414 Calibrator

4-15. The schematic diagram of the calibrator is given in Figure 5-5. This circuit consists of three parts: a tunnel diode square wave generator, a transistor switch, and a calibration network.

4-16. 6.3 volts AC is applied through R201 to tunnel diode CR201, which generates a square wave at line frequency. Transistor switch Q201 is off during the time of the positive half-cycle of the square wave (when the voltage at the base is close to zero), and the collector voltage is thus at a level set by DC voltage divider R206-R207-R208. When the negative-going portion of the square wave is applied to the base of Q201, the transistor conducts heavily, effectively shorting the collector to ground. The output of the calibrator is thus at zero volts. At the end of the negative input half-cycle, the base of Q201 returns to zero, the transistor is switched off, and the output returns to its previous value.

4-17. Tunnel-diode bias current is supplied through resistor R202. The bias current sets an operating level for the diode which affects the symmetry of the square wave output. Cal Adj R205 is used to set the DC voltage at the collector of Q201 to -10 volts when

Figure 4-3. High-Voltage Power Supply Block Diagram

high ^ voltage adjust

Figure 4-3. High-Voltage Power Supply Block Diagram the transistor is off. Breakdown diode CR202 reduces the output impedance, and provides temperature compensation for the circuit. Voltage divider R207/ R208 reduces the 10-volt output to 1 volt. Both 10-volt and 1-volt outputs are available on the front panel of the Model 140A.

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