US2264608A

US2264608A - Means and method for relaying frequency modulated signals

Info

Publication number: US2264608A
Application number: US313496A
Authority: US
Inventors: Edwin H Armstrong
Original assignee: Individual
Current assignee: Individual
Priority date: 1940-01-12
Filing date: 1940-01-12
Publication date: 1941-12-02
Anticipated expiration: 1958-12-02
Also published as: BE473156A; FR946307A; FR946096A; GB623219A; GB623221A; NL67372C; NL68041C; BE472677A

Description

Patented Dec. 2, 1941' MEANS METHOD FOR RELAYING FRE- QUENCY MODULATED SIGNALS Edwin H. Armstrong, New York, N. Y. Application January 12, 1s4o,-seriamo. 313,496

10 Claims.

wide band frequency modulation system described in my U. S. Patent #1,941,069 which have been assigned frequencies in the range of 42.5 to 43.5 megacycles. The various channels assigned are 200 k. c. in width, and hence the frequency difference between the frequency of the signal which is to be received and the frequency which is to,

be retransmitted may be very small, requiring therefore highly selective receivers. In addition to the high selectivity of the receiver, it is usually necessary to adopt the further precaution of placing the receiver at some distance from the transmitting antenna to prevent various forms of interference from the very high signal level encountered in the immediate field of the transmitter.

There are two types of relay transmitting systems. In one the received signal is detected and converted into its original form, and this current is used to modulate the second transmitter. In the other the received radio frequency current is heterodyned to the frequency to be transmitted, amplified up and transmitted without ever being converted into the audio or other signaling current. In each type of relaying certain difliculties are encountered. In the first type reception would be carried out with a superheterodyne which can be readily designed to give the necessary selectivity to prevent interference from the transmitter, but which suffers from the fact that in the detection at the receiver and the remodulation of the local transmitter a certain amount of distortion is encountered. This. while hardly detectable in a single relay, eventually places a limit on the number of retransmissions which could be undertaken. In the second type reception would be accomplished without converting to audio, and the frequency to be transmitted would be produced by adding to, or subtracting from,

the incoming frequency a locally produced current which might be of the order of 400 k. c. The desired frequency component would be selected out and transmitted and in this way the distortion incident to detection an modulation would be avoided. The circuits required to select 65 the desired frequency for retransmission and the circuits required to protect the receiver from the effects of the transmitter would of necessity have to be extremely selective and consequently critical to adjust and maintain.

The present invention has for its object the provision of means whereby the difficulties of both types are overcome and the advantage of both retained. In accordance with the present method the incoming signal is heterodyned down to some relatively low intermediate frequency at which the desired selectivity may be obtained, converted to a frequency which differs from the initial intermediate frequency by an amount equal to the difference between the received frequency and the frequency to be radiated and heterodyned up by the same amount by which it was initially heterodyned down. In order that changes in the frequency of the heterodyning currents used in the conversion of the incoming frequency down to its initial intermediate frequency and in the conversion of the second intermediate frequency up to the outgoing frequency may not produce undesired drift therein, both of these heterodyning currents are obtained from the same oscillating source so that any drift in the frequency of one current occurs simultaneously in the other. The operation of the system may be more exactly understood from the following explanation.

Referring now to the figures which form a part of this specification, Figs. I and II represent the complete relay system in which the apparatus shown in Fig. I will usually be separated from the apparatus of Fig. II by a distance of some hundreds of feet, Fig. 11 being located at the transmitter. While this is not essential under all conditions, where weak signals are to be relayed it is desirable and the arrangement of apparatus required for separated locations is, therefore, depicted.

Referring now to the arrangement of Figs. I and H, it has been assumed that a frequency of 42.8 megacycles isto be received and retransmitted as 43.4 megacycles. I represents the receiving antenna connected through filter 2 to a balanced converter 3. This converter is supplied with a heterodyning current of 41.8 megacycles as shown. 4 represents a filter having a pass band sufilcientlywide to pass the .90lL- 1100 k. c. current produced by the interaction of the incoming 42.8 megacycles signal with a 100 k. c. deviation, and the fixed frequency of 41.8 megacycles. 5, 6, I and 8 represent selective amplifiers, 9 a limiter for the 1000 k. c. current, It! a filter, ii an amplifier, and 12 a second filter connecting to a line I3. It has been assumed that a distance of several hundred feet separated the arrangements of Figs. I and II. The line l3 which terminates at l8 in Fig. II should preferably be a concentric or balanced line. Referring now back to the heterodyning current of 41.8 megacycles, this is initially derived from a current supplied over a line II from an oscillator at the other location at 10.45 megaeycles through an amplifier l5, through a doubler it which raises it to 20.9 megacycles and a second doubler H which supplies 41.8 megacycles current to the amplifier l8.

