Model
8903B
Service
Attenuator
2
is
set to either
0
or
20
dB.
Amplifier
3
has a gain of 20
dB.
Attenuator
3
is
set
to
either
0
or
20
dI3.
The Buffer amplifier has
0
dB
gain. The High-Pass Filter
is
set to
0.3
Hz in the ac level measurement mode
to ensure accuracy down to 20
Hz;
otherwise,
it
is
set
to
13
Hz
to speed up measurement time. Amplifier
4
has
20
dB
gain. The output from Amplifier
4
is the rear-panel MONITOR output signal, an input to the Counter
(since this
is
the point of greatest sensitivity in the ac level measurement mode), and the input
to
the Output
RMS/Average/Quasi-peak Detector.
The Output RMS/Average/Quasi-peak Detector is used
to
set the gain of the output path (Attenuators
1,
2, and
3) and, after processing by the Controller,
its
output
is
also used
as
the numerator of the distortion measurement
or
the denominator of the SINAD measurement. The detector can also be configured
to
respond
to
the absolute
average or quasi-peak value of the
signal
instead of the true RMS value. The Output Overload Detector senses the
output of the Output RMS/Average/Quasi-peak Detector. If the output exceeds 3V, the gain of the output path
is set to
0
dB.
The state of the output can also be read by the Controller, which then takes appropriate action.
The SINAD panel meter
is
normally used only in the SINAD measurement mode when the SINAD
is
between
0
and 18
dB.
On that range, since the gain of the output path
is
0
dB,
SINAD is equal
to
the log of the ratio
of
the output of the Pre-Notch RMS Detector
to
the output of the Output RMS/Average/Quasi-peak Detector. The
mathematical manipulation takes place in the SINAD Log Ratio Meter Amplifier. The Meter Transconductance
Amplifier converts the voltage at
its
input
to
a current to drive the SINAD meter.
If
SINAD
is
not in the range
0
to
18
dB
or
if some other measurement mode has been selected, the Controller either pegs the meter
to
full scale
or
switches
it
off via the Meter Peg/Off Switch.
DC
Voltmeter
Various dc voltages are routed to the Voltmeter Input Selector switch. The outputs from the Pre-Notch
RMS/Average Detector and the Output RMS/Average/Quasi-peak Detector are measured through Ripple Filters
when measurements are made on noisy ranges. The
DC
Voltmeter consists of the Counter, the Controller, and
the Voltage-to-Time Converter. The output of the Voltage-to-Time Converter controls one of the input gates of
the Counter. The voltage-measurement cycle begins when the Controller gates the
2
MHz
Clock into the Clock
Counter. Counting proceeds until the Voltage-to-Time converter produces a Stop Count pulse which opens the
Stop Count Gate. The time lapse between the gating
of
the clock and the opening
of
the Stop Count Gate is
proportional
to
the dc input voltage. The count thus accumulated during the interval
is
proportional to the input
voltage.
Source
The source consists of a 10
Hz
to
100
kHz
Oscillator and output level-setting circuits. Oscillator tuning
is
programed
by
the Controller which uses the Counter to check frequency. The Oscillator output is self-leveled
to
about
3
Vrms via an automatic leveling control (ALC) circuit. The circuit senses the level with a peak Level
Detector. The output from the detector is compared against a dc reference voltage by the Level Error amplifier.
The output of the amplifier drives the level-varying circuitry in the Oscillator
to
eliminate the error condition.
The amplifiers and attenuators in the output path are:
0
Amplifier 1, an active attenuator with a gain of
0
to -6
dB
settable in 256 steps;
0
Amplifier
2,
an active attenuator with a gain of
0
to
-17.5
dB
settable in
2.5
dB
steps;
Attenuator A, a passive attenuator with
0,
20, and
40
dB
attenuation;
Output Amplifier Driver, a non-programmable buffer with
a
gain of
1.9
(+5.58
dB);
e
Floating Output Amplifier with a gain
of
about
+l
dB;
and
0
Attenuator
B,
a passive attenuator with
0
or
20
dB
attenuation.
In the signal-to-noise ratio measurement mode, Amplifiers
1
and
2
and Attenuator
A
are switched between a
normal and an “off” state. (The Controller reads the Output RMS/Average/Quasi-peak Detector
for
both states
and computes the ratio.) In the
“off
state the gain of Amplifier
1
is lowered by
6
dB,
Amplifier
2
is
set to
-60
dB
gain, and Attenuator
A
is shorted
to
ground. This
“off”
state
also occurs when a source level
of
OV
is selected.
reu.20JUN87
8A-3
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