HP 8340A Service Manual page 240

input to the Sample and Hold circuit is used directly to tune
and phase-lock the YO.
(a} When the instrument is in the CW/MANUAL mode or in the
swept mode for YO sweep widths less than 5 MHz. This
means that the YO is phase-locked during the entir� sweep
for sweep widths less than 5 MHz.
(b) Just prior to the sweep in the swept mode for sweep
widths greater than 5 MHz.
YO at the beginning of each sweep.
Hold Mode - This is used for sweep widths of greater than 5
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MHz. The YO is allowed to achieve phase-lock at the beginning
of each sweep.
causes the YO TUNE voltage to be held constant
while the YO is swept. This sequence is repeated at the
beginning of each YO sweep.
The mode of operation of U3 is selected by the control signal
LLEY (Low=Lock Enabled,
Block L. When this signal is a TTL LOW,
enabled. When LLEY is a TTL HIGH,
The sample and hold re,
which translates directly into improved YO sweep accuracy. R52 is
used to allow the IC to operate with the large 2.0 uF hold
capacitor, C24. CR7 and CRB are used to bypass R52 during large
signal operation
current-booster-charging circuit is also activated at this time.
C35 is a compensation capacitor required for stable operation
during large signal operation,
hold circuitry.
FM Coi1 Driver
The FM Coil Driver provides the tune signal for the YO FM Coil.
It consists of two parts:
100 Hz High Pass Filter.
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frequency portion
signal to be applied to the YO FM Coil.
on the A55 YO Driver board has a 100 Hz Low Pass Filter
applying only the low frequency portion of the error signal to
the Main Coil.}
(provided by C26,
GND.
Output Amplifier.
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frequency portion of the error signal to provide the drive
�324
Model 8340A - service
This occurs:
Afterwards the Hold mode is enabled which
YO) from the Logic Inverter/Translator,
U3, was chosen for its low droop rate,
(lock acquisition). An on-chip
and it also provides part .of the
I
This filter allows only the high
(greater than
There is also a ground transformation
R33, and R34) from Reference ground to FM
This circuit amplifies and filters the high
Scans by HB9HCA and HB9FSX
This tunes. arid phase-locks the
the sample mode is
the hold mode is enabled.
100 Hz) of the error voltage
(The YO Tune circuitry
(using C24)