US2545197A - Relay satellite broadcast system - Google Patents

Description

' Patented Mar. 13, 1 951 RELAY SATELLITE BROADCAST SYSTEM Woodrow Darling, Merchantville, N. .L, assignor to Radio Corporation of America, a corporation of Delaware Application March 27, 1947, Serial No. 737,608

5 Claims. (Cl. 250-6) This application discloses a broadcast system and has as its main object provision of a simplified satellite broadcast system wherein the main transmitter station supplies synchronized energy to the auxiliary station which covers an area not well serviced by the main station.

In the prior art of satellite systems, substantially completely separate transmitters are supplied in the main and booster stations, the only common part of the system being the modulation source which supplies modulation for the carrier wave at the main transmitter and modulating currents for'transmission by line to the booster station for modulation of the satellite transmitter. In these systems, considerable interference is produced in any overlapping areas serviced because of phase delays or displacements between the modulated energies. Also, noise is admitted during carrier beat nulls in a small region where the field intensities are approximately equal.

A more detailed object of my invention is to provide essentially a single transmitter system which produces the signal modulated carrier to be broadcast at the main station and also supplies a carrier locked in fixed frequency relation therewith for relaying to the secondary, booster or satellite station. Then the interference by phase displacement is reduced in areas serviced by both systems.

Where broadcasting of signals is to be carried out, the FCC assigns the main broadcasting transmitter frequency and also a band of frequencies wherein the relaying is to take place. Changes may be permitted in both the main transmitter channel frequencies and the relay frequencies and a further object of my invention is to provide a simplified satellite broadcast system wherein the broadcast frequency may be changed readily and wherein the frequency at which relaying may be carried out is changeable in small steps to suit the conditions at hand.

These objects are attained in accordance with my present invention by using the exciter of an existing station, or providing an exciter at a new station, to supply excitation to a mixer. An additional source of oscillation is also provided and supplies oscillatory energy to the mixer. A side band-is selected from the mixer and used to excite the local broadcast transmitter which may be in existence or provided.

A side band is also selected from the mixer and upplied after multiplication to a relay radiating antenna after being modulated to a small extent by oscillatory energy from the said oscillator of .fixed frequency. At the secondary station these modulations are recovered after the high frequency relayed wave has been amplified and divided if desired and are supplied to a mixer alo g with the modulated wave after an appropria e reduction in frequency to derive from the mixer output, a side band which is of the same frequency as the energy broadcast from the main station. The two broadcast waves are thus locked in synchronism by the same fixed frequency source and the distortion discussed above is prevented or reduced since there can be no phase displacement between the audio envelopes in the overlapping areas serviced. The synchronism also prevents cancellation of carriers, modulated or unmodulated, and admission of noise or interfering signals during periods of cancellation. If the listeners antenna happens to be at a voltage minimum, a small movement of M4 would place his antenna in a region of maximum signal.

There has been a trend in the relaying art to higher and higher frequencies. Working at higher frequencies creates problems in tube efliciency and circuit design which makes use of the higher frequencies very diflicult.

A further object of the present invention is to provide a simplified satellite system wherein relaying takes place at frequencies low enough to be efficiently handled by tubes of the lighthouse type coupled by conventional circuits to an antenna of the parabolic type, which operates efficiently at these frequencies.

In describing my invention in detail, reference will be made to the attached drawings where Fig. 1 illustrates an embodiment of my invention,

while Fig. 2 illustrates an amplifier which may be used in my relaying circuits.

Various frequencies may be used in the broadcast transmitter and in the relaying transmitter and I do not propose to limit operation of my system to any particular frequencies. However, to simplify the disclosure, frequencie which might be used have been given. Assume, for example, that the broadcast system is of the FM type operating at 88 megacycles and I wish to relay from the main station to the satellite station at 950 me. The exciter IQ of Fig. 1 may comp-rise a source of modulation, a source of oscillatory energy of carrier frequency and means for frequency modulating the same to derive a 91.5 mc. wave directly or after frequency multiplication. The modulator may be of the direct frequency modulation type such as disclosed in Crosby U. S. Patent 2,279,659, dated April 14, 1942, or may be of the type wherein phase modulation,

