[600MRG] GPSDO?

[Hans Summers]

Hi Jim

In my opinion a 1pps signal is fine for GPSDO purposes. No need for 10kHz
which is hard to find these days.

In 2006/7 I designed a GPSDO project, using a 10MHz OCXO. You can read
about it here: http://www.hanssummers.com/gpsref . I used an Atmel AVR
processor programmed in assembler (now I would use C for it). I used a 1pps
signal from a cheap GPS module. The achieved accuracy of the 10MHz output
was 0.002Hz. Probably with more effort I could improve on that, but I left
it as it was since time is always short and that already seemed an
excellent result. Scaled down to 500kHz that would be +/-0.0001Hz accuracy!

Similarly in my Ultimate3 kit http://www.hanssummers.com/ultimate3 I use
the 1pps signal from a GPS module to calibrate the 125MHz reference
oscillator of the DDS.

One point to bear in mind when purchasing GPS modules is the accuracy of
the 1pps signal. Some of the modules using Sirf III or Sirf IV chipsets
appear to have rather inaccurate 1pps signals, of the order of 1us. Even
then, that won't matter much for 600m I'm sure, but leads to too many Hz of
uncertainty up on the 10m band. I was able to resolve this with software
methods similar to the old Huff Puff VFO stabilisation technique - see
http://www.hanssummers.com/ultimate3/ultimate3hp.html . But I think it is
better to use a higher precision GPS module to start with, and I am now
supplying the SKM52 http://www.hanssummers.com/ultimategps2 which is
specified to have a 10ns RMS accuracy on its 1pps signal.

73 Hans G0UPL
http://www.hanssummers.com

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Message: 2
Date: Sat, 22 Feb 2014 12:43:02 -0500
From: Jim Miller <jim at jtmiller.com>
To: "600mrg at w7ekb.com group" <600MRG at w7ekb.com>
Subject: [600MRG] GPSDO?
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I've been browsing a bit for GPSDO options. I'm not sure I really need one
so my first question is what sort of accuracy is really required for the
630m band using some of the codecs now in use or proposed?

I've just received my OCXO from Paul and I'll get it going this week. Is
stability all that is required or would it help to have accuracy as well?

The Thunderbolts and kin have gotten pretty pricey. Any affordable
recommendations welcome.

The Sparkfun site has lots of bits to play with GPS but none that I've
found offer 10Khz outputs rather just the 1pps outs. Any thoughts on
whether the 1pps might be suitable for locking a TXCO or OXCO with results
satisfactory for our needs?

Thanks

Jim ab3cv