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" <[EMAIL PROTECTED]> To: "Discussion of precise time and frequency measurement" <time-nuts@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 > > _______________________________________________ > time-nuts mailing list > time-nuts@febo.com > https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > > > > -- > No virus found in this incoming message. > Checked by AVG Free Edition. > Version: 7.1.408 / Virus Database: 268.13.8/489 - Release Date: > 20/10/2006 > > -- 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 _______________________________________________ time-nuts mailing list time-nuts@febo.com https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
