Re my posting earlier this year about excessive jitter in my ICOM 706 Mk II
oscillator:
I've checked a number of 706 Mk II and 706 Mk IIG tcvrs, all with the
"hi-stab" osc. installed.
II's
Same jitter as mine. Seems to be two different jitters superimposed - one
square-wave pulse-like jumping of amplitude 30-50 Hz (at 145 MHz) of period
2-3 mins, the other almost a sinewave amplitude 20-30 Hz period 30 mins or
so. The amplitude and frequency of the square-wave is affected by the fan
and I could reduce it somewhat by wrapping polyfoam insulation round the
osc. Disconnecting the fan made amplitude smaller but did not eliminate
it. Both slightly different between different samples.
IIG's
One jitter only - a sawtooth of amplitude 5 Hz (at 145 MHz) with period of
2-3 minutes not linked to the operation of the internal fan. Pretty much
the same between samples.
The IIG jitter seems OK considering it would only be 0.005 Hz at 136 kHz
but the II would be 0.05 Hz which might or might not matter.
For ordinary HF SSB/CW operation or FM on 2m neither would matter of course.
Layout of the IIG oscillator/synthesizer board is quite different from the
II, also the TCXO for the IIG is much bigger and a different part number.
The TCXO for the II is claimed to be a "numerically-controlled" type, that
for the IIG is not. Does anyone know the difference? I thought all tcxo's
were just thermistor-controlled varactor diodes in series with the crystal.
Seems to indicate ICOM discovered the problem and had to redesign the board
to get rid of it. Actually there are also major differences in positioning
and layout of other boards and components to the extent that at a quick
look you might think they were two different machines.
Have also tested other machines including a Vertex FT-817 which was quite
remarkably stable - no jitter and only a slow thermal drift around 20 Hz
(at 145 MHz) between 15 and 25 degs Centigrade. It did not have the hi-stab
option. The best of the bunch by far was the ICOM 756 PRO Mk II which was
totally jitter-free and only drifted 1 Hz (at 50 MHz - does not cover 2m)
for an ambient change of 15-25 deg C. There is no hi-stab option for this
machine - doesn't need one by the look of it!. Anybody know if it
incorporates some drift-cancelling scheme like the Barlow-Wadley loop in
the old Racal RA17? Do any others?
Transceiver tests for the mags ought to include a stability check on the
oscillator in future.
Walter G3JKV.
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