[hpsdr] GPS 1PPS board

Mon Apr 6 16:26:35 PDT 2009

Hi John,

After thinking thru things a little bit and also redaing the post of Chris
(Albertson) I understand the point you both are making.

I think as a reasonable start, a good XO like the HP-10811 (see Alberto's /
my answer) and a simple cheap GPS like the Jupiteru UT+ for US$ 10 and a
simple control circuit ( 10^3 divider and PLL) to start with will be OK?
Later maybe improve on it by buying a really good GPS when they become
available for reasonable prices as surplus?

Thank you for explaining things!

73,

Henry.

> -----Original Message-----
> From: John Ackermann N8UR [mailto:jra at febo.com] 
> Sent: dinsdag 7 april 2009 0:39
> To: Henry Vredegoor
> Cc: 'Chris Salinas'; 'SDR'
> Subject: Re: [hpsdr] GPS 1PPS board
> 
> 
> Not really -- no matter what system you use, the short term stability 
> (less than 1000 seconds or so) will be dominated by the XO 
> performance. 
>   The GPS can't improve that.  What the GPS does is (slowly) steer to 
> the right frequency there, and hold it there over long time periods. 
> The magic is setting things up so that the GPS takes over at 
> the point 
> where its performance matches that of the XO.
> 
> It doesn't take any magic circuitry to do that -- G3RUH gets truly 
> amazing performance from a very simple analog loop circuit, 
> but he uses 
> a fairly expensive OCXO.
> 
> Most of the complexity in the fancy GPSDOs is to get what's called 
> holdover performance -- basically, to learn the XO's aging 
> rate and even 
> temperature response so that if the GPS signal is lost the system can 
> keep steering the frequency to hold the performance for a day 
> or more. 
>   That holdover performance isn't as big a deal now because there are 
> plenty of GPS satellites up there, so a design like the G3RUH 
> can just 
> ignore it.
> 
> John
> ----
> 
> Henry Vredegoor said the following on 04/06/2009 06:28 PM:
> > Hi John, All,
> > 
> > So if I understand you correctly, you trade a more 
> expensive, higher quality
> > (short term stability) XO and simple cheap GPS/locking 
> circuitry for a
> > (maybe-) less expensive, lesser quality XO and more complex, better
> > GPS/locking circuitry?
> > 
> > I am speaking of frequency control as primary use for HPSDR 
> and precision
> > timing as a nice bonus for secondary use.
> > I can however imagine that ADC sample time stamping for 
> sophisticated
> > receiving techniques could require more precise and higher 
> resolution
> > timing?
> > 
> > 73,
> > 
> > Henry.
> > 
> > 
> > 
> > 
> > 
> > 
> > 
> >> -----Original Message-----
> >> From: hpsdr-bounces at lists.hpsdr.org 
> >> [mailto:hpsdr-bounces at lists.hpsdr.org] On Behalf Of John 
> >> Ackermann N8UR
> >> Sent: maandag 6 april 2009 23:02
> >> To: Chris Salinas
> >> Cc: SDR
> >> Subject: Re: [hpsdr] GPS 1PPS board
> >>
> >>
> >> ***** High Performance Software Defined Radio Discussion List *****
> >>
> >> If I did the math correctly, that value is 1x10e-10.  That's 
> >> actually a 
> >> couple of orders of magnitude less than the accuracy of the GPS 
> >> constellation; it's generally accepted that averaging over a 
> >> day or so, 
> >> you can get to within parts in 10e-13.
> >>
> >> 1x10e-10 is a fairly common performance spec for simple 
> >> GPSDOs.  Better 
> >> ones, like the surplus HP and Trimble units many hams 
> have, are more 
> >> like 1x10e-12.  But both are way better than any HF or VHF 
> work could 
> >> ever require, so most of this discussion is in "time-nuts" 
> >> territory. :-)
> >>
> >> But the key thing is that any frequency system based on GPS has to 
> >> average over a long time period, usually 1000 seconds or 
> >> more, because 
> >> the jitter on the GPS signal is much worse than 1x10e-10.  
> >> Even really 
> >> good GPS timing receivers like the Motorola M12+ have about 30 
> >> nanoseconds of jitter.  A unit that has 1 microsecond of 
> >> jitter bounces 
> >> around by 1x10e-6, or 1 Hz at 1 Mhz, potentially every second.
> >>
> >> So, you need to average the signal using a PLL with a very 
> long time 
> >> constant, and use that to steer a crystal oscillator that is 
> >> stable over 
> >> shorter time periods, to make a workable frequency standard 
> >> out of GPS.
> >>
> >> And there the key is that a 10 kHz pulse rate makes the 
> PLL easier to 
> >> implement -- but in the end it won't be any more accurate 
> >> than the 1 PPS 
> >> signal because the 10 kHz pulses are themselves only 
> >> "disciplined" once 
> >> per second by the GPS engine.
> >>
> >> 73,
> >> John
> >> ----
> >>
> >> Chris Salinas wrote:
> >>> John,
> >>> I just read a website on this TU00 model and that they 
> measured the 
> >>> accuracy of +/- .000001 Hz at the 10 Khz pin. Too me it 
> >> sounds stable 
> >>> enough for a majority of work.
> >>>  
> >>> I ordered one yesterday just too play with and do some 
> >> measurements. Can 
> >>> always use it for APRS if it doesn't hold water.
> >>>  
> >>> I'll let you all know what I find.
> >>>  
> >>> Chris N0TTW
> >>>
> >>> --- On *Mon, 4/6/09, John Ackermann N8UR /<jra at febo.com>/* wrote:
> >>>
> >>>
> >>>     From: John Ackermann N8UR <jra at febo.com>
> >>>     Subject: Re: [hpsdr] GPS 1PPS board
> >>>     To: "Henry Vredegoor" <henry.vredegoor at gmail.com>
> >>>     Cc: "'Graham / KE9H'" <KE9H at austin.rr.com>, "'Chris Salinas'"
> >>>     <n0ttw at yahoo.com>, "'HPSDR discussion list'" 
> >> <hpsdr at lists.hpsdr.org>
> >>>     Date: Monday, April 6, 2009, 3:09 PM
> >>>
> >>>     I'd put it a little differently:  the 10kHz signal does 
> >> not make it
> >>>     possible to get *better* performance than a GPS with a 
> >> 1 PPS signal,
> >>>     but it allows much simpler circuitry since filtering a 
> >> 10 kHz signal
> >>>     is much easier than filtering 1 PPS, particularly if 
> >> using analog
> >>>     components.
> >>>
> >>>     However it's important to realize that the GPS part 
> of any GPSDO
> >>>     affects only the long term frequency stability.  When 
> >> measuring over
> >>>     time periods shorter than around 1000 seconds, the 
> >> performance of
> >>>     the XO is paramount.  One of the problems with 
> building a cheap
> >>>     GPSDO is that good XOs are expensive, and therefore the 
> >> short term
> >>>     stability is pretty much  tied to the builder's budget 
> >> or scrounging
> >>>     skills.
> >>>
> >>>     The control loop should (a) optimize the "cross-over" 
> >> point where
> >>>     the XO stability matches the GPS system stability; 
> and (b) avoid
> >>>     messing up the short term stability through excess 
> >> noise applied to
> >>>     the XO's tuning voltage.
> >>>
> >>>     John
> >>>     ----
> >>>
> >>>
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