Message-ID: <003601c297e1$c3ce2ce0$045bfea9@RSGB613192>
From: "Andy talbot" <G4JNT@THERSGB.NET>
To: rsgb_lf_group@blacksheep.org
References: <5.1.0.14.2.20021129180949.00a19cb0@mail.pncl.co.uk>
Date: Fri, 29 Nov 2002 19:59:00 -0000
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Subject: LF: Re: Osc drift
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I don't know about the IC756, but the IC746 derives all frequencies from one
master oscillator at 30MHz.  The high stability option for this is an
ovenned crystal, and takes several minutes to get to a good stability.  It
does not hunt at all, just stays absolutely flat.  I set mine to within
0.2ppm two years ago and it is holding exactly the same frequency now.

By the way Walter, thanks for the GPS module data - they keep getting
better.

Andy  G4JNT


----- Original Message -----
From: Walter Blanchard <blanch@pncl.co.uk>
To: <rsgb_lf_group@blacksheep.org>
Sent: Friday, November 29, 2002 7:02 PM
Subject: LF: Osc drift


> 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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