From: [email protected] [mailto:[email protected]] On Behalf Of Jerry Gaffke via Groups.Io
Sent: Sunday, November 4, 2018 12:51 PM
To: [email protected]
Subject: Re: [BITX20] Si5351 correction question

?

Here's a randomly chosen datasheet for a 25mhz crystal out of Mouser:
? ??https://www.mouser.com/datasheet/2/96/008-0308-0-1160929.pdf
On page 3, it say the "Frequency Stability Tolerance" is +/- 50ppm
over an operating temperature range of -20C to +70C.
That's 50ppm/(20+70) = 0.555 ppm per degree C, or 0.555*9/5 = 1.0 ppm per degree F.
(I'm assuming frequency vs temperature is a linear relationship, should be close to correct.)

Mark was seeing his correction factor change by 1170 ppb, or 1.170ppm.
Using this crystal, we might see such a frequency shift when the temperature changes
by just over 1 degree Fahrenheit.

Mike said he saw 200hz of drift during warm-up while operating at 14mhz.
That's? ?1e6 * 200/14000000 = 14.3 ppm.
This particular crystal from Mouser could shift that much if the temperature inside the rig
changed by 14 degrees Fahrenheit (or 8 Centigrade).
Other (cheaper?) crystals can be worse in this respect.
How the crystal oscillator circuit inside the si5351 responds to temperature is also a factor,
this could add or subtract from that crystal frequency shift.

Anyways, use a TCXO controlled si5351 board if worried about stability.
Lots of the TCXO's available are digitally controlled and make adjustments in discrete jumps
of several hz, you want a good old fashoned analog TCXO for radio stuff.

A cheap solution might be to attach a temperature sensor out near the crystal and si5351.
The processor reads the temperature sensor and adjusts how it programs the si5351 accordingly.
The si5351 allows those adjustments to be made in increments of around 0.01ppm, or 0.14 hz at 20m.
Some of the other SiLabs parts (si5338, si5341, si570, ...) allow these increments to be
at least an order of magnitude smaller, to where there would be no perceivable jump.?

Jerry, KE7ER


On Sun, Nov 4, 2018 at 09:05 AM, Mark Pilant wrote:

Hi Jerry.

Makes me wonder how much of a temperature change is involved here.

This may be on target.

I did a little more testing this morning and a different correction number was
needed to get the correct frequency. Actually between the two from yesterday.

Two temperature factors may come into play. First, my "shop" is in the basement
and we had a fairly cool night. Second, everything had been powered off
overnight, so it was all from a cold start.


I just wanted to close the loop on this one.

73