[time-nuts] The VE2ZAZ GPSDO

[Dave Brown]

Tks Bruce

I did wonder why on earth he didn't use a timing rx-and that raised 
questions re the remainder of the design, but I've not had a chance to 
read it through completely as yet. My initial impression was that too 
many corners were being cut. Said's comment re the 7805 tempco was 
another one I hadn't thought about.  Should be able to better that 
without too much effort.

DaveB

----- Original Message ----- 
From: "Dr Bruce Griffiths" <bruce.griffiths at xtra.co.nz>
To: "Discussion of precise time and frequency measurement" 
<time-nuts at febo.com>
Sent: Saturday, October 21, 2006 5:00 PM
Subject: Re: [time-nuts] The VE2ZAZ GPSDO


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