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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