Referring now to Fig. II, l9 represents the end of the line carrying the 1000 k. c. intermediate frequency current, 20 a band pass fliter, 2| an amplifier, and 22 a balanced converter. A local oscillator 23, preferably Xtal controlled, excites an amplifier 24 which supplies the converter 22. The output of the converter is supplied through a band pass filter 25 having a pass band of 1500-1700 k. c. to an amplifier 26 whose output is fed into a balanced converter 21. The heterodyning current for this converter is obtained from a 10.45 megacycle oscillator 28 through an amplifier 29. 34 represents a band pass filter of 200 k. 0. width and a midfrequency point of 12.05 megacycles. 35 represents an amplifier for this frequency, and 36 a second converter. This converter is supplied with a heterodyning current of 31.35 megacycles derived from the 10.45 megacycles source and raised to the 31.35 megacycles value by the tripler 32. 33 represents an amplifier for this frequency. 31 represents an amplifier for 43.4 megacycle output current of the

converter

35, 38 the power amplifier, and 39 the antenna. Referring back to the oscillator 28, an amplifier 30 is excited by current derived therefrom, and supplied to a line 3| which terminates at H in Fig. 1. From current delivered by this line the frequency multiplication system l-l8 produces the initial heterodyning current.

It will be observed from the foregoing explanation that if the frequency of the first station is maintained on its exact frequency that the second station must of necessity likewise be on its correct frequency, since the only element in the relay whose frequency stability in any way has a bearing on the stability of the frequency of the second station is the 600 k. c. oscillator. Since this is such a small fraction of the values which we are considering, any

variation which could normally occur in it is bound to be numerically insignificant in the final result. As previously explained, any drift in the frequency of the 10,450 k. c. oscillator is of no importance since it is self-cancelling.

I have described what I believe to be the best embodiments of my invention. I do not wish, however, to be confined to the embodiments shown, but what I desire to cover by letters Patent is set forth in the appended claims.

I claim:

1. The method of relaying frequency modulated signals ,on slightly different wave lengths which consists in receiving the wave of the first length, heterodyning the received wave down to a low intermediate frequencyby a heterodyning current which is a higher harmonic of a given low frequency oscillator, converting the low intermediate frequency to another frequency whose difference therefrom is equal to the frequency difference between the received and transmitted waves, raising the last-named frequency to the frequency to be transmitted by adding to it successively the frequency of the fundamental and an appropriate harmonic of said given low frequency heterodyning oscillator and transmitting currents of the resulting frequency.

2. The method of relaying frequency modulated signals on slightly different wave lengths which consists in receiving the wave of the first length, heterodyning the received wave down to a low intermediate frequency by a heterodyning current which is a harmonic of a given low frequency oscillator, amplifying the currents of low intermediate frequency, converting the amplified currents of low intermediate frequency to an other frequency whose difference therefrom is equal to the frequency difference between the received and transmitted waves, raising the lastnamed frequency to the frequency to be transmitted by adding to said last named frequency energy controlled by said given low frequency heterodyning oscillator and transmitting currents of the resulting frequency.

3. The method of relaying frequency modulated signals on slightly different wave lengths which consists in receiving the wave of the first length, heterodyning the received wave down to a low intermediate frequency by a heterodyning current which is a harmonic of a low frequency oscillator generating a substantially constant frequency, amplifying the currents of low intermediate frequency, converting the amplified currents of low intermediate frequency to a second low frequency whose difference therefrom is equal to the frequency difference between the received and transmitted waves, filtering and amplifying the currents of the second low frequency, converting the amplified currents of the second low frequency to the frequency to be transmitted by adding to said last named frequency energy controlled by said low frequency heterodyning oscillator and transmitting currents of the resulting frequency.

