Agilent Technologies 8564EC, Service Manual

Page: 487 / 512

626 Chapter 12
RF Section
A15 RF Assembly
5. On the spectrum analyzer, press
SGL SWP
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
Ω
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 spectrum analyzer 10 MHz reference to internal by pressing
AUX CTRL
,
REAR PANEL
, and
10 MHz EXT INT
so that INT is
underlined.
2. Use a second spectrum analyzer to confirm the presence of a 10 MHz
signal at the following test points:
A15J303 ...................................................................
≥−
10 dBm
A15J304 ..................................................................
≥−
10 dBm
A15J301 ....................................................................
≥−
2 dBm
3. Check for a 1.3 Vp-p waveform at A15J302 using an oscilloscope. See
Figure 12-9 on page 627 .
4. Check that the signal at A15J301 is 10 MHz
±
40 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 spectrum analyzer 10 MHz reference to external by pressing
10 MHz EXT
.