Fluke Biomedical 956A-201-M2 - Manual (Page 64)

3-38
Signal Input Circuitry
Detector input pulse circuitry consists of an input buffer, high/low discriminators, signal detection, anti- jam
and signal multiplexer circuits. The following paragraphs describe circuit operation in detail.
Buffer Amplifier
The detector input is connected to J6. The input impedance is 50 ohms to match the signal cable and the
detector's output impedance. Jumpers JP4 and JP5 are used to select the proper pulse polarity as shown
below.
Jumper Function Remarks
JP4 1-2 Pulse Polarity (Input) For negative input
JP4 2-3 Pulse Polarity (Input) For positive input (factory
set)
JP5 1-2 Pulse Polarity
(Shield)
For negative input
JP5 2-3 Pulse Polarity
(Shield)
For positive input (factory
set)
The detector input signal (with appropriate polarity jumpers installed) is applied to unity gain buffer
amplifier U90. VR8 is used to fine adjust for unity gain. Regardless of input signal polarity, U90-6 outputs
positive going pulses. VR9 is a zero offset adjust for U90. The buffer amplifier output is provided to the J7
connector (for use by analyzer option circuitry) as well as the high and low discriminators. TP-Pulse is
available as a test.
Discriminators
The low level discriminator is comprised of comparator U91B and associated circuitry. VR11 is used to set
the trip threshold. The adjustment range is 50 mV to 1 volt, which can be measured at the low
discriminator test jack. As the positive pulse, applied to the input, passes through the trip threshold, the
output (U91-6) is forced low. When the pulse returns through the trip threshold, the output U91-6 returns
high and is ready to accept another input pulse. Pulses below the trip threshold do not trigger the output.
The high discriminator is comprised of comparator U91A and associated circuitry. VR10 is used to set the
trip point. The adjustment range is 3.5 to 7.5 volts, which can be measured at the high discriminator test
jack. As the positive pulse, applied to the input, passes through the trip threshold, the output (U91A) is
forced low. When the pulse returns through the trip threshold, the output (U91A) returns high and is ready
to accept another input pulse. Pulses below the trip threshold do not trigger the output.
The normal factory settings are:
• Low Disc.: 0.500 Vdc
• High Disc.: 7.000 Vdc
Figure 3-6 shows signal "A" below both discriminators, signal "B" between discriminators, and signal "C"
above discriminators.