HP 8340A, Руководство по обслуживанию

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Model 8340A - Service
(Block
B).
The input to the discriminator is used to trigger a
very stable 1.6 microsecond pulse. The current source is
activated by this pulse, and outputs a fixed amount of current
for 1.6 microseconds. These current pulses are averaged to yield
some net current flow that is proportional to the frequency of
the input to the discriminator� the higher the frequency, the
more frequent the 1.6 microsecond pulses, and the greater the
average current. The gain of the discriminator (current
out/frequency in) can be adjusted by changing the magnitude of
the current pulses, but the width will always be a constant 1.6
microsecconds.
PULSE GENERATOR A
The PULSE GENERATOR functions as a one shot multivibrator.
However, in a standard multivibrator the pulse width is
determined by an RC time constant and the trigger point may vary
with circuit components and also with noise. In this case, pulse
width (about 1.6 microseconds) repeatability is critical. The
signal at TP2 then is a 1.6 microsecond pulse, with a repetition
rate equivalent to the input frequency. This signal will be
integrated in the Current Source (Block
B)
to produce an average
current related to the input frequency. The 150 to 300 kHz (ECL
level) input from the A40 PLL2 VCO assembly is amplified and
level shifted by Q6 and Q7. At the begining of a cycle, both
inputs to U7D are LOW. When Q7 collector goes HIGH, U7D output
goes LOW saturating Qll and causing resonator L4, ClS, and Cl6 to
ring at 5.2 MHz. This damped oscillation appears at Q9 collector,
is clipped by QlO, and used to drive counter U6.
QlO is a comparator. The signal appears across LS driving the
base of QlOA and QlOB in opposite directions. U6 is preset to a
count of 6. When the count reaches 8, the "8" output of U6 goes
high. This is fed back to U7D pin 11, holding its output LOW
•
After 1.6 microseconds, the count reaches 16, the 8 output goes
LOW again, U7D output goes HIGH turning Qll OFF resulting in the
resonance being damped by Rl7. At the count of 16, U7C and U7B
reset the counter to 5. Before the oscillation is fully damped,
the counter gets clocked to 6. U6 should always be reset to
either 5 or 6 depending upon how quickly Qll can dampen the
resonator. The actual number is not relevant, since the pulse
output starts at the count of 8, effectively ignoring the first
several pulses, and the width will always be a constant 1.6
microseconds.
U7D output going LOW starts the resonator ringing. If for some
reason the counter did not reach an 8 count, U7D output would be
held low causing a latch-up condition. When U7D output is HIGH,
U7A output turns CRl ON discharging Cl4. When U7D output goes
LOW, CRl is reversed biased and Cl4 starts to charge through U6
8-262
Scans by HB9HCA and HB9FSX