[time-nuts] Navsync CW-12 GPS Board - I don't believe it!!

[SAIDJACK at aol.com]

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 
oscillator.
 
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  
answer?  
(perhaps because there is a small dead band in the phase  detector)


-- 
These are my opinions, not necessarily my  employer's.  I hate  spam.