Fluke 900 - Manual (Page 14)

Theory of Operation FLUKE 900 SERVICE MANUAL
During the test phase, signals received by the H3 are routed immediately to a comparison circuit
to be matched against one of two possible signals. In the case of a signal extracted fix)m a DUT
input pin, the comparison is made with itself ( no failure should appear ). In the case of a DUT
output pin signal, the comparison is made with the identical RD pin signal. The RD pins are
continuously monitored by a resistive load to determine whether tiiey are input or output and
thus how to route the equivalent DUT signals. At each signal transition, the DUT/RD
discrepancy is timed to see if it exceeds a set FMASK value. If so, and if not overridden by a Gate
or Trigger setting, the faults are latched and displayed on the monitor LEDs and LCD Screen.
2.2 Interface Buffer
Pages 3,4,S,6 of the IB schematics show the signal path for each of the 28 channels that can come
from a DUT. lOK resistors connect each channel to an output bit on the cascaded shift registers
shown on page 1. In actual usage, these are resistors pulled up to 5 volts by default to resolve
floating imconnected inputs on a UUT which could otherwise confuse test results. In addition,
the check for a low condition on a pin is made while attempting to pull it high; the high check is
made with the lOK resistor pulled down to ground.
After the impedance matching components and protection diodes, each chaimel feeds a
comparator referenced to a bias voltage. The ouqjuts are differential E(X levels for relative
noisefiee transmission to the High Speed Board.
The shift registers shown on page 2 are used to control the bias setting for threshold, the Reset
line setting and external frequency multiplexing. Eight bits drive a DAC which resolves a 0 to 5
volt range into 100 mV increments. Five bits control the polarity, voltage and status of the Reset
driver (Page 1). Three bits route one of several lines through an analog multiplexer to a
voltage-controlled oscillator (U25). The measurement of the VCO frequency on the HSB is used
to determine clip size ( from a characteristic voltage divider in each clip), threshold verification
and VCC accuracy.
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