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Model 8901B
Service
AM Demodulator Assembly (A6)
The down-converted signal from the IF Amplifier
is
filtered by a 2.5 MHz Low-Pass Filter. The AM
IF Buffer drives the AM Demodulator. The FM
IF
Buffer drives the
FM
Demodulator and rear-panel
IF OUTPUT connector.
The AM is demodulated by means of a precision, half-wave rectifier in an automatic level control (ALC)
circuit. The buffered IF signal
is
amplified by a Current-Variable Amplifier then rectified (detected)
by the AM and
IF
Average Level Detector. The detected signal, after carrier filtering, represents the
carrier level (dc component) plus AM (ac component). The ac component accurately represents the AM
only if the dc component is known or set to a known level. The detected signal is filtered and amplified
by the Level Amplifier and Carrier Filter. The signal is then compared to a constant ALC Reference
by the
BW
Control and Level Comparison Amplifier. The output of this amplifier is the carrier level
error. The error voltage is amplified by the Resistor Drive Amplifier which sets the current input to
the Current-Variable Amplifier. Current-variable resistors in the amplifier adjust the amplifier gain to
cause the dc component of the carrier to equal the ALC Reference.
The amount of filtering in the Bandwidth Control and Level Comparison Amplifier determines the
minimum AM rate which can be accurately demodulated. An ALC Bandwidth Control line sets the
ALC loop for a fast or slow response. The feedback loop may also be defeated by the ALC Defeat line.
Special Function 6 controls the ALC loop.
The second output of the AM and IF Average Level Detector is buffered by the AM Output Buffer.
One output of the buffer is fed to the rear-panel AM OUTPUT connector. The other output is fed to
the audio circuits for filtering and audio processing.
The output of the FM
IF
Buffer is detected by two detectors. The IF Peak Detector output is read by
the Voltmeter. It is used in the automatic tuning routine and for making Tuned
RF
Level measurements
using the
IF
Peak Detector. (Refer to the n n e d
RF Level
detailed operating instruction in the
Operation and Calibration Manual. The IF Present Detector is used to stop the LO sweep during
a signal search (independent of the Controller).
The Voltmeter also receives IF level information from the output of the Level Amplifier and Carrier
Filter. The IF level is measured after completion of tuning to confirm that the AM ALC loop is
operating within range.
The IF Level measurement mode (invoked by pressing the S (shift) and IF LEVEL keys) compares
the level from the Level Amplifier and Carrier Filter with the ALC reference and displays the ratio of
the two (normally 100% unless the IF signal is too low or Special Function 6.2 is used).
The voltage from the Resistor Drive Amplifier is an indication of the ALC current driving the input
resistor circuit. It is used for setting the Input Attenuator, (see Service Sheet BD2) when the ALC is
on. (When the ALC is off, the Input Attenuator is set using the
IF
Peak Detector for FM or the level
read on the IF AVG Level line for AM.)
FM Demodulator (A4)
The signal from the FM IF Buffer drives the FM IF Limiters. The limiters strip AM and noise 08 the
IF to minimize demodulation of AM by the FM Demodulator (called incidental FM). The three stages
each have 22 dJ3 of gain. The output of the limiters
is
a squarewave which drives a Precision Limiter.
This limiter clamps the upper and lower levels of the squarewave to highly-stable references required
by the Charge-Count Discriminator. For each cycle of the IF signal, the discriminator passes a fixed
quantity of charge through the feedback resistor of an amplifier. The voltage developed at the amplifier's
output
is
proportional to the amount of charge delivered per unit of time. Fluctuations in IF frequency
(FM) produce fluctuations in the voltage at the output of the discriminator. The demodulated FM
passes through the FM Output Amplifier and on to the audio circuits for further filtering and audio
processing.
The Squelch Switch grounds the output of the discriminator whenever the IF level detected by the
Squelch Detector
is
insufficient. This squelch action attenuates the large noisy output that results when
8D-26
Service Sheet BD3
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