“Do-it-yourself” lock-in amplifiers - radio and audio frequencies

Abstract

We describe two specific lock-in amplifiers, one designed for radio frequencies (rf), and the other for audio frequencies (af). Both have been built and tested by chemists with limited electronics backgrounds. The af unit, which uses commercial circuit modules, was designed for an electron-spin-resonance spectrometer locking a klystron's frequency to that of a resonant cavity. The lock-in amplifier has been adapted to nuclear-magnetic-resonance work by combining it with a marginal oscillator. The frequency range is 40 to 20 000 Hz, but this is easily extended. The rf unit, designed for measuring differential pressures on the order of 0.1 μ with an accuracy of several percent, can detect capacitance changes on the order of 10 −18 F. Although designed for 2.7 MHz, it can be adapted to other radio frequencies. The rf unit uses Nuvistor tubes to minimize the instability problems normally encountered at radio frequencies.