[time-nuts] NavSync CW12 and CW25

David t_list_1_only at braw.co.uk
Thu Aug 7 16:50:02 EDT 2008


I've been playing with a CW12, it basically appears to do what they say but
I will not confirm any numbers for the moment, it certainly compares to my
VLF derived 10MHz signals. I was looking to build a GPSDO and 10MHz from the
GPS Rx seemed a better starting point than 1PPS.

I'm only now near to completing part two of the exercise, phase locking a
10MHz double oven XO to the CW12 10MHz, the phase noise on the CW12 output
is rather significant and needs cleaning for most applications. The ssb
numbers I see from the CW12 (using an HP8560E) relative to the 10MHz carrier
are :

1kHz -80 dBc/Hz
10kHz -102 dBc/Hz
100kHz -109 dBc/Hz
1MHz -126 dBc/Hz

The signal is far from a pure line, close in there is a set of sidebands
based on harmonics of 132 with the fundamental at -22dBc, there are various
synthesis artefacts. This unit clearly needs filtering.

The PLL I've built to clean it up is essentially a simple 2nd/3rd order loop
with the phase comparison at 78kHz (sigs divided by 128), my model claims
the 3dB closed bandwidth will be 0.3 Hz and sideband suppression about 127dB
at 1kHz offset in principle. In reality I'll need to play with it to look at
the real world limitations of acquisition, loop BW, analogue offsets etc .

The OCXO I'm locking is a Temex part, a 'Doc1478-D" which seems to be from
their S110 series. It claims to manage -145dBc at 1kHz offset. There were on
ebay at low cost a while ago, everything else has largely been built from
scrap which has biased the design ever so slightly, the less it costs the
more amusing the project if you are not doing it for a customer.

The slight disappointment has been the lack of support information about the
CW12 on the web, when I spoke to them in March they said their site was
going to be updated by the end of that month, clearly they have dropped the
ball somewhere but the email support seems responsive. I think I've seen the
CW25 appear in a recent GPSDO from Quartzlock, have a look at :


This appears to use a DDS/PLL to clean the reference clock rather than a
large and expensive OCXO.

Starting afresh I may have gone for the CW25 but I've got the CW12 so not
thought much more about it.

David Mackenzie (GM4HJQ)

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