[time-nuts] Navsync CW-12 GPS Board - I don't believe it!!
ed_palmer at sasktel.net
Wed Jan 4 14:29:39 UTC 2012
I've never said that the CW12 was a GPSDO. What I expected to get was a
noisy NCO that had an average frequency of 10 MHz. What I got was a
worthless noise generator.
What's the point of saying that it's "steered by GPS" when it's
off-frequency. What does that even mean? Does it steer the frequency
to keep the error constant?
For some reason, this makes me think of the GPS systems that say "Turn
left now!" when you're in the middle of a bridge. :-)
On 1/4/2012 2:50 AM, SAIDJACK at aol.com wrote:
> These Conner Winfield (Navsync) parts are not GPS Disciplined Oscillators,
> many folks are initially fooled by this.
> They use digital phase hopping techniques to generate something close to
> 10MHz. You can see the 10MHz 100ns phase jump around on a scope.
> I believe their FTS250 and 125 modules use this output to drive a VCXO
> cleanup oscillator to try to remove the phase jumps, phase noise, and spurs.
> From their literature:
> "This ... module has an onboard, programmable NCO oscillator that outputs a
> synthesized frequency up to 30MHz steered by a GPS receiver"
> So it's a noisy digitally steered NCO, not a clean phase locked analog
> The frequency error can be up to +/-100ppb according to their literature
> while the unit is trying to align itself to GPS (that's a massive drift of
> 100ns per second!)
> I believe the uBlox 6T frequency output is generated in a similar manner.
> Many folks try these GPS NCO's because they are cheaper than real GPSDO's
> and seem to work similarly, then find out they don't perform as well...
> In a message dated 1/4/2012 00:32:33 Pacific Standard Time,
> hmurray at megapathdsl.net writes:
>> I compared the OCXO output to the 10 MHz output of both a Z3801A and a
>> Tbolt and discovered that the CW-12's 10 MHz output is about 1.5e-11 (i.e.1.5e-4 Hz) low in frequency. I emailed Navsync and they replied:
>> "The CW12 Motorola Binary and NMEA versions both do not phase align the
>> frequency output so the long term drift that Ed is seeing is expected."
> Is there a timing version that does the right thing?
>> Incredible. It's 'steered by the GPS receiver' - that's a direct quote
>> from the data sheet and the user manual - but it's not quite on frequency
>> and that's fine with them.
> Is it consistently off that much, or does it wander around the right
> (perhaps because there is a small dead band in the phase detector)
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