Abstract
The instrument described is designed to plot automatically in polar co-ordinates the transfer functions of electrical networks or servo systems over the frequency range 0.2–200c/s. It consists basically of:(a) A variable-frequency resistance-tuned oscillator which delivers a constant voltage to the system under test.(b) A narrow-band-pass RC filter tracked with the oscillator to eliminate noise from the output of the system under test.(c) An RC 90°-phase-shifting circuit tracked with the oscillator to obtain a radial measure of output voltage on an oscilloscope.(d) A 12in cathode-ray display unit with pulse brightening which shows output voltage, phase and frequency in the form of a Nyquist plot as the frequency of the oscillator is varied.Eight ganged 10-turn helical resistors are used for tracking the oscillator, filter and 90°-phase-shifting circuits.Owing to tracking errors, the filter results in maximum inaccuracies the plot of ±0.4 dB amplitude and ±2° phase shift. The band width of the filter is 20% of the operating frequency
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