[time-nuts] Looking for GPSDO for home use

Tom Van Baak tvb at LeapSecond.com
Thu Apr 17 17:14:27 EDT 2014


> Having done extensive testing on the ublox 6M that you can buy with antenna 
> for less than $ 23 new from DX.com I can say with certainty that the 1 Hz 
> pulse  is better than 1 E-9 accurate, closer to E-10. Depending on the 

Hi Bert,

There are lots of cheap GPS/1PPS receivers out there these days. Having tested many GPS receivers myself, let me help you out. I understand the number $23. But the number "1 E-9" by itself means nothing. Like, are you talking time or frequency? There's a big difference.

Tell us if your number is timing accuracy or units of frequency accuracy or stability. Also either way, the measurement interval (or tau) is critical:

a) If it's timing, then the units are usually seconds; e.g., 1 ns, peak-to-peak, or rms or even something like TDEV(tau) -- and tau is important.

b) If it's frequency, then the units of absolute frequency accuracy or relative frequency stability; e.g., 0.01 Hz out of 10 MHz over some tau, or MDEV/ADEV(tau) -- and tau is critical.

I say this because:

a) If the 1e-9 or 1e-10 number you quote is rms time accuracy, then you have a GPS receiver better than what NIST or USNO has!

b) If the 1e-9 or 1e-10 number you quote is frequency stability over a day, then you have the worst GPS receiver we've ever seen!

So please clarify your units, or share raw data, or plots, or something. It's rather important.

/tvb




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