Service Model 8903B
SINAD Meter Circuits
The
SINAD
meter displays the log of the Pre-Notch RMS/Average Detector minus the log of the Output
RMSjAveragelQuasi-peak Detector. The Pre-Notch RMS/Average Detector produces a current which flows
through R57 and logging transistor Q4A. The voltage developed across the base-emitter junction
of
Q4A is
proportional
to
the log of
its
collector current. Similarly, the Output RMS/Average/Quasi-peak Detector produces
a current which flows through R56 and logging transistor Q4B and develops a base-emitter voltage proportional to
the log of
its
collector current. The voltage at the base of Q4A (with respect
to
ground)
is
equal to the difference
between the base-emitter voltages of Q4A and Q4B. The feedback amplifier, Q17C, amplifies this difference and
produces the output. Note that the collectors and bases of both Q4A and Q4B are all biased near ground potential.
Thermistor
EtTl
thermally compensates
for
the
drift
of the
two
logging transistors.
The voltage from the SINAD Log Ratio Meter Amplifier is converted to
a
current by the Meter Tkansconductance
Amplifier which drives the meter. CR15 protects the meter against reverse current. R83 and C51 add heavy filtering
to the often noisy
SINAD
measurement signal.
The Meter Peg/Off Switch receives commands from the Controller via the Latch Assembly which enable, gently
peg,
or
switch
off
the meter via the Meter Transconductance amplifier, depending on the measurement being made
and the SINAD measurement range. A low at the inverting
(-)
input of U19A or U19B switches the respective
output
FET
(Q6 or Q5) on pulling the output connection
to
either ground
or
i-2.4V.
Output
Overload
Detector
The Output Overload Detector compares the level from the Output RMS/Average/Quasi-peak Detector
to
a
reference set by R60 and R61.
If
the level exceeds the reference, U19D switches
FET
Q7
on
which resets the
Overload Flip-Flop (see Service Sheet
12)
and reduces the gain
of
the Output Amplifier to
0
dl3.
8B-44
rev.20JUN87
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