Advertisement
Model 8901B
Service
Option
Typical Measurement
Typical Channel
Number
Application
Spacing
032
Adjacent Channel Power
12.5 kHz
033
Adjacent Channel Power
20
or
25
kHz
035
Adjacent Channel Power
60 kHz
037
SSB Noise
-
by the Controller, keeps the IF signal within the linear range of the IF RMS Detector and increase the
dynamic measurement range to 95
dB.
Measurement
Bandwidth
8.5
kHz
16 kHz
30
kHz
2.5
kHz
At frequencies above 300 MHz, a low-noise, external LO is recommended for best performance. The
LO is conveniently switched by the LO Input Switch. (This feature is not allowed in combination with
Option 003, rear-panel LO connections.)
AM Demodulator
The
AM
Demodulator is an automatic leveling control (ALC) loop with a relatively slow response time.
The IF signal is amplified and detected by the AM and IF Average Detector, and the dc component of
the detected signal is compared to a stable, dc reference. If the dc voltage is different from the reference,
the difference is amplified by the ALC Feedback Amplifier which drives the Current-Variable Amplifier
to force the detected voltage to equal the reference.
Since the AM on the IF carrier is too fast for the ALC loop to respond to, the ALC loop produces an
ac voltage in the detector which is proportional to the AM. The ALC Bandwidth and Defeat line can
be used either to completely defeat the ALC action or to speed up the ALC response time in response
to variations in IF level (at the sacrifice of accuracy at low AM rates).
After demodulation, the recovered AM is filtered and processed by the Audio Circuits. The unfiltered
AM from the AM and IF Average Detector (containing both ac and dc components) is sent to the rear-
panel AM OUTPUT connector. The dc component from the detector is measured by the Voltmeter
(1)
to determine setting of the RF input attenuators when the ALC loop is switched off, (2) to determine
the IF Level in the IF Level measurement mode (where it is compared to the ALC reference).
FM Demodulator
The FM Demodulator consists of IF Limiters and an FM Discriminator (frequency-to-voltage
converter). The limiter-amplifier provides 66
d B
of gain and limits the output voltage swing to
approximately
1
Vpp to reduce the influence of AM and noise on FM measurements. The signal
from the limiters also drives a Counter input for measuring the IF frequency. The FM Discriminator
produces a voltage linearly proportional to the IF frequency, and the FM variations in the IF frequency
appear as an ac component on the output. The ac component is amplified, filtered, and then processed
by the Audio Circuits. The output from the FM Discriminator (with both ac and dc components)
is
sent to the rear-panel FM OUTPUT connector. The filtered dc component is used to tune the LO in
the track-tune mode.
Audio Circuits
The signal from the AM or FM Demodulators is filtered by a 260 kHz Low-Pass Filter. Before the audio
signal is measured or sent to the MODULATION OUTPUT/AUDIO INPUT connector,
it is
processed
further by various filters, amplifiers, and attenuators. For FM, the audio may also be de-emphasized. For
@M the signal
is
integrated. Factors which control the audio processing are; measurement mode, selected
features, audio level, input frequency, and any selected special functions. Table 8D-2 summarizes the
types of signal processing.
Service Sheet BD1
8D-3
Advertisement
