A21 Ocxo (Option 003 Only - HP 8560A Service Manual

A21 OCXO (Option 003 only)

Spectrum analyzers with Option 003, Precision Frequency Reference, use an oven-controlled
crystal oscillator (OCXO) instead of the standard temperature compensated crystal oscillator
(TCXO), 1oca e t d on the Al5 RF assembly. Jacks J305 and J306 on the Al5 RF assembly are
located where the TCXO would normally be installed.
The oven in the OCXO is powered only when the spectrum analyzer is powered on; there is no
standby mode of operation. The OCXO's oscillator operates only when the internal frequency
reference is selected. Control line HEXT (High = EXTernal frequency reference) is inverted
by A15U303B (Refer to the Al5 RF Assembly Schematic Diagram, Block M, sheet 2 of 4)
to generate LEXT. 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:
Disconnect W49 (Coax 82) from A155305. Connect the output of W49 to the input of
1.
another spectrum analyzer.
2.
Check that the fundamental frequency is 10 MHz and that the power level is 0 dBm
generate spurious responses on the roller oscillators on the Al4 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 [mCTRL) 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.