[Collins] Re: {Collins] Colpitts oscillator

[Richard Keller]

Right Jerry, I forgot the "L" in the formula for Fo (sq root LC) which just
compounds the problem. Unfortunately the MFR did not provide tuning slugs.
The coils assy was adjusted and glued, then waxed at the factory. You may
have hit the nail on the head, though my options are limited.

I spent many hours yesterday taking readings at various frequencies and
various ranges, the result adds more to the original findings. It is just
slightly high in freq at the low end of the dial on each range and almost
right on in the middle of the dial. At the high end of each range, the freq
is about 2-3% low. Sounds like the bandspread is not good but considering
it is a generator of just average accuracy (50 years ago by the way) and
not a high quality receiver or professional lab instrument, I guess the
final answer would be to just live with it. I must keep in mind that it is
quite old and although thecomponents are still of close tolerance, it would
take a complete re-engineering to improve it.

I guess that sometimes the technician in me wants to tweak everything and
make it better. Sometimes it is more work than it is worth. With a freq
counter I can get very close to the desired freq and with the generator's
fine stability, tests can be done on most any ham equipment up to the mid
90's in origin.

Thanks for the advice. It is greatly appreciated.
73, Dick KF4NS
St. Petersburg, FL 33714 USA
Keep the glow!

Message: 2
Date: Wed, 1 Sep 2004 11:42:17 -0500
From: Dr.Gerald Johnson <geraldj at ispwest.com>
Subject: [Collins] Re: {Collins] Colpitts oscillator
To: rkeller at ij.net
Cc: collins at mailman.qth.net
Message-ID: <SAK.2004.09.01.dtrftkha at default>
Content-Type: Text/Plain; charset=ISO-8859-1

Its not necessarily excess C that keeps the frequency down, it could be
excess L. Generally when tracking, its important to set the slugs in the
coils at
the LF end of the scale first. Because the variable capacitor is at or near
maximum, the trimmer C has least effect. Then use the trimmer to set the
high end. There will be some interaction so you repeat.

You can add a capacitor in series with the variable, but then you sacrifice
its maximum range rather drastically because to have effect at the HF end
of the tracking range, the capacitance of the capacitor is small, maybe 20
pf, so to cut that down significantly you might need a 20 pf series
capacitor.
Then at the 400 pf end you still would have that 20 pf capacitor and the
effective tuning C would be only 19 pf. So you would have reduced the C
swing from 20 to 400 pf down to 10 to 19 pf. That won't cover more than a
1.4:1 frequency range where it did wider before.

It might be necessary to reduce stray C by shortening leads around the
switch and the active device.

73, Jerry, K0CQ, Technical Advisor to the CRA

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Entire content copyright Dr. Gerald N. Johnson, electrical engineer.
Reproduction by permission only.