HP 8560A, Service Manual

Page: 396 / 450

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 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
oscillator operates only when the internal frequency
reference is selected. Control line HEXT (High = frequency reference) is inverted
by (Refer to the Al5 RF Assembly Schematic Diagram, Block M, sheet 2 of 4)
to generate LEXT. LEXT is sent to the OCXO via pin 4. When LEXT is low, the
oscillator in the OCXO will be turned off.
Replacement 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.
2.
3.
4.
Disconnect W49 (Coax 82) from Connect the output of W49 to the input of
another spectrum analyzer.
Check that the fundamental frequency is 10 MHz and that the power level is 0
Also check that the harmonics are at least -25 Excessive harmonics can
generate spurious responses on the roller oscillators on the Al4 Frequency Control
assembly.
If the OCXO has no output, check pin 1 for Vdc. Check pin 4 for a
TTL-high level.
If pin 4 is at a TTL-low level, press and REAR PANEL. Press
10 MHz EXT INT until INT is underlined. pin 4 should read a TTL-high level.
Press 10 MHz EXT INT until EXT is underlined. pin 4 should read a TTL-low
level.
Synthesizer Section