Gain and Noise Figure of a Variable-Capacitance Up-Converter

Since microwave receivers frequently must operate at a very low input signal level it is important to obtain the smallest possible noise figure. One way of accomplishing this is to use a low-noise amplifier at the input of the receiver. In this article the role of a nonlinear-capacitance p-11 junction diode in performing such a function is analyzed. The performance of a variable capacitance as a frequency converter lias been extensively discussed in the literature. 1,2 ' 3 Theory indicates that when the variable capacitance is used for conversion between a low frequency and an upper sideband (a noninverting frequency), the maximum available gain is f2/fi , f i being the input frequency and / 2 being the output frequency. Hence in up-conversion (modulation) a power gain results, with the power added to the input signal being supplied by the beat oscillator. In down-conversion a power loss results. (These statements apply when no signal power is transferred at the lower sideband or at any of the harmonic images). The situation is different when one of the two frequencies involved is a lower sideband (an inverting frequency). A negative resistance may then appear at both sets of terminals, * S u p p o r t e d in p a r t by t h e U. S. Arm}' Signal Corps u n d e r c o n t r a c t DA 36-039 sc-73224. 989