HP 8340A Service Manual page 190

pin 14 towards Vee until the U6 input circuit trips,
and preventing a latch-up condition.
The pulse width at TP2 should be equivalant to eight pulses at
TP3.
CURRENT SOURCE
With the tuning voltage input
voltage divider from -7V through R6,
input to U4 pin 3 at about +13V. U4 and Q4 form a voltage
controlled current source,
flowing out of Q4 drain is proportional to the voltage on U4 pin
3. The ratio of current to voltage is calibrated by R9,
Adjust. The output of the current source flows through Q5A when
the "8" output of U6 is LOW. If an input pulse has triggered the
pulse generator,
microseconds. Since Q4 and its associated circuit is a constant
current source,
leaving no current for Q5B. When Q5A is shut OFF all the current
flows through Q5B. Q5B current is integrated by C20 and goes
through a low-pass filter to the summing point of the
discriminator loop. The average value of this current is directly
proportional to the input frequency,
MHz. The 0.3 MHz Adjust
the discriminator gain by changing the amplitude of the 1.6
microsecond current pulses into the filter.
The tuning voltage input
Detector (Pl pin 28)
discriminator by +1% by changing the voltage at U4 pin 3,
thereby changing the amplitude of the 1.6 microsecond current
pulses.
C21 is used to stabilize the two grounds when Q5A/B is switching.
-7V SUPPLY C
Ul3 maintains a constant current through reference diode CRl. R35
sets this current to approximately 7.7mA. The 6.3V reference is
amplified due to the ratio of R33 and R34 to yield -7V. Q8
provides a low impedance,
PRETUNE D
The pre-tune circuit tunes the discriminator loop approximately
to the desired frequency. The phase lock loop then applies small
corrections to get the frequency exact. The A60 Processor outputs
a 10-bit binary word representing the pretune frequency and then
strobes the data into latches U9 and Ul2 by outputting address
Model 8340A - Service
B
(Pl pin 28)
where the magnitude of the current
the current is switched through Q5B for 1.6
when Q5A is ON all the current flows through Q5A,
(R9) in the U4 feedback loop adjusts the
(N2 Tune)
is also able to adjust the gain of the
high current output.
Scans by HB9HCA and HB9FSX
at zero volts,
Q3A/B, R7 to +20V sets the
being about 1.5 mA at 0.3
from the A41 P LL2 Phase
reseting U6
the
.3 MHz
and
8-263