In my opinion you are expecting more of the transmitter than it was designed
to give. A carrier current transmitter wouldn't have to maintain the
broadcast standard of plus or minus 20 Hz. A drift of 200 Hz would never
have been noticed on an all American five radio. Given a strong received
signal beats with other stations on the same channel wouldn't be an issue
either.
Regards.
Max. K 4 O DS.
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----- Original Message -----
From: "Magnus Danielson" <[email protected]>
To: <[email protected]>
Sent: Friday, June 21, 2013 1:32 PM
Subject: Re: [time-nuts] Oscillator temperature compensation
On 06/21/2013 06:59 AM, Joseph Gray wrote:
Can you show some pictures of the oscillator?
The wiring is point-to-point, so I don't think a picture is going to tell
you much.
Is there a tunable inductor in the oscillator circuit?
As I mentioned, nothing tunable there.
Who makes the unit?
It is an LPB RC-6A carrier current AM transmitter. It was used at the
local
university many years ago. I was told they used to have several. I am
rescuing it from oblivion.
The closest to a schematic I find is this:
http://i30.photobucket.com/albums/c343/1073Dave/Schematics/LPBRC-5ATransmitter.jpg
Hooking a trim-pot to either of the caps next to the crystal should allow
you to trim it, should be a good start.
If you really need temperature compensation, a first degree compensation
should help. I haven't seen anything matching the X or Y cut crystal you
most probably have. The only plot I have for an X-cut shows a mostly
linear shift. A simple trimable first degree compensation should not be
too hard.
Otherwise you might just as well lock it up instead. Divide 10 MHz down to
20 kHz (divide by 500) and 660 kHz to 20 kHz (divide by 33) and then a
phase-comparator of choice. The varicap is just inserted under the foot of
one the caps around the crystal oscillator. Use a PI-active loop.
Cheers,
Magnus
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