US2468038A

US2468038A - Frequency modulation stabilization system

Info

Publication number: US2468038A
Application number: US735961A
Authority: US
Inventors: Clavier Andre Gabriel
Original assignee: International Standard Electric Corp
Current assignee: International Standard Electric Corp
Priority date: 1947-03-20
Filing date: 1947-03-20
Publication date: 1949-04-26
Anticipated expiration: 1966-04-26
Also published as: GB638336A; BE482222A; ES182282A1; FR963218A

Description

April 26, 1949. A. G. CLAVIER FREQUENCY MODULATION STABILIZATION SYSTEM Filed March 20, 1947 2 Sheets-Sheet 1 6cm 6 2 a a A WM WM 0 0 6? B5 6 W r a w w TED FREQ.

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April 1949- A. G. CLAVIER I 7 2,468,038

FREQUENCY MODULATION STABILIZATION SYSTEM Filed March 20, 1947 2 Sheets-Shet 2 INVENTOR. ANDRE G. (ZAV/fk Patented Apr. 26, 1949 UNITED STATS TENT OFFICE FREQUENCY MQDULATION STABILIZATION SYSTEM Application March 20, 1947, Serial No. 735,961

i Glairns.

This invention relates to the frequency stabilization of systems of the type in which pulses, which may be modulated in a given characteristic in accordance with the instantaneous amplitudes of a signal to be transmitted, are used to frequency modulate the carrier wave.

Various systems, single and multichannel, have been proposed, as for example, in my copending application A. Clavier, Serial No. 735,960, filed March 20, L947, for Frequency modulation com" munication system, and in which pulses, modulated in given characteristic, are used to frequency modulate a carrier wave.

An object of the present invention is the provision of means for frequency stabilizing systems of the type to which this application refers.

The above-mentioned and other features and objects of this invention will become more apparent and theinvention itself, though not necessarily defined by said features and objects, will be best understood by reference to the following description of the invention, taken in connection with the accompanying drawings, in which:

Fig. 1 is a block diagram of a multichannel transmission system utilizing both time division and frequency division multiplexing, and particularly illustrated as used for cable transmission;

Fig. 2 is a block diagram of a single channel transmission system utilizing intermittently frequency modulated waves; and

Fig. 3' is a set of curves used in describing the operation of the foregoing systems.

The transmission system of Fig. 1 is adapted to handle a relatively large number of different channels. The channels are treated in separate groups, each group being distinguished from the other by being transmitted over a different portion of the entire frequency band employed. These groups will be referred to herein as group or frequency channels. Within each group of frequency channel, multiplexing is accomplished by time division, that is, each group handles a number of signals which are mixed, utilizing the principle of time division, and are then trans-- mitted within the unique portion of the fre quency band belonging to said group.

The foregoing will be best understood with reference to Fig. 1 in which only a limited number of groups and a limited number of channels are illustrated for the easier understanding of the operation of the system. A plurality of signo.1 sQurc s: H8. are. d vided i to and tr d. as h eegrol p Ql1cws;-sources let form roup 2 icrmsrounlil; and i 18 for roupv These signal sources are used to modulate pulses which may be derived from a common pulse generator l9, feeding into a pulse distributor 20. The pulse distributor 20 feeds the pulses sequentially to lines 2l-36. The pulses fed on each of the lines 2I-26 are modulated in each of the groups by the signals from a separate one of the sources. For example, the pulses fed along line 2! are modulated by the signals from source I, 1, I3. Those pulses fed along line 22 are modulated by the signals from sources 2, 8, I l. The pulses along the other lines are correspondingly modulated.

In the specification and the accompanying claims, the terms modulated or modulation with respect to pulses or pulse trains are used broadly to signify the variation of a characteristic of such pulses or pulse trains in accordance With the instantaneous Values of the signal, and are not limited to any specific techniques or apparatus for producing this result. In this connection, the terms amplitude modulated or amplitude modulation signify such variations with respect to amplitude.

Since the groups A, B and C are similar in operation and in arrangement only group A will be described. Referring now specifically to group A, amplitude modulation of the pulses is accomplished in modulators 21-32, the pulses from lines Zip-26. feeding modulators 21-32 respectively and the signal sources l-B respectively, also respectively feeding the modulators 27-32. These modulators may be any of various wellknown types and may for example, consist of normally blocked amplifiers Which are unblocked by the pulses. The outputs from modulators 21-32 Which are in the form of trains of-amplitude modulated pulses are fed into the A mixer 33 where the trains of pulses are interleaved. Similar arrangements of modulators in groups B and C are provided and the resultant pulse trains are mixed in the B mixer 34 and the- C mixer- 35. The multichannel amplitude modulated pulse outputs of mixers 33 to 35 are then fed to frequency modulators 36:38 respectively Where they are used to frequency modulate the carrier wave energy in

oscillators

39, 48 and 4|. The frequency bands covered by oscillators 39, 40 and ll differ somewhat so that said bands While adjacent, do not overlap each other, thereby providing for frequency division between'group. channels A, B and C as will be. more fully described hereinafter.

Since, the output. of the. transmission system a; of Fig. l. is; to be transmitted by means of cable,

:r it is preferred to beat down the output of oscillators 39-4! and for this purpose, use is made of an oscillator 42 of a slightly different frequency output than oscillators 39-4l. The oscillator 42 is preferably a highly stable oscillator and for this reason is preferably crystal-controlled. The outputs from oscillators 39-4! are fed into mixers 43-45 where they are mixed with the output of oscillator 42 to produce beat output frequencies which are applied from each of the mixers 43-45 to the same transmission medium, which may be a cable 46.

