US2241933A - Utilization of broadcast waves for obtaining high frequency carrier - Google Patents

Description

May 13, 1941.

W. VAN B. ROBERTS UTILIZATION OF BROADCAST 2,241,933 WAVES FOR OBTAINING HIGH FREQUENCY CARRIER Filed Dec. 1, 1938 Ml/[Tl-SMGE AMPLIFIER TO REMOVE MOM/LA TION [4NDOPT/0M4L FREQUENCY MUL T/PL [ER AMPLIFIER AND SELECTOR FREQUENCY MUL T/PL IER AMPLIFIER AND ssLsqrolz ATTORNEY.

Patented May 13, 1941 UTILIZATION OF BROADGAST WAVES FOR OBTAINING HIGH FREQUENCY CARRIER Walter van to Radio tion of Delaware Application December 1, 1938,

2 Claims.

The present invention provides a method of and means for obtaining a number of constant frequencies for use as carrier frequencies in the transmission of messages, without the need for a local oscillator. The invention finds particular application in the field of amateur transmitters, but is not limited thereto since frequencies outside the amateur band can be obtained by a similar process.

In accordance with the invention, the necessity for a local oscillator for the generation of a constant frequency for transmitting purposes is dispensed with by utilizing waves from broadcasting stations which are received and combined to produce any one of a number of frequencies within the desired band. More specifically, the method of the invention involves selecting two broadcasting stations which have suitable known frequencies, receiving the waves, passing the same through sharply selective circuits, and then impressing the same on a mixer or frequency converter tube from which the sum or difference frequency (i. e., the beat frequency) is derived. By properly multiplying the frequency of one of the received waves prior to impressing the same on the mixer or frequency converter tube, it is possible to obtain a large number of constant frequencies for use as carrier frequencies in the transmission of messages. Each of these large numbers of frequencies is linearly related to the original frequencies of the broadcast station; in other Words, the beat frequency utilized as a carrier may be expressed as equal to nflifz where ii is one of the broadcast frequencies, is is the other, and n is the number of times frequency f1 has been multiplied.

A better understanding of the invention may be had by referring to the following description, which is accompanied by a drawing whose single figure illustrates suitable radio receiving and frequency converting apparatus for achieving the results of the invention.

In the drawing, there is shown an antenna 1 for receiving the broadcast waves and for impressing the same on suitable amplifier and selector circuits 2 and 3, shown here in conventional box form. Transformers 4 and 5 serve to couple the antenna I to these amplifier and selector circuits. Both circuits 2 and 3 are preferably designed to discriminate somewhat against the side band frequencies of the selected stations so that the modulation of the voltage impressed on the subsequent stages is reduced to less than 100% at all times. In the output of B. Roberts, Corporation of Princeton, N. J., assignor America, a corpora- Serial N0. 243,366

amplifier and selector circuit 2 there is provided a frequency multiplier 6, preferably a frequency doubler, which feeds voltage of the multiplied frequency to one input grid of a mixer or frequency converter vacuum tube 1. The other input grid of tube 1 is fed by voltage of a different frequency obtained from amplifier and selector circuit 3. These two voltages of different frequency beat, or heterodyne, within mixer tube 1 to produce a plate current having, in addition to the frequencies of the input voltages, the sum and difference frequencies. The output circuit 8 of the mixer or converter tube 1 is tuned to select only one beat frequency, e. g., the frequency equal to the sum of the input frequencies; and the voltage developed in the plate circuit is passed on to a multistage amplifier 9 at least the final stage of which operates with saturation excitation so as to remove substantially all modulation. The term limiter can be used to designate the final stage having such a saturation characteristic. The output frequency of multistage amplifier 9, which may be resonated in tuned circuit I0, is then a known constant frequency available for transmitting purposes. In some cases the amplifier or frequency multiplier (doubler) stages of the apparatus will of itself be sufficient to remove the modulation. In order to remove modulation completely, however, it is necessary that the selection of the two voltages of different frequency applied to converter tube 1 should be sufficiently sharp so that neither voltage ever falls to zero during the modulation cycle.

