Often, increases in signal-to-noise ratio and basic sensitivity are gained by the use of carrier modulation. Further advantage can be taken of this process by the use of varactors. These are suitably biased solid-state diodes and transistors, usually used to provide capacitance reactances. Their capacitance is changed by applying a voltage. The change in capacitance available by applying to their terminals the electrical output of a transducer--such as many of the sensors for light, heat, sound, mechanical stress and deformation--can be used in suitable modulating circuits. The magnitudes of reactance available from varactors fall conveniently into the range of values suitable for the carrier frequencies now accessible by means of another stable, mass produced solid-state device, the crystal clock oscillator.
In general, the complexity of the carrier circuit to be chosen depends upon the sensitivity and noise floor requirements. This paper describes circuits for applying the modulation process, and discusses the hierarchy of choices for the methods used. The authors have built circuits with an input noise level equivalent to 0"3pV for signals over the entire audiofrequency range, having a gain in excess of 50 dB.
PRINCIPLES OF OPERATION
This class of amplifiers makes use of the fact that capacitive reactances are provided by suitably biased solid-state diodes and transistors. Normally, 'varactors', as some of these devices are called, serve as tuning or trimming