Agilent Technologies 856290216 service manual Sweep Generator Circuit

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 CRlOl, CR102, and CR103 should be reverse-biased.

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 CRlOl, CR102, and CR103 should all be turned on with about 7 mA of forward current.

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:

sweeptime = capacitance(C14) cufrInt

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