HP 8562E, Service Manual

Page: 424 / 482

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 Al5 RF assembly. Connectors and on the Al5 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 =
frequency reference) is inverted by to generate LEXT. (Refer to
the Al5 RF assembly schematic diagram, block M, sheet 2 of 4.) 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. Disconnect W49 (Coax 82) from 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
Also check that the harmonics are at least -25 Excessive harmonics can
generate spurious responses on the fractional N oscillator on the Al4 frequency control
assembly.
3. If the OCXO has no output, check pin 1 for Vdc. Check pin 4 for a
TTL-high level.
4. If pin 4 is at a TTL-low level, press and . Press
10
EXT
until INT is underlined. pin 4 should read a TTL-high level.
Press 10 until EXT is underlined. pin 4 should read a TTL-low
level.
1 l-50 Synthesizer Section