[time-nuts] The VE2ZAZ GPSDO

Dr Bruce Griffiths bruce.griffiths at xtra.co.nz
Sat Oct 21 00:00:43 EDT 2006

Dave Brown wrote:
> Interested in any comments on this GPSDO -
> http://www3.sympatico.ca/b.zauhar/GPS_Std/GPS_Std.htm#GPS_Receivers
> DaveB, NZ
Why anyone would bother to create something like this escapes me.
Just another example of unintelligent engineering.
If one is foolish enough to insist on using a non timing GPS receiver 
then one cant really expect too much in the way of stability.
A good GPS timing receiver isn't that much more expensive and the 
improved performance more than justifies the extra cost.

Since it is not clear how the frequency was measured any conclusions 
must be somewhat tentative.
1) The frequency stability over a few hours appears worse than that of 
the crystal alone.
2) The short term DAC stability needs to be very high -low pass 
filtering the PWM output of the PIC is inadequate the switching levels 
of the PWM output need to be tightly regulated.
3) The DAC resolution needs to be very high (> 20 bits) for the 
frequency stability to approach the limits set by a good crystal and GPS 
timing receiver.
4) Using an RS485 transceiver with its built in input attenuator will 
degrade the SNR of the 10MHz  clock.
5) Using the same buffer IC to buffer different frequencies will produce 
undesired phase modulation of its outputs.
6) Using a ripple counter without resynchronising its output signals 
will phase modulate the divided down outputs.
7) The apparent absence of a ground plane should allow the circuit to 
effectively radiate RF noise and harmonics.
8) The bypass capacitors on the supplies will be relatively ineffective.

There are GPS disciplined crystal oscillators available which have a 1s 
Allan deviation of 2E-11(8E-11 @1000s, 1E-12 @ 1day), the performance of 
this circuit falls woefully short of this.

No allowance appears to have been made for weeding out spurious 
measurements. What happens if a PPS pulse is missing or has an 
abnormally large timing error?

Degrading the resolution by ignoring sawtooth timing corrections and 
using a 100ns resolution timer to measure the PPS pulse position 
relative to an internal (10/2^16) MHz clock just throws away the 
inherent timing precision (~10ns or better) of a good timing GPS receiver.
GPS timing receivers that internally correct for the sawtooth error are 

There are no Allan deviation plots for either the GPS derived PPS signal 
or for the OCXO.
These are necessary for intelligent design.


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