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Theory of Operation-AA
501A
Table 3-3
GAIN AND SWITCHING THROUGH U2031
Option 01
The output of the distortion amplifier enters the main
board through J1042 driving the weighting filters and the
distortion amplifier ranging level detector. The detector,
composed of U4030A and U4030B, full wave rectifies and
filters the distortion amplifier output. This dc signal goes to
the logic board to control auto-ranging of the distortion
amplifier.
Internal Connections
pins
13 to 12 and 3 to 1
13 to 14 and 3 to 5
13 to 15 and3 t o 2
1 3 t o 1 1 a n d 3 t o 4
Logic Level
Pins
9
10
0 0
0 1
1 0
1 1
The weighting filters consist of U2023A, U20238,
U3021 B, U3021 A and associated resistors and capacitors.
The signal from the distortion amplifier passes through the
330 kHz filter before passing to the remaining filters. Output
from the filters is multiplexed by U1021 to the input of
buffer, U4020A. Table 3-4 is
a
truth table for U1021.
Table 3-4
TRUTH TABLE FOR U1021
Total
Gain Through
Dist Amp
+6 dB
+26 dB
+46 dB
+66 dB
U2041
Gain
0 dB
0 dB
0 dB
$20 dB
The highpass filter (three pole 400 Hz Butterworth) is
composed of U4020B, C4012, C4011, C4010, R4012,
R4010, and R4011. This filter is driven by U4020A. When
the highpass filter is disabled, U3020 connects pins 1, 13,
14, and 15 thus shorting the output of U4020A directly to
the input of U4020B. R4013, R4014, and C4013 provide
10 Hz response compensation for low frequency accuracy.
B
0
0
1
1
to the Cx input through protection components R3022,
CR4020, and CR4021. U2030B connects to either the AUX-
ILIARY INPUT or the output from U4020B depending upon
the state of the EXT control signal.
After filtering, the signal is converted to a dc voltage by
both rms and average techniques. Rms conversion is ac-
complished in U3031 (pin 10 out) using an implicit comput-
ing approach. The averaging capacitor is C3032. A low pass
filter, U2040A, reduces readout jitter due to low frequency
noise or ripple.
A
0
1
0
1
The averaging rectifier is U2030A with diodes CR2031
and CR2032. The rectified output is smoothed and filtered
by U2040B, C1040, and associated components. The aver-
age detector output connects to U2040A via Q3040 in the
average response mode, overriding the rms converter.
Filters and AC-DC Converters (Option 02
Instruments Only)
@
ON CHANNELS
The output of the distortion amplifier enters the main
board through J1042 to drive the filters and the distortion
amplifier ranging level detector. This detector, composed of
U4030A and U4030B, full wave rectifies and filters the dis-
tortion amplifier output. This dc signal goes to the logic
board to control the distortion amplifier autoranging.
X O
X I
X2
X3
The filters consist of U2023A, U2023B, U2021 B,
U2040A, U2040C, and U2040D with associated resistors
and capacitors. The signal from the distortion amplifier
passes through the 330 kHz filter before passing to the
80 kHz LO PASS and AUDIO BAND PASS filters. The
weighting filter input is taken directly from the distortion am-
plifier output. R2035 provides gain calibration adjustment
for the CClR weighting filter. Output from the filters are
multiplexed by U1021 to the input of buffer U4020A. Table
3-4 is a truth table for U1021.
Output from the highpass filter, U4020B, connects to the
front panel Function output connector and the Cy channel of
U3020. The AUXILIARY INPUT on the front panel connects
Y O
Y1
Y2
Y3
WEIGHTING
30 kHz LOWPASS
(Bandpass Option 02 only)
80 kHz LOWPASS
330 kHz LOWPASS
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