Some Aspects of Radio Reception at Ultra-High Frequencies: Part V. Frequency Mixing in Diodes

Although the diode is one of the simplest forms of vacuum tube, the behavior of the diode mixer in superheterodyne reception has not been well understood. One reason for this is that the conversion process is more complex than in other mixers in that it is bilateral, a radio-frequency input voltage giving an intermediate-frequency output current and the resulting intermediate-frequency output voltage in turn giving a radio-frequency current in the input. Analysis of the behavior leads, however, to a very simple equivalent circuit consisting of a symmetrical π circuit of three conductances whose magnitudes are determined by the average diode conductance and by the conversion conductance of the diode. The present paper derives this circuit and uses it to find the conversion loss of the converter stage both with and without input circuit loss. The results, although arrived at independently, are in agreement with the recent publication of James and Houldin. If the conversion loss is to be held small, the diode must be operated so as to obtain the highest ratio of conversion conductance to average conductance. The upper limit of this ratio is unity and this is attained only when the mixer-stage impedance is infinite. Thus, circuit losses prevent the attainment of the condition of no conversion loss, in practice. The signal-to-noise ratio of a receiver whose input stage is a diode converter is not determinable accurately because of uncertainties in the diode noise behavior.