The Operation of Frequency Converters and Mixers for Superheterodyne Reception

Abstract

This paper presents a general picture of superheterodyne frequency conversion followed by a detailed discussion of the behavior of tubes used with different types of oscillator injection. The general picture shows that the different methods of frequency conversion are basically similar for small signals. A strong local-oscillator voltage (which may be a pure sine wave) causes a periodic variation (which is usually nonsinusoidal) of the signal-electrode transconductance. The coefficient of each Fourier component of the transconductance-versus-time relationship is just twice the conversion transconductance at the corresponding harmonic of the local-oscillator frequency. For most tubes the conversion transconductance g c at the oscillator fundamental is approximately 28 per cent of the maximum transconductance. At the second harmonic, g c is about 14 per cent, and at the third harmonic it is about 9 per cent of the maximum transconductance. Fluctuation noise and input resistance at high frequencies of the different methods of conversion may be found from the time average over the oscillator cycle. Using these general concepts, we discuss the detailed behavior of three conversion methods. In the first method, signal and local-oscillator voltages are impressed on the same electrodes. This method gives best signal-to-noise ratio, but has the disadvantage of bad interaction between signal and local-oscillator circuits. In the second method, the local-oscillator voltage is impressed on an electrode which precedes the signal electrode along the direction of electron flow.