[time-nuts] time-nuts Digest, Vol 41, Issue 36
Scott Dennis
sddengineer at gmail.com
Tue Dec 11 19:22:19 EST 2007
To TVB: Really enjoyed the article! Especially the part about the GREAT
experiment. But it was nice just to see your picture after all these
years. Think I'll camp out here and see what kinds of esoterica you guys
come up with here.
--Scott Dennis AL7EM
P.S. Yes, I did eventually graduate.
On Dec 11, 2007 3:59 PM, <time-nuts-request at febo.com> wrote:
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> Today's Topics:
>
> 1. Re: is there a "best bet" advanced hobbyist buildable
> GPSDOdesign? (Scott Burris)
> 2. Re: need recomendation for a portable 10mhz reference
> oscilator (John Ackermann N8UR)
> 3. Re: Fury Interface Board simulation results (Bruce Griffiths)
> 4. Re: WIRED Time Nuts (Keith E. Brandt, M.D.)
>
>
> ----------------------------------------------------------------------
>
> Message: 1
> Date: Tue, 11 Dec 2007 15:38:31 -0800
> From: "Scott Burris" <slburris at gmail.com>
> Subject: Re: [time-nuts] is there a "best bet" advanced hobbyist
> buildable GPSDOdesign?
> To: "Discussion of precise time and frequency measurement"
> <time-nuts at febo.com>
> Message-ID:
> <4f3d38470712111538k5f5cbcd3l3ba39433f0a660b6 at mail.gmail.com>
> Content-Type: text/plain; charset=ISO-8859-1
>
> On Dec 11, 2007 12:53 PM, Bruce Griffiths <bruce.griffiths at xtra.co.nz>
> wrote:
>
> > Tom Van Baak wrote:
> > >> Hi,
> > >>
> > >> Like many, I've acquired a fair amount of surplus test equipment off
> of
> > Ebay
> > >> which could use the services of good master frequency standard. So
> I'm
> > >> looking to discipline an HP 10811 VXCO to provide this.
> > >>
> > >> Any general consensus about the best design for a hobbyist to build?
> > >> I'm familiar with the Brooks Shera design, the G4JNT Jupiter-T
> design,
> > >> the TAC-2 circuit, and the VE2ZAZ design. I take it from discussions
> > >> I've seen in the archives of this list that the VE2ZAZ design makes a
> > >> number of simplification/performance tradeoffs.
> > >>
> > >> Is there a design I haven't listed which is "better" than the others?
> > >> I'm quite familiar with microcontrollers, FPGAs, spinning my own
> > >> PCBs, etc, so I'll roll my own if I have to, but I'd prefer to build
> > >> a variation on someone's tried and true design.
> > >>
> > >> I'm aware of products like the Fury, but I'd like something I could
> > tinker
> > >> with, and the cost is hard to justify for a hobbyist.
> > >>
> > >> Scott
> > >>
> > >
> > > Scott,
> > >
> > > Hard to say which is better at this point; there are a number
> > > of variables, not the least of which is the intrinsic short-term
> > > stability of the OCXO you use.
> > >
> > > Do have a close look at James Miller's GPSDO:
> > >
> > > http://www.jrmiller.demon.co.uk/projects/freqstd/frqstd.htm
> > >
> > > I recently tested one and it makes it to 1e-13 at one day, which
> > > is really nice for a simple, cheap, homebrew GPSDO.
> > >
> > > /tvb
> > >
> > >
> > > _______________________________________________
> > > time-nuts mailing list -- time-nuts at febo.com
> > > To unsubscribe, go to
> > https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
> > > and follow the instructions there.
> > >
> > >
> > Tom
> >
> > What about the short term performance?
> > Its relatively easy to achieve a stability of 1E-13 for an averaging
> > time of 1 day, achieving good short or medium term stability is more
> > difficult.
> >
> > If you want simplicity and higher performance you can do far better with
> > fewer parts,
> > An expensive high resolution DAC can be replaced with a software
> > sigma-delta DAC that has higher resolution.
> > The complex phase detector can be replaced with a D flipflop.
> > Add a microprocessor plus an opamp or 2 to filter and scale the EFC
> > voltage and thats about all thats required in addition to a good GPS
> > timing receiver.
> > For improved performance a hardware circuit to correct the PPS sawtooth
> > error will improve the medium term stability significantly when using a
> > high performance GPS timing receiver that provides an estimate of this
> > error.
> >
> > Both the Brooks Shera and the James Miller designs have inadequate phase
> > error measurement resolution to achieve good short and medium term
> > stability.
> > However, this is only noticeable when using high performance GPS timing
> > receivers (M12+T, M12MT etc) and a high quality OCXO (10811A etc).
> >
> > Bruce
> >
> > _______________________________________________
> >
>
> OK, so for the DAC piece, why not just use an NXP LPC ARM chip for the
> microcontroller, and use a 32bit PCM output followed by a low pass filter
> as
> the VXCO EFC? The DAC just needs high resolution, not accuracy, right?
