Re: Frequency reference for the K3 directly off a GPS

Posted by Sverre Holm (LA3ZA) on Sep 18, 2015; 9:49pm
URL: http://elecraft.85.s1.nabble.com/Frequency-reference-for-the-K3-directly-off-a-GPS-tp7606275p7607984.html

I have done some more experimenting now with my poor man's 10 MHz reference. The ublox Neo-7 GPS is a lousy frequency reference in most respects, except that it is accurate in keeping exactly 10 million periods per second. My experience is that the K3 has enough internal averaging to deal with that.

Using a ublox Neo-7 (USD 12 on Ebay) and a 74HC04 as a driver to feed the 10 MHz to the 50 ohm input of the K3EXREF the K3 accepts the input and I see the star in REF*CAL blinking.

The reference frequency now starts at 49,379,680 and stabilizes to 49,279,650 after some 15-20 minutes. My initial setting for the reference was 49,379,640 which I determined manually when the K3 was new in 2009 or so.

I am going to write this up on my blog with schematics eventually.

Sverre Holm (LA3ZA) wrote

Some recent GPS modules (ublox Neo 7) have a programmable output that in addition to the 1 pps output can be programmed even to output 10 MHz. This output has a lot of jitter as it seems to be derived from a 48 MHz clock which is divided down in such a way as to generate a mix of short and long cycles but so that the average frequency is maintained accurately at 10 MHz.

A description can be found here
https://sites.google.com/site/g4zfqradio/u-blox_neo-6-7 and performance is partly described here:
https://www.febo.com/pipermail/time-nuts/2014-August/086257.html

My question is regarding the K3 external reference input. Is this source good enough to drive the K3 reference? Can the K3 clean up the jitter internally, or does the K3 require a source with much less phase noise?