Agilent Technologies 856290216 service manual MHz Reference, Q604 on

4.With the SIG ID Oscillator on, measure the frequency at A15X602 with a frequency counter and an active probe.

5.On the HP 85623, press (m) until A15J901 pin 13 is at TTL low. Diodes CR603 and CR605 should be forward biased and CR604 should be reverse biased (approximately 6 Vdc reverse bias). Diodes CR501 and CR502 should be forward-biased, disabling the 3rd LO Driver Amplifier.

6.The voltage at the R622/R623 node should measure approximately -5 Vdc, biasing

Q604 on.

7.If oscillator bias voltages are correct, place a 100 R resistor across SAWR U602 input and output. If the SAWR has failed, this will provide the equivalent loss of a correctly functioning SAWR, and the circuit will begin to oscillate.

10 MHz Reference

The spectrum analyzer 10 MHz reference consists of 10 MHz OCXO (Option 103: TCXO) with associated TTL level generator and distribution circuitry. The OCXO (or TCXO) and TTL level generator are turned off when an external 10 MHz reference is used. Also, with the analyzer set to EXTernal frequency reference, U304A output (low) forces the output of U304D to stay high. This allows U304B to control the outputs of U303B, U304C, and U303D. In INTernal frequency reference, U304D controls the outputs of these three NAND gates, and the output of U304B is held high.

Check the 10 MHz reference by performing the following steps:

1.Set the HP 85623 10 MHz reference to internal by pressing (mCTRL), REAR PANEL , and 10 MHz INT.

2.Use a spectrum analyzer to confirm the presence of a 10 MHz signal at the following test points:

A15J303 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ..L-lO dBm

A15J304 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ..~-lO dBm

A15J301 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ..>-2 dBm

3.Check for a 1.3 Vp-p waveform at A15J302 using an oscilloscope (see Figure 12-6).

4.Check that the signal at A15J301 is 10 MHz f40 Hz (with Option 103 TCXO reference) using a frequency counter. If necessary, perform the appropriate 10 MHz reference adjustment.

5.If there is no problem with INTernal 10 MHz reference operation, check EXTernal 10 MHz reference operation as follows:

6.Set the HP 85623 10 MHz reference to external by pressing 10 MHz EXT.

7.Connect a 10 MHz, -2 dBm, signal to the rear-panel 10 MHz REF IN/OUT connector.

8.Check the signals at A15J301, A15J302, A15J303, and A15J304 according to the procedure in steps 2 through 4.

9.If the signals are correct in EXTernal operation, but not in INTernal operation, the problem lies in A21 OCXO (or Option 103 TCXO), its voltage reference, or the TTL level generator. Check these areas as follows:

a. On the spectrum analyzer, press 10 MHz INT .

12-20 RF Section