> Or would the switching noise from the processor modulate the control
> voltage?
> I would hope the filter would clean any such noise, but I'll be the first
> to
> admit
> that the farther we get into the analog domain, the more I'm out of my
> comfort zone.
>
> I'm still trying to wrap my brain around the phase detection piece of
> this.
> I've studied the Shera controller with it's 24Mhz oscillator and divided
> down
> sample of the VXCO and I'm can't get past thinking that this ends up
> adding jitter. With more modern parts can't the phase be measured more
> directly? What about sampling both the VXCO and 1PPS at a 200MHZ rate?
> That should determine the phase difference within no more than a 10ns
> inaccuracy.
>
> Or use a pulse stretching technique to amplify the short time intervals
> into
> something
> more easily measured, although that's beyond what I'm familiar with.
>
> I've read the PTTI presentation about using a DS1020 delay line to
> de-sawtooth the
> 1PPS signal -- that's a pretty interesting idea. At least the chip is
> available in Qty 1,
> at $30!
>
> It just seems that the designs I've seen could use a refresh with some
> more
> modern
> circuitry. At the very least the Shera controller could have much of its
> logic put into
> a single CPLD these days.
>
> Scott
>
>
> ------------------------------
>
> Message: 2
> Date: Tue, 11 Dec 2007 18:38:55 -0500
> From: John Ackermann N8UR <jra at febo.com>
> Subject: Re: [time-nuts] need recomendation for a portable 10mhz
> reference oscilator
> To: Discussion of precise time and frequency measurement
> <time-nuts at febo.com>
> Message-ID: <475F1F8F.7050604 at febo.com>
> Content-Type: text/plain; charset=us-ascii
>
> Hal Murray said the following on 12/11/2007 05:57 PM:
> >> As great as GPSDO are, the problem with using them in a portable
> >> environment is the time it takes to do a survey and then get to final
> >> lock; mountain-toppers may not be in one place long enough.
> >
> > What if you use a navigation GPS unit rather than a timing unit?
>
> To get decent (for some definition of "decent") timing, you need to
> operate the receiver in "0-D" mode, telling it where it is and letting
> it solve just for time. I haven't done the experiments myself but I
> believe that the time accuracy is pretty dramatically degraded when
> operating in 2-D or 3-D mode.
>
> > How long does a survey take (in good conditions) and what sort of time
> > constant would you want to use with a navigation box?
>
> It depends on the receiver, but usually it's a few hours to a full day.
> Of course, if you use external software you can use whatever time
> period you want.
>
> John
>
> > Does anybody have graphs of the PPS wander from a navigation unit?
> >
> >
> > I like the lock-to-rubidium suggestion.
> >
> >
>
>
>
>
> ------------------------------
>
> Message: 3
> Date: Wed, 12 Dec 2007 12:42:46 +1300
> From: Bruce Griffiths <bruce.griffiths at xtra.co.nz>
> Subject: Re: [time-nuts] Fury Interface Board simulation results
> To: Discussion of precise time and frequency measurement
> <time-nuts at febo.com>
> Message-ID: <475F2076.7000000 at xtra.co.nz>
> Content-Type: text/plain; charset="iso-8859-1"
>
> George
>
> The circuit schematic for a BJT version of the JFET frequency is attached.
> The biasing is a little more complex as it is necessary set each
> frequency doubler BJT collector current at about 1mA or so to maximise
> conversion gain.
> The input impedance is also around 50 ohms.
> Have also minimised the number of different component values used.
> Other transistors may be substituted but some care is required as the
> reverse voltage across the base emitter junction is about 2V for a
> +13dBm input.
> Some filtering of the output waveform is required.
> The intended output load is 50 ohms.
>
> The 200 ohm emitter series resistors together with the transformed
> source impedance ensure that there is sufficient degeneration to keep
> the phase noise low.
>
> Obtaining suitable transistors for this circuit should be much easier
> than obtaining suitable JFETS for the JFET doubler.
>
> Whilst a diode doubler (at higher frequencies this may be the only
> sensible option) could be used it would need both an input amplifier and
> an output amplifier.
>
> Bruce
> -------------- next part --------------
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> ------------------------------
>
> Message: 4
> Date: Tue, 11 Dec 2007 17:58:53 -0600
> From: "Keith E. Brandt, M.D." <wd9get at amsat.org>
> Subject: Re: [time-nuts] WIRED Time Nuts
> To: Discussion of precise time and frequency measurement
> <time-nuts at febo.com>
> Message-ID: <20071211235910.744D01BCB33 at mail7.mdx.safepages.com>
> Content-Type: text/plain; charset="us-ascii"; format=flowed
>
> So, what's the 'hockey puck' clock mentioned in the article?
>
>
>
>
> ------------------------------
>
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> End of time-nuts Digest, Vol 41, Issue 36
> *****************************************
>
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