[time-nuts] VCXO center frequency offset

paul swed paulswedb at gmail.com
Fri Jan 20 13:32:35 EST 2012


Could be they are over 9 years old and I think FE only ever suggested 7
years or less.
Granted we run till they are dead. Then fix'em and run'em some more.
So with that in mind I noticed something on start up today after adding a
large heat sink. Nothing to do with what I saw.
When the 10 Mhz signal sweeps downward before lock I actually saw the
signal die for a fraction of a second in the sweep.
So perhaps what you are running into is indeed a failure mode for the worst
case.
Take the cap out and see what happens. (That should be a challenge its
awful small)
Regards
Paul
WB8TSL

On Fri, Jan 20, 2012 at 1:20 PM, beale <beale at bealecorner.com> wrote:

> Yes... if the C217 trimmer was not already at the very minimum setting! I
> was thinking of possibly removing it altogether, maybe adding back a
> smaller fixed C.  All three of my units are pretty low in VCXO center
> frequency, and in fact I wonder if that's the reason they were on the
> surplus market in the first place.
>
> >  -------Original Message-------
> >  From: Bob Camp <lists at rtty.us>
> >  Based on the frequency plot of your unit, it swings about 35 Hz high and
> >  about 170 Hz low as it sweeps. You could bump the trimmer on the VCXO
> and
> >  move it up by about 70 Hz or so to keep it from running out of range.
> Since
> >  the VCXO is trending down in frequency as it warms up, this may be
> needed
> >  sooner rather than later ...
> >
> >  -----Original Message-----
> >  Great detective work Scott! I'll bet Channel 4 is voltage to the VCXO.
> It
> >  is driven up and down at startup searching for the lock, then it settles
> >  down, but the oscillator components are also affected by temperature so
> the
> >  VCO voltage would logically track the temperature too.  At least it
> seems a
> >  reasonable theory.
> >
> >  For example, look at this plot of the 10 MHz output frequency after
> >  powerup. It has a similar look to your Channel 4, except for the
> post-lock
> >  drift.
> >
> https://plus.google.com/photos/109928236040342205185/albums/5680473650837554
> >  113/5681715799377076466?banner=pwa
> >
> >  -John Beale
>
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