.
>  "The original TXCOs were really a challenge since
>   you had to use analog circuitry to match the
>   tempco of the crystal and other circuitry"
.
Motorola mastered this technique late in their
"Motrac" series ("Motorola Transistor Research Line"
of 2-way FM commercial radios) - with the last ovenized
crystals used about 1965. All the newer product utilized
a technique in the form of little modules known as
"channel elements" versus discrete "crystals" placed
in small ovens.

To build these solid-state "channel elements" it was a
matter of characterizing each xtal and determining it's
"curve" (see Peter's postings) and selecting the
correct valued compensation cap to placed in series
with thermistors (with various characteristics) along
with other ceramic caps that had also known variation
over temperature ...

I've uesd the PTC thermistor myself - to keep a 10.24 MHz
ref osc on frequency over temperature in an automotive
install on a VHF Engineering 2-Meter synthesizer that gave
"frequency agility" to a 1965 "E"-series Motorola Motrac
in the mid-70's (predating the compact, Japanese synthesized
radios by a few years!) ...

RF Jim



----- Original Message -----
From: "Harold M Hallikainen" <haroldhallikainen@JUNO.COM>
To: <PICLIST@MITVMA.MIT.EDU>
Sent: Wednesday, August 14, 2002 4:32 PM
Subject: Re: [OT]: What Test Gear to measure xtal frequency to 1Hz


> On Wed, 14 Aug 2002 13:14:51 -0500 Jim <jvpoll@DALLAS.NET> writes:
> >    "oven-compensated crystal oscillator"
> >
> > 1) Do you know what this could do to your power budget?
> >
> > 2) How about required warm-up times after power-up before
> >    seeing those quoted accuracy specs?
> >
> >   RF Jim
>
>         I haven't done much RF work recently. Back when I was in
broadcast,
> where we're allowed about 20 ppm (20 Hz for AM, 2 kHz for FM), the
> crystal was often mounted in a glass envelope with a vacuum. They seemed
> to have no trouble staying on frequency. An AM would drift a few Hz
> months to month.
>         The 950 MHz studio to transmitter link we used had the transmit
crystal
> for the PLL reference in an oven. This worked great until the thermostat
> got stuck. The main transmitter dropped off the air because the STL
> transmitter frequency went far enough from the receiver frequency that we
> lost the 110 kHz control subcarrier.
>         I remember years ago seeing a PTC thermistor that clipped on a
crystal
> can. It would self heat to hold the crystal at temperature. I think it
> was from murata. Of course, it'd be best to hold the oscillator and the
> crystal at a constant temperature.
>         The original TXCOs were really a challenge since you had to use
analog
> circuitry to match the tempco of the crystal and other circuitry. Now, I
> expect they use an eeprom to hold a table of trim values to bring it back
> on frequency for each temperature in the range.
>
> Harold
>
>
>
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