[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 -
> 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
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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