" by corrected audio frequency, is carried out as in Crosby U. S. Patent 2,104,318 dated January 4, 1938, with multiplication as desired to give the necessary phase and frequency swing. The exciter output after amplification, if desired, is supplied to a mixer 12 and it is mixed wth oscillatory energy of fixed frequency from a source M. The mixer l2 may be a simple demodulator and the source it may be conventional and may comprise a crystal oscillator operating at 3.5 me. or

at lower frequencies in which case, appropriate 3 frequency multipliers are included in unit Hi. The lower side band at 88 mc., at theoutputof the mixer 12, isselected by tuned circuits or other selecting means and supplied ,to a local broadcast transmitter system l6 operating at'88mc. The transmitter has the necessary amplifier stages. The upper side band is selected from the mixer l2 and is supplied to amplifying and frequency multiplying stages in units l8 and 20 and to an amplitude modulated wave energy relay in one of these units, say unit 20. In the example given, it is assumed that the multipliers in relays l3 andifi have a multiplication factor of 10 so that a 950 me. wavemodulated in frequency is transmitted to the satellite or'booster staion. In

l8 or20, the relayed Wave is also amplitude modulated by oscillations from the source M, in the example given, of 3.5 me. be of small percentage, for example, 10% to The multipliersheremay be conventional as may be the amplifiers and the amplitude modulator i9. The relayed oscillatory energy is picked up and supplied to a divider system'including units 22 and E i. Some of-the divided energy is selected from the divider orone thereof and supplied to a detector 20 wherein the amplitude modulations of 3.5 me. are recovered and supplied to a frequency multiplier in unit 28. Amplification may be carried out as desired in units 22 and/or 24 and/or 28. The dividers22 and 24 in the example given will divide by the factor it which is the multiplication factorof the units 58 and/or 20 at the broadcast station. The 95 mo. wave at the output of divider 24 is selected out by selector circuits of the tuned resonant type and supplied to a mixer which'is also excited by oscillatory carrier energy multiplied in unit 28 so that it is of a frequency of 7 me. The difference frequency is selected at the output of mixer 30 and fed to the secondary broadcast transmitter 88 wherein it is amplified as desired for radiation.

If the system is considered, it .will beseen'that the carrier wave transmitted at the satellite or booster transmitter 32 is locked in phase with the energytransmitted at the main transmitter station 16. Moreover, the frequency of excitation of the broadcasting station may be changed by changing the frequency of the 'eXciter oscillator l0. Then the relayed frequency will be changed unless the frequency of the source'in M is appropriately changed. By making appropriate change in the frequency of source M, the frequency at which relaying takes place need not be changed. However, in many cases, it is desired to change the relay frequency. Then'the source l4 may be changed and if the transmitter frequency is to be fixed, a compensating change is made in the frequency of the source [0. The manner in which the frequencies at which relaying and broadcasting takes place may be changed, will be obvious from my description and drawing. For example, if the exciter source in H] has an output of frequency F0, then the broadcast transmitter frequency vF'I would be the differ- -..ence between F0 and the :output frequency 'Fc This modulation may v of source [4, while the input to the multiplier in 18 would be equal to Fo-l-Fc and the relay frequency FR. would be equal N (Fo-i-Fc) and so forth.

Since the relayin system makes use of frequencies below the ultra high frequency band, good efficiency of operation may be obtained by the use of known tubes such as tubes of the lighthouse type and by somewhat conventional circuits already in use in the art. Moreover, radiation problems are simplified because a parabolic type of antenna worksvery 'well at frequencies of the order of 1000 me. per second. In this respect, note that my'system has a fundamental important advantagein the elimination of the need for frequency discriminators and other circuits of the like critical nature wherein tuning, symmetry and balance are diificult to attain, and wherein phase shift of the modulation frequencies may be caused due to circuit dissimilarities and so forth. Moreover, in my system, remodulaticn never takes placebecause the original modulated waveisused to excite the main transmitter and to provide the relayingenergy which excites the secondary transmitter.

As to the transmission, there is no audio frequency phase shift because the circuits used including the mixers, multipliers, dividers and so forth are sufficientlyibroad totreat all modulation frequencies uniformly. Thus, there is no relative attenuation of'the side bands and they retain'the original audio amplitude and phase in the booster transmitter and'in overlapping service areas. :Moreover, 'since the carriers of the main 'andbooster transmitter are synchronized, phase shift of the 'audio envelope is not caused bythetransmission and thus there is no audio'beat, due :to audio envelope displacement, in the receivers of :the system.

As stated above, amplifiers may be used throughout the system as desired and in Fig. 2,

I have shown an amplifier appropriate for use in orpreceding the dividers 22 to which is fed the relayed I signal -;at 'the satellite station. Such an amplifier may comprise a tuned circuit 40 feeding the high frequency energy to a crystal detector 42 of a simple type. This detector circuit is also supplied by oscillatory energy from a source 45 by way of transformer T. In the example given, the frequency of :source 46 might be 900 me. and the incoming wave-of the order of l000-mc. as pointed outabove. Then the intermediate frequency amplifier in 48 would select the difference frequency which is a 100 me. wave whichjcanbe amplified veryreadily. This Wave would then be supplied to a mixer'50 wherein it would bebeatwith a'900 mc. wave from source iii-to raise its frequency-back to the, frequency at which. itis relayed. The output of the mixer 50 would then be supplied'to the frequency divider 22.

