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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.