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Model 8901B
Service
Audio Distortion Analyzer
The input to the Distortion Analyzer is selected by the Output Switch. The input can be either
the internal demodulated RF signal or an audio signal input
to
the front-panel MODULATION
OUTPUT/AUDIO INPUT connector. The distortion measurement is limited to audio signals with
a frequency of 400 Hz or
1
kHz.
The distortion measurement consists of measuring the rms level of the t o t a l audio signal then
remeasuring the signal after it passes through a notch filter that removes the fundamental of the
signal-leaving the distortion components, hum, and noise. Both measurements use the RMS-to-DC
Converter to obtain true r m s measurements. The dc level from the RMS-to-DC Converter is measured
by the Voltmeter. The Post-Notch Amplifier and the Audio Amplifier optimize the signal level into the
RMS-to-DC Converter. The Controller computes the distortion as the ratio of the output of the Notch
Filter to its input (with corrections made to account for amplifier gain). (In the SINAD measurement
mode, the reciprocal of distortion is computed.)
Audio Counter
The input to the Audio Counter is selected by the Output Switch. As with the Audio Distortion
Analyzer, the input can be either the internally demodulated RF signal or an audio signal input
to
the
front-panel MODULATION OUTPUT/AUDIO INPUT connector.
The audio signal is first conditioned by the Schmitt Trigger to make it compatible with the digital
circuits it drives. When a frequency count is to be made, the Controller, after clearing both the Counter
and Audio Counter and after opening the Audio Switch, arms the Counter Control Logic. The first
signal pulse from the Schmitt Trigger causes the Counter Control Logic to close the Audio Switch and
the Counter Gate switch (in the main Counter). The number of cycles of the audio signal are counted
by the Audio Counter. The number of time-base reference pulses are counted by the main Counter.
After a fixed period of time, the Controller readies the count to to stop. The next signal pulse from the
Schmitt Trigger causes the two switches to open. The counts in the two counters are then read back,
and the audio frequency is computed and displayed.
Local Oscillator
The heart of the LO is a 320 to 650 MHz High-Frequency, Voltage-Controlled Oscillator (HF VCO).
After passing through the programmable
LO
Divider, the HF
VCO
signal becomes the
LO
drive
to
the
Input Mixer. The LO Divider is programmed to divide the HF VCO by powers of two from
2-I
to 2*,
(that is, from a times 2 divide by 256). Thus the LO can tune from 1300 MHz to
1.25
MHz in ten
octave ranges. A fixed divide-by-eight output from the LO Dividers is the LO (HF VC0+8) input to
the Counter.
There are four tuning modes:
0
manual tuning and low noise,
0
automatic tuning and low noise,
0
automatic tuning and signal tracking,
0
manual tuning and signal tracking.
Manual Tuning and
Low
Noise. Consider the sequence followed
for manual tuning. When a frequency
is entered from the Keyboard, the LO is configured as in Figure 8D-1. The Digital-to-Analog Converter
(DAC) is connected to the HF VCO tune input as shown. Knowing the desired frequency, the Controller
computes the octave number (n) for the LO Divider and sets the DAC to its midrange. Then, an iterative
sequence
of
counting the LO and adjusting the DAC is carried out until the LO is near the correct
frequency.
Service Sheet BD1
8D-5
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