US 1878614 A
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Sept. 20, 1932. y H. A. WHEELER v1,873,614
Y IE'HOD 0F OPERATING SUPERHBTERODYHE RECEIVERS Filed Feb, 24. 1931 INVENTOR HAROLD A. WHEELER A BY Qww. ML-Mx.. 1 'ATTORNEYS Patented Sept. 20, `19232 UNITED STATES PATENT OFFICE HAROLD A. WHEELER, OF GREAT NECK, NEW YORK, ASSIGNOR TO HAZELTINE' CORPO- RATION, A CORPORATION OF DELAWARE i METHOD OF OPERATING SUPERHETERODYNE RECEIVERS vApplication led February 24, 1931. Serial No. 517,742.
The principles of operation of the superheterodyne receiver are well known. The
, received signal having a certain high radio 'quencies frequency is applied to a so-called first detector, usually after amplification in a radiofrequency amplifier tuned to the signal frequency. A local'oscillator in the receiver, having a frequency slightly different from the signal frequency, is also coupled to the first detector. In this detector a signal is generated having a new frequency which is called the intermediate frequency. This intermediate frequency is equal to the difierence between the signal and oscillator frequencies. The oscillator frequency is so chosen relative to the signal frequency that the intermediate frequency has a value above the audible range but substantially `lower than the signal frequency. For this reason amplification and a certain. amount of selectivity can be accomplished at the intermediate frequency more easily than would be possible lat the signal frequency. Therefore an amplifier tuned to the intermediate fre` quency is connected in the circuit following the first ceiver includes the so-called second detector followed by an audio-frequency amplifier which correspond to the detector and audiofrequency amplifier usually employed in any type of receiver for high-frequency signals. While this description applies to a representative superheterodyne receiver it is understood thatany number of variations may be employed without departing from the superheterodyne type.
It is also well knownv that such superheterodyne receivers are responsive to two different carrier frequencies, one of equal to the sum of the oscillator and intermediate frequencies and the other of which is equal to the difference between these 'fre- In practice it is customary to select one of these two carrier frequencies by the use of tuned radio-frequency circuits preceding the first detector. lIn thefollowing description 'it is assumed that 'a desired signal is being received on one of these two.
carrier frequencies and that the tuned radiofrequency circuits preceding the first detecdetector. The remainder of the re which is tor are tuned to this carrier frequency. It is signal frequency are equally distant from the oscillator frequency,one higher and the other lower).
l In a broadcast receiver wherein signals must be received over a wide frequency range it is desirable to minimize the number of radio-frequency circuits which must be tuned to the signal frequency. L'On the other hand it is difficult to secure with a smallnumber of such tuned radio frequency circuits a sufficient amount of discrimination against interfering signals'whose :frequencies are approximately equal to the ima-ge frequency.
It is the purpose of this invention to provide a method of operating a superheterodyne receiver, which will under certain conditions reduce the amount of interference from this cause. A
It is common practice to allot ato broadcast stations in a given section of the country a group of frequencies which are equally spaced 50 kilocycles apart over the broadcast band. The reason for this practice is to minimize interference between different nearby stations, one of which is being received in a broadcast receiver. When a superheterodyne receiver is tuned to the transmitting frequency of. one of these nearby stations, the intermediate frequency can be so vchosen that the image frequency will lie intermediate the transmitting frequencies of two other nearby stations separated in frequency by 50 kilocycles. In this way thereceiver is made responsive to only one nearby station, i. e. the one transmitting atthe desired carrier frequency. Y
The image frequency and the desired carrier frequency areA separated by a frequency quency.
to be realized the difference between the 'image frequency and the desired.v signal frequency mustbeequal also to an odd multiple loo of 25 kilocycles which is one-half the freuency separation 'of the nearby stations in t e above example. Therefore the intermediate frequency must e ual an odd multiple of 12.5 kilocycles whic is one-quarter the frequency separation.
There are other factors which'are equally important in the choice of an intermediate frequency but which need not be described in detail. A ,representative value for the intermediate'frequency which would fulfill the above requirements is approximately 312.5 kilocycles. If the desired signal is received on a frequency of 600 kilocycles, for example, the oscillator frequency might be 912.5 kilocycles and the image frequency 1225 kilocycl'es. According to the preferred practice two other nearby stations would be operating on frequencies of 1200 and 1250 kilocycles respectively. The image frequency would lie midway between 1200 and 1250 kilocycles and the receiver would not be responsive to interference fro'm either of these stations. v
The essential elements of this invention are therefore as follows: A number of nearby stations are equally spaced in the broadcast band with a certain frequency separation. The in# termediate frequency is chosen to be an odd multiple of one-quarter the frequency separation.- It is thereby assured that the image y frequency will not correspond to the frequency of an undesired nearby signal. Interference at the image frequency is thereby reduced to minimum. f
The single figure illustrates graphically the method here intended. Graph A depicts at fa, fb, fc, etc. the several transmitting fre-- quencies of nearby radio-broadcasting stations. These transmitting frequencies are successivelyA displaced by a constant frequency interval of fx which in the example given above amounts to kilocycles.
In graph B the heavy line designates the receiver as being tuned to a desired transmitting frequency, in this instance fb. Design'ating `the oscillator frequency by f., and
the image frequency of fb by fb', then in order to ensure that the image frequency of fb will not coincide with any of the remaining transmitting lfrequencies f1, jk, fm, etc. the following relation should exist between the frei quency spacings:
where n is an integral number and f, is the difference between theLgelected carrier and the oscillator frequencies. With this condition fullled ther-image frequency fb corresponding to the tuned frequency fb will be situated, as shown in the figure, midway between the transmitting frequencies f. and fk of two other broadcast stations, while the oscillator frequency is displaced .from the transmitting frequency f., by the interval WhatIclaimis: i v 1. The. method of operating a superheterodyne receiver to receive any one of a numberv of stations equally spaced in the frequency spectrum, comprising the choice of an intermediate frequency substantially equal to an odd multiple of one-quarter the frequency separation of successive stations in said spectrum, whereby the image frequency fallsk of broadcast stations spaced 50 kilocyclesapart in the broadcast band, comprising the choice of an intermediate frequency substantially equal to an odd multiple of 12.5 kilocycles, whereby the ima e frequency falls midway between two un esired station frequencies when the desired one of said broadcast stations is being received.
In testimony whereof I affix my signature.
` HAROLD A.