HP 8562E Service Manual page 420

10. Set a microwave source to the following settings:
Frequency . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2107GHz
Amplitude . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . -5dBm
11. Connect the microwave source to A15UlOOJl.
12. Measure the signal at U103 pin 1 using an active probe/spectrum analyzer combination,
13. If a 94.7 MHz signal, approximately -14 dBm, is present, but the signal at A15JlOl is
low, suspect U103.
14. When U104 pin 3 is at TTL low, U104 pin 6 should near -15 Vdc and PIN diodes
15. Set HP 85633 to the following settings:
Center frequency . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . , . . . . . . . . . 89.3 MHz
Span . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . OHz
16. Check that U104 pin 3 is at a TTL high and U104 pin 6 is greater than +7 V. PIN diodes
17. Disconnect the power splitter and reconnect W32 to A15JlOl.
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 ps to 2,000 seconds. The sweep times are generated in
two different ranges, a 50 ps to 30 ms range and a 50 ms to 2,000 second range. The 50 ~LS to
30 msec 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, U308, 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, Ql shorts out C16. The D to A converter U307, has
zero output current. U334A is a buffer with zero offset, because there is no current coming
out of U307. The buffering of U334 makes the base-emitter voltages on Q3A and Q3B 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 Q3B is 200 PA, therefore the
emitter current of Q3A is 200 /.LA and the sweep ramp is generated by C14. The sweep time is
given by the formula:
The DAC setting is increased for longer sweep times. This increases the current sunk by
the DAC output U307 pin 4, which increases the emitter voltage on &3A, decreasing the
base-emitter voltage drop. Q3A acts as an exponentiator and reduces its collector current,
creating a slower sweep ramp.
For the shorter sweep times, 50 ps to 30 ms, Ql is opened putting Cl6 in series with C14.
This changes the effective capacitance from 1 PF to 1,000 pF, or a reduction of 1,000 to 1.
11-46 Synthesizer Section
sweeptime = capacitance(C14) cufrInt