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Model 8901B
Service
The LF VCXO drives the Sampler at a nominal (but tunable) 2 MHz rate. The other input to the
Sampler is the HF VCO. The Sampler drives the HF VCO tune line through the Tune Integrator and
Amplifier. The HF VCO is thus phase locked to a harmonic of the LF VCXO, which greatly improves
the noise and frequency stability of the
HF
VCO.
Before closing the phase-lock-loop, the DAC is set near the low end of its range. When the loop
is
first
locked, the LO frequency is slightly low, but after an iterative sequence of counting the HF VCO and
tuning the LF VCXO by the DAC, the
LO
is brought to within 500 Hz of the desired frequency. During
the process of fine tuning the LO, the DAC may reach the end of its tuning range. If this happens,
the Controller will break the lock loop, set the DAC to the other end of its range, and lock will be
re-established to a different harmonic of the
LF
VCXO.
Automatic Tuning and
Low Noise. The automatic tune mode is similar to the manual tune mode
except the
LO
is first swept from the top to the bottom of each octave range by the Sweep Current
Source. See Figure 8D-3. If the LO sweeps past a signal at the
INPUT,
the down-converted signal
appears in the IF and
is
detected by the IF Detectors. The signal on the Stop Sweep line immediately
turns off the Sweep Current Source. With no input to the Tune Integrator and Amplifier, the HF VCO
will remain approximately tuned to the input signal, and the frequency of the
LO
(and thus the input)
can be determined by the Controller. Once the signal has been found after a sweep of all octaves, it
is found four more times by sweeping just the octave where it was first found and
two
octaves above
it. This is necessary in case the signal has AM which was in a deep trough when the fundamental
of
the
LO
passed through and was out of the trough when the strong third harmonic of the LO passed
through.
Figure 80-3.
LO
Configuration:
HF
VCO Sweep
Having now found an input signal, the Controller manipulates the LO through a series of tuning
sequences to search for the fundamental of the input that was found. Once the fundamental of the
input signal is identified, the LO is tuned to approximately
1.5
MHz above that signal. The Controller
then accurately counts the LO and the IF and thus determines the frequency of the input signal. (Signal
frequency = LO frequency
-
intermediate frequency.)
Service Sheet BD1
8D-7
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