MAGNE-SONIC 300 SERIES 4-20mA LEVEL ... - Clark Reliance

See Figure 4-1 for a simplified schematic diagram pertaining tothese descriptions:1. The power supply section (not shown) converts line powerto appropriate voltages for circuit operation.2. The OSCILLATOR section produces the triangle wavesignal used to measure the probe impendance and severalsquare wave signals used to synchronize other parts of thecircuit.A voltage controlled square wave oscillator operates at afrequency four times that of the measuring signal. Two"flip-flops" divide the oscillator frequency and producefour square waves with a 90° phase difference betweeneach successive signal. One of these square waves isapplied through a resistor to a capacitor. The result, a lowvoltage triangle wave with low distortion, becomes theSHIELD signal.Figure 4-1 Instrument Operations Schematic DiagramThe SHIELD signal is gated with analog switchescontrolled by two of the square wave signals to adifferential amplifier which produces an output voltageproportional to the peak-to-peak voltage of the SHIELDsignal. This voltage is compared to a reference voltage by