"I recall seeing a self regulating
     thermistor based crystal heater
     years ago. It might have been from
     murata or CTS. It was a PTC thermistor
     with a clip that clipped it onto the
     crystal case. Apply a DC voltage ..."

Crude, but will work with the PTC thermistor
soldered to the xtal can ... had a VHF Engineering
Synthesizer II (144-148 MHz range with a 10.24 MHz
reference xtal osc) so equipped to ward of drift
seen during cold Michigan winters in the late
70's ... drifts of greater than 4 KHz (at channel
frequency or F-sub-o) were being experienced w/o
the PTC being added ...

As it turned out half of the drift was actually due
to the TTL INVERTER used as the 'active' element in
that xtal osc circuit - a single bipolar transistor
eliminated the amount of 'time delay' versus
temperature and the xtal circuit 'oscillated' 5 KHz
higher in frequency with the single transistor design
versus the TTL gate ... a new xtal was required
since an xtal at 10.24 MHz can't be 'pulled' quite
that far ...

I went through numerous cold-soaks in the freezer and
even several hot-cycles in a full size oven evaluating
each change ...

Jim


----- Original Message -----
From: "Harold M Hallikainen" <haroldhallikainen@JUNO.COM>
To: <PICLIST@MITVMA.MIT.EDU>
Sent: Wednesday, October 03, 2001 11:58 AM
Subject: Re: [EE]: crystal oven cct?


>         I recall seeing a self regulating thermistor based crystal heater
years
> ago. It might have been from murata or CTS. It was a PTC thermistor with
> a clip that clipped it onto the crystal case. Apply a DC voltage and it
> heats up to a specified temperature. Put the whole thing in an insulated
> can to reduce power dissipation.
>         Another trick is to use the LM399H voltage reference as a
termperature
> controlled heater. I don't know if the temperature is the best for the
> crystal (at the lowest tempco point), but the LM399H has a built-in
> heater and temperature controller so its voltage reference doesn't have
> to deal with varying temperature. I think you could cut the insulating
> case off the LM399H, glue it to the crystal case (or solder it?), then
> put the whole thing in an insulated case. Should hold the crystal
> temperature pretty stable.
>         On crystal ovens... back when I worked in broadcast stations, we
had a
> 950 MHz Studio to Transmitter Link that used a thermostat based crystal
> oven. One day the FM station dropped off the air. A trip to the site
> showed that the remote transmitter control had lost the 110 kHz control
> subcarrier off the STL. A look at the discriminator voltage showed
> SOMETHING was way off frequency (either the transmitter or the receiver),
> possibly causing the subcarrier to go outside the IF passband. Back to
> the STL transmitter site. Found the thermostat on the crystal oven had
> stuck, heating the crystal enough to knock the transmitter way off
> frequency. Disabled the oven until a replacement was received (this was
> one of those little octal based things that held two crystals). All was
> well...
>
> Harold
>
>
> On Fri, 28 Sep 2001 18:08:14 -0500 Scott Newell <newell@CEI.NET> writes:
> > >element. I would then mount the whole unit inside a block of foam
> > >insulation. I would like to hold the crystal at a non critical
> > >temperature between 35-50C. Any ideas?
> >
> > The temperature is critical.  You want to hold it at the turning
> > point of
> > the specific crystal where the Tc is zero.  Most common cheap AT cut
> > rocks
> > are ground to have their turning point near room temperature.
> >
> > I'd probably use a thermistor bridge circuit mounted in contact with
> > the
> > heating device.
> >
> > You might also read through:
> >         http://www.karlquist.com/osc.pdf
> >
> >
> > newell
> >

>

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