4. Means for relaying frequency modulated signais on slightly different wave lengths comprising, in combination, a converting device, means for energizing said converting device by the received wave of the first length, a first low frequency oscillator, frequency multiplying means coupled to said oscillator for generating a harmonic of the oscillator frequency, means conmeeting said frequency multiplying means to the converting device whereby currents are generated therein of a low intermediate frequency, a second converting device having its input connected to the output of the first converting device, a second low frequency oscillator connected to the input of the second converting device and arranged to generate a frequency equal to the frequency difference between the received and transmitted waves, means coupled to the output of the second converting device for increasing the frequency of the currents therein to the frequency to be transmitted, said coupled means comprising means for generating a harmonic frequency of the first low frequency oscillator, and means for transmitting currents of the resulting frequency.

5. Means for relaying frequency modulated signals on slightly different wave lengths comprising, in combination, a converting device, means for energizing said converting device by the received wave of the first length, a first oscillator. frequency multiplying means coupled to said oscillator for generating a harmonic of the oscillator frequency, means connecting said frequency multiplying means to the converting device whereby currents are generated therein of a low intermediate frequency, a second converting device having its input connected to the output of the first converting device, a low frequency oscillator connected to the input of the second converting device and arranged to generate a frequency equal to the frequency difference between the received and transmitted waves, a third converting device connected to the second conlength, generating a fundamental frequency and a higher harmonic of such fundamental frequency, heterodyning the received wave down to a first intermediate frequency by a heterodyningcurrent of said higher harmonic frequency, generating a radio frequencywhich is equal to the frequency diflerence between the received and transmitted waves, increasing said first intermediate frequency to a substantially higher inverting device, means for applying to the input of the third converting device a voltage component having the fundamental frequency of the first oscillator and also a voltage component which is a harmonic' of the fundamental frequency, amplifying; means connected to the output of the third-converting device, and means for transmitting the amplified currents.

- 6. The method-of relaying frequency modulated radio signals on slightly different wave lengths which consists in receiving the wave of the first'length, heterodyning the received wave down to a low intermediate frequency by a heterodyning'current which is a harmonic of'a given lowfrequency oscillator, ,raising the lowintermediate frequency to the frequency to be transmitted by adding thereto a plurality of frequencies, one frequency of said plurality being ,equal to thedifference between the frequencies of the received and transmitted waves and the remaining frequencies of said plurality being controlled by the said low frequency oscillator. A

'7. Means for relaying frequency modulated radio signals on slightly different wave lengths comprising, in combination, means for receiv ing the wave on the first length, a lowfrequency oscillator, means for generating harmonic fretermediate frequency by adding thereto said fundamental and said radio frequencies, increasing said higher intermediate frequency to the frequency to be transmitted by adding thereto at least a higher harmonic of said fundamental frequen y, and transmitting currents of the resulting frequency.

9. A system for retransmitting a frequency modulated wave at a frequency which difiers slightly from the frequency at which it comes in, said system comprising means for heterodyning the incoming frequency down to a desired low intermediate frequency, means for heterodyning the low'intermediate frequency current upto the desired frequency of retransmission, a

' quency which differs slightly from the frequency quencies of said oscillator, means connected to said receiving means and to said harmonic generating means for heterodyning the received wave down to a low intermediate frequency, means for generating a frequency which is equal to the difference in frequencies between the received and transmitted waves and frequency converting means connected to the output of said last named means, to the output of said heterodyning means and to an output of said harmonic generating means for raising the low intermediate frequency to the frequency to be transmitted.

8. The method of relaying frequency modulated signals on slightly different wave len ths which consists in receiving the wave of the first at which it comes in, which consists in generating a heterodyning frequency, generatinga difference frequency equal to the difference be- "tween the frequencies of the received and transmitted waves, heterodyning the incoming frequency by said heterodyning frequency down to a desired low intermediate frequency, heterodyning the low intermediate frequency current up to the desired transmitting frequency by means of a plurality of frequencies whose algebraic sum is equal to the aggregate of said heterodyning and said differencefrequencies, and eliminating from the outgoing wave the effects of, amplitude variations in said incoming wave. I

EDWIN- H. ARMS'I'RQNG.