[hpsdr] GPS 1PPS board

Henry Vredegoor henry_vredegoor at hotmail.com
Mon Apr 6 17:23:42 PDT 2009


Hi Graham, All,

It seems your post is just my final idea after the conversations tonight of
what could be a "beginners system" of a precision frequency source!! (I
didn't read it before posting my reply to John!)

And I share your fear of becoming a time-nut......

Is there an antidote?

  ;-)


73,

Henry.



> -----Original Message-----
> From: hpsdr-bounces at lists.hpsdr.org 
> [mailto:hpsdr-bounces at lists.hpsdr.org] On Behalf Of Graham / KE9H
> Sent: dinsdag 7 april 2009 1:05
> To: Chris Albertson; HPSDR discussion list
> Subject: Re: [hpsdr] GPS 1PPS board
> 
> 
> ***** High Performance Software Defined Radio Discussion List *****
> 
> Chris:
> 
> Yes that is the basic concept, and it will work, but then you need
> to start thinking about what accuracy you want and the effect of
> the jitter.
> 
> I have a homebrew GPSDO using a UT+ receiver, with about
> +/- 80 ns jitter on the 1 PPS signal, gating a simple counter,
> managing a 10 MHz HP10811 ovenized oscillator.
> 
> A cycle at 10 MHz is 100 ns, so 100 ns jitter is a variable error of
> up to one cycle on top of any other implementation errors.  
> Your receiver
> with 1 us jitter would be ten times as bad.
> 
> So with 100 ns jitter, you get a variable count error of +/- 1 cycles
> on top of whatever the real error is.
> 
> If you count for one second then the system will hover at 10 MHz
> plus minus 1 cycle, or about 0.1 ppm accuracy.
> 
> If you count for 10 seconds, then the system will hover at 
> +/- 0.1 cycles
> or 0.01 ppm accuracy.
> 
> I count for 2048 seconds and the system hovers at +/- 2 milliHz
> or 0.2 ppB accuracy.  Plenty good enough for what I need.  The
> HP oscillator ages at 5 milliHz per day, so plenty good enough to
> actually watch the crystal age over the course of a single day,
> and watch the system steer to correct it out.
> 
> But not good enough to win a frequency measuring contest, because
> those guys measure sub 1 milliHz.
> 
> But, measuring phase instead of simple counting would improve
> my accuracy by another order of magnitude.
> 
> Performing the granularity/sawtooth correction would add another
> order of magnitude of accuracy.
> 
> And there are probably better ways to steer it, rather than a
> simple proportional error correction in such a noisy system.
> Perhaps adding some trend analysis for the aging and mathematical
> averaging of the remaining noise in the correction signal.
> 
> The best GPSDOs are measured at several orders of magnitude
> better than mine.   Hmmmmm. The slippery slope to becoming
> a "Time Nut" is easy to slide down.
> 
> If you use your Rockwell receiver, depending on how you run it,
> it will likely be on the order of one magnitude worse than 
> mine, but your
> system might meet your needs just fine.  And for $9, you will
> have a lot of fun learning, and can upgrade anytime you want.
> 
> --- Graham / KE9H
> 
> ==
> 
> Chris Albertson wrote:
> > ***** High Performance Software Defined Radio Discussion List *****
> >
> > On Mon, Apr 6, 2009 at 9:11 AM, Graham / KE9H 
> <KE9H at austin.rr.com> wrote:
> >
> >   
> >> Although a cheap way to get started, this is a navigation 
> receiver, not a
> >> timing receiver,
> >> and it will not be a good choice for frequency control.  
> This one's spec is
> >> that the 1 pps
> >> signal has a jitter spec of +/- 1 microsecond.  Timing 
> receivers, such as
> >> the Motorola
> >> Oncore UT+ have a timing mode, that provides jitter around +/- 50
> >> nanoseconds,
> >> without sawtooth correction, and sub 10 nanoseconds with sawtooth
> >> correction.
> >>     
> >
> > It's not that bad, the purpose of the 1PPS signal is to correct for
> > long term drift by providing a direct real time link to a national
> > standard frequency reference.
> > What the controller might do is count how many times the signal from
> > your 10Mhz oven-ized
> > XTAL oscillator crosses zero in a one second period.   
> Subtract this count from
> > the expected number.  You will have a small positive or negitive
> > number.  The idea is that you very slowly adjust the oscillator so
> > that tens of thousands of these small numbers add up to 
> exactly zero.
> > This corrects for effects like the age of the crystal or the
> > temperature sensor on the crystal oven slowly drifting over time.
> > These effects are slow enough that you can average over 
> many thousands
> > seconds.
> >
> > Basically what you are doing is never "jerking" your 10Mhz OSC.  You
> > are instead using the 1PPS signal to slowly adjust the "faster or
> > slower" knob that we used to see on the back of clocks.
> >
> > Yes a better GPS unit will be better because your 10Mhz oscillator
> > will converge faster because it needs to average few 1 
> second periods.
> >   
> 
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