Designating the frequency of oscillator 42 as Fe, then the frequency of oscillator 39 is preferably Fo+AF1, that of oscillator 40 is Fn+AF2, and that of 4| is Fo-I-AFc. AF1, AF2 and AF3 represent three bands of frequencies preferably adjacent each other but not overlapping. The output of mixers 43-45 is AF1, AF2 and AF3 respectively. For the purpose of center frequency stabilization, each of the oscillators 39-4! is treated separately and in a similar manner. Accordingly only the stabilization of oscillator 39 will be described. In accordance with a feature of the present invention, stabilization is produced by selecting portions of the frequency modulated carrier waves which are not frequency modulated, that is, the portions between those at which the pulses have produced frequency modulation. For the purpose of selecting said portions, the output of oscillator 33 is fed to a pulse controlled amplifier 4 which is periodically blocked by pulses from generator l9 during the time when frequency modulated portions of the output of oscillator 39 occur. Between the time of occurrence of said frequency modulated portions, energy from the output of oscillator 39 is fed via the amplifier 41 to a stabilization mixer 48 in which such energy is mixed with energy from the stable oscillator 42. The resultant beat frequency is fed to an automatic frequency control system 49 which may be for example, in the form of a discriminator, any deviations from the desired beating frequencyresulting in a direct current voltage which is sent back to the corresponding modulator 36 for stabilization purposes. Similar equipment similarly operating is likewise provided in each of the group channels B and C.

An understanding of the operation of the foregoing system may be aided by an examination of the curves of Fig. 3 wherein W1, W2 and We represent a portion of the signals from the signal source l, 2, and 3 respectively. These produce at the output of modulators 21, 28 and 29 pulses P1, P2 and P3 whose amplitudes vary in accordance with the instantaneous value of the signals of their channels. The pulses P1, P2 and P3 etc. are interleaved in mixer 33 and are then used to frequency modulate the carrier C (Fig. 3). The carrier C has a given center frequency which is modulated by the pulses P1, P2 and P3 to produce frequency modulation at the spaced intervals A1, A2 and As, the extent of the frequency modulation varying in accordance with the amplitude of the pulses P1, P2 and P3. The modulated carrier Wave C is applied to the amplifier 41 which is periodically unblocked so that the portions A4 and A5 between the frequency modulated portions A1, A2 and A3 are selected and fed to the mixer 48 as indicated in curve CI. The wave Cl is then beaten with the carrier wave F0 from oscillator 42. The resultant beat frequency is then used to produce frequency stabilization in the automatic frequency control system 49. To facilitate synchronization of the receiver for a transmi's sion system of the type hereinabove described, the pulses of a given channel, for example channel I, may be provided with a distinctive characteristic, for example a distinctive duration, by suitable means, such as a pulse shaper, to enable selection of these pulses at the receiver, the selected pulses synchronizing distributors at the receiver for proper separation of the various pulse channels, the group channels being preferably first separated by band pass filters.

While I have described frequency stabilization in connection with a multichannel transmission system, it may also be used with single channel systems, for example, as illustrated in Fig. 2. Referring now to Fig. 2, signals from a source 50 are mixed with pulses from a pulse generator 5| in an amplitude modulator 52, which may be similar to those described in connection with Fig. 1, to produce a train of amplitude modulated pulses at its output. This train of pulses is then applied to a frequency modulator 53 where it is used to modulate the frequency of a carrier wave produced in an oscillator 54, the output of the oscillator 54 then being applied to some suitable transmission medium such as for example, an antenna 55. For frequency stabilization purposes, the output of oscillator 54 is likewise fed to a pulse controlled amplifier 56 similar to amplifier 41 of Fig. 1, to pass only those portions of the modulated carrier wave output of oscillator 54 which are not modulated. These selected portions of the output are thenapplied to an automatic frequency control system 51 to control, as indicated by line 58, the frequency of oscillator 54. The

I automatic frequency control system 51 may be any suitable system, and may include, for example, a reference oscillator of relatively stable characteristics such as for example, a crystalcontrolled oscillator, or simpler forms of automatic frequency control may be employed.

While I have described above the principles of my invention in connection with specific apparatus, and particular modifications thereof it is to be clearly understood that this description is made only by way of example and not as a limitation on the scope of my invention.

I claim:

1. A signal transmission system comprising a. carrier wave source, a modulator for frequency modulating said wave solely at spaced intervals thereof leaving unmodulated portions therebetween, means for selecting solely unmodulated portions of the frequency modulated wave after the wave has passed through said modulating means, and means responsive to the frequency of said selected portions for frequency stabilizing said source.

2. A system according to claim 1, further including means for producing a train of pulses, means for amplitude modulating said pulses in accordance with instantaneous values of the signal to be conveyed, and means for applying the amplitude modulated pulses to said frequency modulator to frequency modulate the carrier wave at spaced intervals thereof.

3. A system according to claim 2, wherein the selecting means includes a circuit for coupling the output of said frequency modulator to the frequency stabilizing means, and means controlled by the pulses from the pulse producing means for alternately blocking and unblocking said circuit.

l. A system according to claim 3, including means for normally blocking said circuit and means for applying said pulses to said circuit to 5 6 periodically unblock said circuit, whereby fre- NITE quency stabilization occurs only between the mod- U D STATES PATENTS ulated intervals of the wave. Number Name Date ANDRE G. CLAVIER. 2, 01,978 Bedford May 28, 1940 5 2,296;962 Tunick Sept. 29, 1942 REFERENCES CITED 2,377,326 Crosby June 5, 1945 The following references are of record in the file of this patent: