An automatic electronic Nyquist plotter

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