HP 8562E, Service Manual

Page: 420 / 482

10. Set a microwave source to the following settings:
Frequency . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Amplitude . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . -5dBm
11. Connect the microwave source to
12. Measure the signal at pin 1 using an active probe/spectrum analyzer combination,
13. If a 94.7 MHz signal, approximately -14 is present, but the signal at is
low, suspect
14. When pin 3 is at TTL low, pin 6 should near -15 Vdc and PIN diodes
CR102, and CR103 should be reverse-biased.
15. Set HP 85633 to the following settings:
Center frequency . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . , . . . . . . . . . 89.3 MHz
Span . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
16. Check that pin 3 is at a TTL high and pin 6 is greater than V. PIN diodes
CR102, and CR103 should all be turned on with about 7 of forward current.
17. Disconnect the power splitter and reconnect W32 to
Sweep Generator Circuit
The sweep generator circuitry generates a ramp from 0 to 10 volts during the sweep time. The
available sweep times range from 50
to 2,000 seconds. The sweep times are generated in
two different ranges, a 50 to 30 ms range and a 50 ms to 2,000 second range. The 50 to
30 range is only needed for analog zero span sweeps.
The sweep generator is controlled with an 8-bit latch and the control signal HSCAN. The
latch, controls the sweep rate. HSCAN determines when to reset the scan ramp and
when to let it sweep.
Operation of the 50 ms to 2,000 second range will be described using a 50 ms sweep time as
the example. For a 50 ms sweep time,
shorts out C16. The D to A converter has
zero output current. is a buffer with zero offset, because there is no current coming
out of The buffering of U334 makes the base-emitter voltages on and the
same. These two transistors are matched, so their collector currents should be identical when
their base-emitter voltages are identical. The emitter current of
is 200 therefore the
emitter current of is 200 and the sweep ramp is generated by C14. The sweep time is
given by the formula:
sweeptime = capacitance(C14)
The DAC setting is increased for longer sweep times. This increases the current sunk by
the DAC output
pin 4, which increases the emitter voltage on decreasing the
base-emitter voltage drop. acts as an exponentiator and reduces its collector current,
creating a slower sweep ramp.
For the shorter sweep times,
50
to
30
ms, is opened putting Cl6 in series with C14.
This changes the effective capacitance from 1 to 1,000 or a reduction of 1,000 to 1.
11-46 Synthesizer Section