In the operation of the invention, let us assume that it is desired to obtain a known carrier frequency for transmission in the meter band which extends from 3500 to 4000 kilocycles. A broadcast station of about 1400 kilocycles will be selected in amplifier and selector circuit 2', amplified therein, and impressed upon a frequency doubler 6 so as to provide a frequency of 2800 kilocycles on one of the input grids of the mixer or converter tube I. This amplifier circuit 2 is preferably so selective as to discriminate against the side band frequencies of the selected station so that modulation of the voltage impressed upon the frequency doubler 6 is reduced to less than at all times. At the same time, the other amplifier and selector circuit 3, having similarly sharp selective properties, is operated to pick up any broadcast station in the range from 700 kilocycles to 1200 kilocycles, and to impress this selected voltage upon the other input grid of the converter 1. Since proximately 1400 kilocycles, it is by a frequency quadrupler.

the output 8 of the frequency converter tube 1 will contain a frequency equal to the sum of the two frequencies impressed upon the two input grids, it: will be obvious that any broadcast station within the range from 700 kilocycles to 1200 kilocycles selected by circuit 3 will produce in the plate circuit of the converter tube 1 a frequency lying between 3500 kilocycles and 4000 kilocycles; namely the sum of the doubled or multiplied frequency derived from 6 (in this case 2800 kilocycles) and the selected frequency obtained in the range from 700 kilocycles to 1200 kilocycles. Since the selected frequency in the output of converter tube 1 lying between 3500 kilocycles and 4000 kilocycles is in the 80 meter band, this frequency, after all modulation is completely removed by amplifier 9, can be employed either immediately or after frequency multiplication for use as a carrier frequency in a suitable amateur transmitter. While the adjustments hereinabove described are sufficient to obtain a number of frequencies within the range of 3500 kilocycl'es to 4000 kilocycles, it is also possible to obtain. frequencies in the lowest frequency amateur band, namely, from 1700 kilocycles to 2000 kilocycles by tuning the plate circuit of the converter tube 1 to accept the difference between the two frequencies impressed on its two grids. In this case, a range of broadcasting stations from 800 kilocycles to 1100 kilocycles derived from amplifier circuit 3 will cornbine with the output of the frequency doubler 6 to produce in the output of tube 7 difference frequencies running from 1700 to 2000 kilocycles.

Inasmuch as it may be difficult to provide ordinary tuned circuits of sufficient sharpness to discriminate appreciably against the side-bands of broadcast stations, it is advisable to utilize a certain amount of regeneration in each of the broadcast frequency amplifiers 2, 3 so as to sharpen the response of the respective selecting systems. 7

It will be understood that it is not necessary to select, in amplifier circuit 2, for frequency doubling; a station of exactly 1400 kilocycles, the multiplied frequency ofwhich is to be applied to #3grid of the converter tube, as a considerable departure in either direction from this value is permissible, provided a slightly different portion of the broadcast band is utilized in amplifier circuit 3 for application to the #1 input grid of the frequency converter tube. If, however, there is no strong broadcast signal available at appossible to select a station in the vicinity of 930 kilocycles, provided that a frequency tripler is employed in place of the frequency doubler 6. Similarly, a station in the vicinity of 700 kil'ocycles might be utilized for amplification by circuit 2 if followed In each of these cases, the frequency impressed upon the second 7 input or control grid (#3 grid) of the converter tube will be in the neighborhood of 2800 kilocycles and hence the operation of the system, whether for obtaining frequencies in the 3500 to 4000 range, or in the 1700 to 2000 range, is unaltered.

Once an assortment range from 3500 to 4000 able as above described, be successively doubled, as teur practice, to obtain a corresponding number of frequencies inany of the higher frequency amateur bands, since these higher frequency bands comprise only such obtained by multiplying the frequencies in the meter band by a suitable power of two.

What is claimed is:

1. The method of obtaining a substantially unmodulated radio frequency current of accuratel-y known frequency and high frequency stability which comprises receiving modulated carrier waves of different known frequencies from twobroadcast transmitting stations, sharply selecting each of said carrier waves to substantially reduce the received effective modulations thereof, multiplying the frequency of one of said received waves, producing from said selected and multiplied waves a new radio frequency wave of a frequency which is linearly related to the freuuencies of said broadcast waves of known freof frequencies in the has been made availthese frequencies may is well known in amaquency and substantially eliminating the modulation thereof to produce a current of stable frequency and substantially constant amplitude.

2. High frequency apparatus for generating a WALTER VAN B. ROBERTS.

frequencies as may be I

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