574 Chapter10
Synthesizer Section
A21 OCXO
A21 OCXO
The spectrum analyzer uses an oven-controlled crystal oscillator
(OCXO). It is deleted in Option 103 and replaced by a
temperature-compensated crystal oscillator (TCXO), located on the A15
RF assembly. Connectors J305 and J306 on the A15 RF assembly are
located where the TCXO would be installed in an Option 103. The oven
in the OCXO is powered only when the spectrum analyzer is powered
on; there is no standby mode of operation. The OCXO oscillator
operates only when the internal frequency reference is selected. Control
line HEXT (High = EXTernal frequency reference) is inverted by
A15U303B to generate LEXT. (Refer to the A15 RF assembly schematic
diagram, block M, sheet 2 of 4.) LEXT is sent to the OCXO via A15J306
pin 4. When LEXT is low, the oscillator in the OCXO will be turned off.
Replacement OCXOs are factory adjusted after a complete warmup and
after the specified aging rate has been achieved. Thus, readjustment
should typically not be necessary after OCXO replacement, and is
generally not recommended.
If adjustment is necessary, the spectrum analyzer must be on
continuously for a minimum of 24 hours immediately prior to oscillator
adjustment to allow both the temperature and frequency of the
oscillator to stabilize. Failure to allow sufficient stabilization time could
result in oscillator misadjustment.
Check operation of the A21 OCXO as follows:
1. Disconnect W49 (Coax 82) from A15J305. Connect the output of W49
to the input of another spectrum analyzer.
2. Check that the fundamental frequency is 10 MHz and that the
power level is 0 dBm ±3 dB. Also check that the harmonics are at
least −25 dBc. Excessive harmonics can generate spurious responses
on the fractional N oscillator on the A14 frequency control assembly.
3. If the OCXO has no output, check A15J306 pin 1 for +15 Vdc. Check
A15J306 pin 4 for a TTL-high level.
4. If A15J306 pin 4 is at a TTL-low level, press
AUX CTRL and REAR
PANEL. Press 10 MHz EXT INT until INT is underlined. A15J306 pin 4
should read a TTL-high level. Press
10 MHz EXT INT until EXT is
underlined. A15J306 pin 4 should read a TTL-low level.
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