Service Model
8901B
The Digital-to-Analog Converters (DACs) drive either the LF VCXO (through the LF VCXO Tune
Amplifier)
or
the HF VCO (through the DAC Control Amplifier and HF VCO Tune Integrator and
Amplifier). The DAC outputs a current proportional to the weighting of the bits of
its
digital input.
The amplifiers following the DAC convert the current into a tune voltage.
The
LF
VCXO Tune Filter filters the tune line of the LF VCXO to reduce phase noise in the low-noise
phase lock mode. The filtering
is
necessary because the tune-line input is external to the phase lock
loop.
The Sweep Down Current Source sweeps the HF VCO when the Controller searches for the input
signal. The Sweep Up Current Source is the retrace for the sweep.
The Track Loop Amplifier is used only in the track tune modes.
Its
input is the dc output from the
FM
Demodulator which
is
proportional to the IF center frequency.
If
the input signal changes frequency, the
HF VCO
is
tuned via the Track Loop Amplifier and HF VCO Tune Integrator and Amplifier (in
A23)
to keep the
IF
at a nominal
1.5
MHz.
(Track tuning is not permitted with the
455
kHz
IF.
Thus the
track mode
is
the only tuning mode where the LO "locks" to the input signal (that is, a frequency lock
loop is formed). The Track Loop Amplifier has a different gain for each
LO
range. This compensates
for
the change in
LO
tuning sensitivity caused by the LO Dividers.
8D-20
Service Sheet
BD2
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