Keithley 2015, Service Manual

Page: 80 / 158

Troubleshooting 4-17
Filter
U307, U336, and U337 form a low pass filter with a software-selectable cutoff frequency.
This filter is used to reduce spurious noise in the sine wave output. The filter cutoff frequency is
set by the microprocessor through the FPGA, U334, and opto-isolators U304, U320, and U321.
Outputs
The Model 2015 has two outputs. U305, U306, U307, and U308 form the main sine wave
output stage.
The secondary output may be either an inverted sine wave of the same magnitude and fre-
quency as the main sine wave output, or a 5V pulse output of the same frequency as the main
sine wave. One IC is a comparator that squares up the main sine wave output. Another IC selects
whether the sinewave or the comparator is fed to the output.
K301 selects between either 50
Ω
or 600
Ω
output impedance.