Some Aspects of Radio Reception at Ultra-High Frequency: Part IV. General Superheterodyne Considerations at Ultra-High Frequencies

This paper presents a general survey of the problems encountered in the mixer or converter stage of superheterodyne receivers, particularly at ultra-high frequencies. The application of a strong local-oscillator voltage causes a periodic variation of the signal-electrode transconductance as a consequence of which intermediate-frequency-current components appear in the output circuit when a signal is also impressed upon the signal electrode. It is demonstrated that intermediate-frequency-current components are present in the output, which differ from the signal frequency by integral multiples of the local-oscillator frequency, if the Fourier analysis of the signal-electrode transconductance contains components which are integral multiples of the local-oscillator frequency. Methods of determining the conversion transconductance for so-called fundamental and harmonic conversion are given. It is shown that the noise output and input loading of a mixer stage are given by averaging these quantities over a local-oscillator cycle. A discussion of mixer gain is included, with a demonstration that the gain of a mixer stage is given approximately by the product of the conversion transconductance and the impedance of the output circuit (for high-output-impedance tubes). Considerations regarding image rejection and the undesirability of radiation of oscillator power lead to the conclusion that high intermediate frequencies are desirable. An extended discussion of whether to use an amplifier or mixer stage in the first stage of a superheterodyne receiver is included. If the received signal is strong, one should convert immediately, unless image rejection or the prevention of oscillator radiation necessitate the use of radio-frequency stages.