Range Comparator; Wideband Frequency Counter - Fluke 5790A Service Manual

2-112. Range Comparator

Comparator U11 and U13, and associated parts form a circuit that indicates when the
RMS sensor circuit is being driven beyond normal limits. Each of the 4 Schottky diodes
CR16 through CR19 is biased by current sources Q7 and Q8 to 6 mA each. When the
amplifier U5 is forced by the large input signals to drive more than 6 mA output, the
diode CR16 and CR19 stop conducting. The 12 mA from Q7 flows through CR17 and
split between the sensor input at U15 pin 10, and R126. The sensor receives 7.6 mA
which gives 3 V across the 400 e input of RMS sensor U15 pin 10. The 3 V is a safe
level for the sensor.
When amplifier U5 is forced to drive more than 6 mA output, diodes CR18 and CR17
stop conducting and the 12 mA from Q8 is split between the resistors and sensor input.
The sensor voltage is thereby clamped at 3 V. When the diodes stop conducting in either
direction, the voltage at U5 pin 6 output jumps to the saturated level of 5 V.
When the output of amplifier U5 reaches 2.5 V comparator U11 pin 4 or pin 9 drops
low (pin 4 for 2.5 V and pin 9 for -2.5 V) and pulls the voltage of capacitor C63 down
to the trip level of 1.5 V at the input of U13 (pin 3). The output of U13 at pin 1 drops low
and indicates the need to change to the next highest range. At this signal, the signal at
TP9 (RANGE COMP) causes gate U27 pin 11 to go high and turn on FET Q11. With
Q11 on, the interrupt line at P206 pin 1A is low, thereby telling the digital system to
range up.
If the up-ranging mechanism reaches the 7 V range and U13 pin 1 does not indicate that
the sensor amplifier is within the normal range, an overload condition exists and
Wideband mode is turned off.
Diodes CR20, CR21, Q9, Q10 and associated components form a backup clamp for the
RMS sensor and are activated only if the CR16 through CR19 protection bridge fails. It
clamps the sensor AC input pin 10 to 3.25 V.

2-113. Wideband Frequency Counter

A circuit on the A6A1 assembly conditions signals for use by the frequency counter
circuit on the A15 A/D Amplifier assembly. The counter function for Wideband mode is
provided by buffer Q23 and the counter section of the A6A1 assembly. The output of
RMS sensor buffer amplifier U5 is connected to emitter follower Q23 through resistor
R143. Q23 isolates the counter circuit from the signal measurement path. The output of
Q23 is attenuated by R106 and R141 and passed to the A6A1 assembly input at pin 14 of
the SIP connector. The input is sent to comparator U1 at pin 2. Comparator U1 produces
an output of 3.4 V (HIGH) or 0.3 V (LOW) whenever the input exceeds the input
threshold of about 30 mV. The output of comparator U1 is therefore at normal logic
levels and can be used by divider U2 to divide down by 16.
For frequencies greater than 1.99999 MHz, the output from the circuit is taken from the
divider U2 when quad switch U3 closes the switch from pins 3 to 2 and connects the
signal to output resistor R11. When quad switch U3 closes the switch from pins 14 to 15
the output is sent on the COUNTER line across the Motherboard to the frequency counter
circuit of A15 A/D Amplifier assembly, where the frequency is measured for display on
the Measurement Display.
For frequencies below 2 MHz, the output from the circuit is taken from comparator U1
when quad switch U3 closes the switch from pins 10 to 11 and passes the signal to R11
and out the COUNTER line as before. Resistors R6, R10, and resistor networks Z2 and
Z3 set the bias and signal levels needed on the COUNTER output line. The signal level
on the COUNTER output line is 400 mV. Below 2 MHz, additional filtering of the input
signal is provided by capacitor C14 which is switched into the circuit by PIN diode CR1.
CR1 is turned on by Q2 when Q1 is turned off by the digital control signal FILT. The
Theory of Operation
Analog Section Detailed Circuit Description
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