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9-1
Chapter 9
Functional Circuit Description
Introduction
The excitation signal to the primary coils of the LVDT is generated by an oscillator made
of Z2A and Z2B. The output of the oscillator (at TP7) is a 24 volt peak-to-peak sinusoid
at a frequency of 5.1 KHz. This signal is attenuated by R5 and R6 and buffered through
Z1B. The excitation signal (TP6) to the primary coil has a magnitude of 4 volts (P-P).
The output of the LVDT is measured differentially and amplified by Z4B. This error
signal is then again amplified by Z4A. The information present at the output of Z4A is a
sinusoid whose amplitude and phase (0o or 180o) is a function of the position of the core
in the LVDT. This information is half-wave rectified by Q1. Q1 is operated by the
squarewave generator Z1A. The result at TP5 is a positive or negative half-wave sinusoid
whose amplitude is a function of core displacement from the null position and whose
polarity is a function of the direction of the core displacement. The signal at TP5 is then
integrated by R8 and C5. This DC level is then amplified through Z3. The gain of Z3 is
controlled by the GAIN adjustment. The ZERO adjustment provides a bias voltage added
algebraically to the error signal via Z3.
Resistor R29 and diodes CR3 and CR4 provide current and voltage limiting to the null
meter. R30 is a current limiter to the recorder output jack. Resistors R25, R26, R27, and
R28 determine the ZERO control sensitivity and span.