[time-nuts] questions on uncompensated crystal oscillators
SAIDJACK at aol.com
SAIDJACK at aol.com
Wed Jul 5 14:12:48 EDT 2006
In a message dated 7/5/2006 08:30:44 Pacific Daylight Time,
johnday at wordsnimages.com writes:
I would agree with you Said, 10ppm is a lot to ask and let's be
honest, production trimming is not so difficult these days. Without
going to 10ppm you can get your crystal manufacturer to group your
crystals by cut angle. This will give you a basic temperature
coefficient curve - for a given angle the tempco curves are quite
similar. We used to do this with fundamental mode crystals at between
12 and 20MHz.
You can then have a set of compensation tables and drive the varicaps
from a DAC to keep the crystal under control. I have done this using
small micro's, like the Zilog Z8, or using a bit of really crude
ADC-Table-DAC circuitry in an FPGA and it works quite well. This was
done to avoid having to use TCXO's in SCADA/DATA type radios.
For higher performance equipment we used to have the compensation
table built by the test system during temperature cycling.
John
Hi John,
great idea to compensate for temperature using the varicaps in a
free-running system.
The basic design was for locking an MPEG video stream to a broadcaster's
27MHz master clock using the MPEG time stamps in a digital PLL loop.
When free-running (eg playing back video from a hard disk etc), the
temperature compensation should work great to keep the 27MHz within specs.
bye,
Said
More information about the time-nuts
mailing list