Superheterodyne Frequency Conversion Using Phase-Reversal Modulation

In radio reception using the superheterodyne principle the incoming signal is changed in frequency by the converter stage of the receiver to a new and lower frequency known as the intermediate frequency. The electron tubes used in the converter stage have been characterized in the past by poor performance as compared with that of tubes used for amplification. This paper describes a new principle whereby frequency conversion may be accomplished with substantially improved performance over that available from conversion methods heretofore used. The principle of conversion herein described is to reverse the phase of the signal output periodically at a rate which differs from the signal frequency by the intermediate frequency. This may be done either by continuous variation of phase or by continuous variation of tube transconductance from positive to negative. The result is a conversion transconductance which is twice as high as had heretofore been believed ideal. Furthermore, if the phase-reversal rate is made by any integral multiple of an applied local-oscillator frequency, equally good conversion is obtained at a harmonic of the local oscillator without spurious responses at any other harmonic than the one chosen. An electron tube with a multihumped characteristic has been devised as a means to this end since the transconductance characteristic will then vary from positive to negative as the control voltage is varied. An analysis of such a tube is carried out in detail, including the effect of fluctuation noise.