulation system embodying the invention.

It has been found that with this system, signal to noise ratios of a relatively high order are obtained which are superior to conventional double amplitude or amplitude and frequency modulation systems. 'I'his is true under various conditions of operation, including response on weak impulse noise or strong impulse noise. The line-up tuning of the sub-carrier and second discriminator circuits are not critical. With an increase in band width, the signal to noise ratio imooY 500 mc., while the sub-carrier frequency of 50e kc. is frequency modulated by audio frequency signals with a maximum frequency deviation or swing of 25 kc. on either side of its center frequency.

A guard band of 25 kc. is provided on either side of the main frequency modulated signal, thus making the pass band of the first or main carrier intermediate frequency amplifier of the order of 200 kc. as above noted, and the pass band of the sub-carrier or second intermediate frequency amplifier of the order of 100 kc.

- In the example shown in Fig. 4, the main carrier is frequency modulated by the sub-carrier frequency with a maximum frequency deviation proves more rapidly than in direct proportion to the band width and with wide band reception of vthe main carrier and narrow band reception of the sub-carrier, the tuning is not as critical for maximum noise reduction as in conventional frequencyA modulation systems.

I `claim as my invention:

1. The method of communicating by a radio signal which comprises producing a frequencymodulated wave, the frequency of which is above one hundred megacycles for wide band modulation, producing a frequency-modulated sub-carrier highfrequency signal, frequency modulating the main carrier with said sub-carrier frequency signal, the deviation frequency swing of said frequency modulated wave exceeding the center frequency of the frequency modulated subcarrier signal, and the deviation frequency swing of the sub-carrier frequency signal being less than the maximum modulating frequency, amplifying and demodulatingvthe main carrier wave assassin l l 3 tially equal to the transmitted frequency band, a

second intermediate frequency amplifier connected with said rst intermediate frequency amplifier responsive to the sub-carrier intermediate frequency and having a pass band of less than twice the highest initial modulating frequency, a frequency discriminator and detector interposed between said intermediate frequency amplifiers for deriving the frequency-modulated sub-carrier signal, and a second frequency discriminator and detector coupled to said second intermediate frequency ampliner for deriving the modulation 3. An ultra high multiple frequency-modulation signal system comprising in combination,l

means providing a modulating signal, means providing a high frequency sub-carrier modulating signal, means providing an ultra high frequency main carrier wave, `means for frequency modulating the main carrier wave with the sub-carrier signal providing a deviation frequency swing at least vof the order of 10:1 with respect to the sub-carrier center frequency, and means for frequency modulating the sub-carrier frequency sig-l' nal with said modulating signal with a deviationfrequency swing substantially of the order of the highest frequency of the first mentioned modulating signal.

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