[time-nuts] The VE2ZAZ GPSDO
tractorb at ihug.co.nz
Sat Oct 21 06:21:04 EDT 2006
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.
----- 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 -
>> 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
> 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
> 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
> 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
> timing receiver.
> 4) Using an RS485 transceiver with its built in input attenuator
> degrade the SNR of the 10MHz clock.
> 5) Using the same buffer IC to buffer different frequencies will
> 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
> effectively radiate RF noise and harmonics.
> 8) The bypass capacitors on the supplies will be relatively
> There are GPS disciplined crystal oscillators available which have a
> 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
> GPS timing receivers that internally correct for the sawtooth error
> There are no Allan deviation plots for either the GPS derived PPS
> or for the OCXO.
> These are necessary for intelligent design.
> time-nuts mailing list
> time-nuts at febo.com
> No virus found in this incoming message.
> Checked by AVG Free Edition.
> Version: 7.1.408 / Virus Database: 268.13.8/489 - Release Date:
No virus found in this outgoing message.
Checked by AVG Free Edition.
Version: 7.1.408 / Virus Database: 268.13.8/489 - Release Date: 20/10/2006
More information about the time-nuts