What is claimed is:

1. In a satellite broadcast system, a main transmitter and a secondary transmitter at a distance therefrom operating at the frequency at which the -main transmitter operates, and means for broadcasting synchronizedsignals from said main and'secondary"transmitter including a source of frequency modulated carrier energy, a source of oscillatory energy of fixed frequency, means for mixing energies from :said'twosources and supplying sideiband energy derived from said mixing means to the main transmitter, means for amplitude modulatingxsideband energy derived from .said means by oscillatory energy of said fixed frequency derived directly from said fixed frequency source, and means for radiating energy derived from the amplitude modulated sideband energy to said secondary transmitter to supply the excitation therefor.

2. In a satellite broadcast system, in combination, a main transmitter, a secondary transmitter, a frequency modulated source of oscillatory energy and a source of oscillatory energy of fixed frequency, means for intermodulating energies from said sources and supplying the difference frequency to said main transmitter, means for supplying the sum frequency to a relay system, means for multiplying the frequency and modulating the amplitude of the energy being relayed in accordance with said oscillations of fixed frequency, means for transmitting the amplitude modulated relayed energy, means for receiving the same and dividing its frequency by a factor equal to the multiplication factor, means for detecting the received energy to derive the amplitude modulations in the form of oscillatory energy corresponding to said first mentioned oscillatory energy of fixed frequency, means for doubling the frequency of said derived modulations, means for mixing the same with said frequency divided wave energy, and means for applying the resulting side band energy to the secondary transmitter.

3. In a satellite broadcast system, in combination, a main transmitter, a secondary transmitter, a frequency modulated source of oscillatory energy for exciting said main transmitter, and means for exciting said secondary transmitter in synchronism therewith including means for producing frequency modulated energy of higher frequency which is synchronized at a multiple of the frequency of said first source, means for modulating the amplitude of the energy of higher frequency by oscillations of fixed frequency,

" means for sending the resultant energy to said secondary transmitter, means for reducing the frequency of the resultant energy and detecting the amplitude modulations corresponding to said oscillations of fixed frequency, means for doubling the frequency of said detected modulations, means for beating the same with the frequency reduced energy, and means for applying a beat note of the resulting energy to the secondary transmitter.

4. In a satellite broadcast system, a main broadcast transmitter, a secondary broadcast transmitter at a distance therefrom operating at the frequency at which the main transmitter operates an exciter, a source of frequency modulated oscillatory energy at the main transmitter, a source of oscillations of a fixed frequency differing from the frequency of the exciter by the broadcast frequency, a mixer excited by oscillations from said last-named source and said ex- 6 citer, means for selecting the lower side band produced in said mixer and supplying the same to said broadcast transmitter, means for selecting the upper side band from said mixer, multiplying its frequency and relaying the same by radiation, means for amplitude modulating the frequency multiplied relayed energy in accordance with oscillatory energy from said fixed frequency source, means at the secondary transmitter for dividing the frequency of the relayed energy and for deriving therefrom oscillatory energy proportional to said fixed frequency, means for mixing said last-named oscillatory energy with the signal modulated energy of divided frequency, 7 and means for selecting a side band of the broadcast frequency and impressing the same on said secondary broadcast transmitter.

5. In a satellite broadcast system, in combination, a main transmitter, a secondary transmitter at a distance from said main transmitter and means for operating said secondary transmitter in synchronism with the main transmitter comprising a source of frequency modulated carrier energy, a source of oscillatory energy of fixed frequency, means for mixing energies from said two sources and supplying side band energy derived from said mixing means to the main transmitter, means for amplitude modulating side band energy derived from said mixing means by oscillatory energy of said fixed frequency derived directly from said fixed frequency source, means for sending energy derived from said amplitude modulated side band energy to said secondary transmitter, means for detecting the same to abstract, from the sent derived energy, energy of said fixed frequency, thereby leaving frequency modulated energy, and means for using said abstracted fixed frequency energy and said frequency modulated energy to obtain excitation for the secondary transmitter.

WOODROW DARLING.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,928,794 Poole Oct. 3, 1933 2,005,795 Mathieu et al June 25, 1935 2,067,353 Snow Jan. 12, 1937 2,094,113 Affel Sept. 28, 1937 2,140,730 Batchelor Dec. 20, 1938 2,292,222 Haigis Aug. 4, 1942 2,344,813 Goldstine Mar. 21, 1944 2,369,268 Trevor Feb. 13, 1945 2,406,932 Tunick Sept. 3, 1946 2,421,727 Thompson June 3, 1947 2,458,124 Wilmotte Jan. 4, 1949 FOREIGN PATENTS Number Country Date 12,261 Australia May 1, 1933

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