[hpsdr] GPS 1PPS board

[Henry Vredegoor]

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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