whenever a GPIB transaction occurs which requires the CPU’s response. (The GPIB address is set by switch bank SW1.)

Power Supplies

The line transformer provides two outputs, 40VAC and 15VAC, both center -tapped. The transformer has dual primaries which may be selected by the voltage selector card in the fuse holder. The 15VAC is rectified by diode bridge BR2 and passed to 5V regulator U909. The output of U909 powers the microprocessor and its related circuitry. The 40VAC output is half-wave rectified by BR1 and regulated by U901 and U902 to provide +20V and -20V. These two dc voltages are then regulated again by 15V regulators U903- U908. Each 15V regulator powers a separate section of the lock-in to reduce coherent pick up between sections. U911 and U912 provide plus and minus 7.5V and U910 generates +5V for the analog circuits.

Internal Oscillator

The internal oscillator is on a small circuit board attached to the rear panel of the instrument. Local regulators, Q1 and Q2, provide power to the board. The VCO input is internally pulled up by R12. This pulls the VCO input to 10V when the VCO input is left open. 2/4 U1 translates the VCO input voltage to provide a negative control voltage to U2, the function generator. P3 adjusts the VCO calibration. U2 is a sine wave generator whose frequency range is selected by the VCO Range switch and capacitors, C4-C6. P2 adjusts the sine wave symmetry at low frequencies. 4/4 U1 buffers the output of U2. P1 adjusts the amplitude of the output sine wave. The output amplitude on the SIne Out is selected by the amplitude switch. The output impedance is 600Ω .

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