[time-nuts] Another "atomic" clock question
John C. Westmoreland, P.E.
john at westmorelandengineering.com
Thu Mar 6 23:36:08 EST 2014
To the Mike that posted:
http://www.pst.netii.net/patents.htm - I tried going to your site - can't
reach it. Is the site operational? I wanted to take a look at your
On Thu, Mar 6, 2014 at 2:40 PM, Bob Camp <lists at rtty.us> wrote:
> In modern GPS modules the sawtooth error is no longer truncated at the 1
> ns level. The have been giving you far more resolution than that for 10
> years now. The resolution is not just useless bits. If you compare the
> result to a cesium standard they do improve the GPS.
> On Mar 6, 2014, at 1:30 AM, Mike at febo.com wrote:
> > Wow! One post and I've got the two top heavyweights against me! Let
> > me introduce myself.
> > I am a retired electronics engineer with over 50 years of experience
> > in instrumentation and metrology. Here is my patent list:
> > http://www.pst.netii.net/patents.htm
> > Among the achievements listed, I claim credit for the first
> > disclosure of the now universal dual-d phase-frequency detector, and
> > for the technique called "Phase Margin Analysis" as applied to hard
> > disk bdrive it error analysis. The technology has evolved
> > tremendously since the 1970's, but this was the first to show that
> > rapid bit error analysis was possible. The internet would not be
> > possible without this basic technique, since it would not be
> > possible to manufacture hard disks fast enough.
> > Another significant invention is Binary Sampling. I will talk more
> > about this later, but some information is on my web site at
> > http://www.pst.netii.net/sampler/index.htm
> > also starting on page 6 of
> > http://www.pst.netii.net/pdfs/tdrpaper.pdf
> > One of the significant advantages of the Binary Sampler is the
> > elimination of Gaussian and Impulse noise. Unlike conventional diode
> > bridge samplers, the performance improves as the frequency
> > increases.
> > After working with the Binary Sampler, I am always dismayed to view
> > the noisy graphs presented in time-nuts and other forums. The noise
> > is hiding the interesting stuff and making it virtually impossible
> > to understand what is actually going on. I Think the Binary Sampler
> > can do a lot to help unravel the issues.
> > I now intersperse replies:
> >> Hi(
> >> While you see a lot of pretty plots in GPS spec sheets showing
> >> clean looking sawtooth sort of offsets marching down the page,
> >> that?s not what I see on a real receiver. The real data, even
> >> compared to a 5071A is much more random. It will indeed ?hang?,
> >> but it also will reverse far more often than the pretty data
> >> sheets suggest. A simple model would be to add the sawtooth to
> >> some sort of random process. The sawtooth comes from the TCXO, the
> >> random looking stuff comes from the GPS solution.
> >> The oscillator in most timing modules is one form or another of a
> >> TCXO. Often they have digital compensation (one way or another).
> >> Their frequency versus temperature curves are not the simple third
> >> order curve you would expect from a bare crystal. They have a much
> >> higher order frequency versus temperature curve (6th, 8th ?). That
> >> makes even the simple ?frequency goes down when temp goes up?
> >> decision pretty tough. If they are doing some sort of auto
> >> correction TCXO based on the GPS it would get even more crazy. In
> >> that case the curve would be changing real time.
> >> Since the sawtooth changes multiple ?runs? per minute in a room
> >> that holds 2C / 30 minutes, you could guess that a control of
> >> 0.01C would be needed to have any luck steering the oscillator.
> >> It?s nowhere near that simple, so that?s not even up to the ?wild
> >> guess? level of confidence. If it?s close, that?s not going to be
> >> very easy all by it?s self. A double loop control is likely to be
> >> needed.
> >> Combine the random jitter with the (possibly) tough temperature
> >> control problem, and frequency reversals - this is a real can of
> >> worms.
> >> ???????????
> >> Way lots easier approach:
> >> 1) You already need a CPU to set up the GPS, read the sawtooth data
> stream and do a control loop. It?s free / same with either approach.
> >> 2) Rip a VCTCXO out of something (or buy one cheap).
> >> 3) PWM control the TCXO, use it as your CPU clock
> >> 4) Generate a PPS with a timer output on the CPU.
> >> 5) Do a cheap / simple / easy TDC on the GPS pps, it will cost less
> that what ever was going to drive the heater.
> >> Now you have a GPSDO with a much lower jitter PPS output. You need
> >> to write from scratch code for the CPU either way. The code for
> >> the GPSDO is probably simpler than the temperature control code.
> >> It?s certainly no more difficult. This way you have an output at
> >> what ever the TCXO frequency is for ?other stuff?.
> >> Bob
> > Thanks, Bob. I don't propose using temperature to control the GPS
> > clock. I plan to use a AD9912 DDS (1GHz 48Bit 4uHz 0.19ps
> > $59 at Newark.)
> >> On Mar 5, 2014, at 6:48 PM, Tom Van Baak <tvb at LeapSecond.com>
> >> wrote:
> >>> I agree with Bob.
> >>> For casual use, "hanging bridges" are not really a problem,
> >>> statistically speaking -- so don't worry.
> >>> Yes, you can apply various techniques to reduce/eliminate the
> >>> rare effect: forced temperature change, forced Vcc change, 2 or 3
> >>> or more shared-antenna receivers, modulating phase, frequency,
> >>> voltage, temperature, etc. But as you spend too much time
> >>> engineering this uncertain hack you maybe start to wonder if the
> >>> real solution is just to apply known digital, numerical
> >>> correction instead of wishful analog cover-up. Been there, done
> >>> that.
> >>> For more serious use, at the tens or unit nanosecond level, the
> >>> robust solution is simply to apply 1PPS sawtooth correction from
> >>> the receiver.
> > The sawtooth error data is truncated at 1 ns. I would like to get
> > far below that error.
> >>> This issue comes up every now and then as people gradually
> >>> transition from casual to serious use. I welcome any hard data or
> >>> plots that demonstrate the difference among all approaches. There
> >>> *is* a slight difference for sure. It's just that most people
> >>> throw in the towel and use sawtooth corrections instead of trying
> >>> to avoid them and cover up with less deterministic methods.
> > Tom,
> > Thanks for your reply. The sawtooth error correction is described in
> > "Timing for VLBI", by Tom Clark and Rick Hambly, at
> > http://www.cnssys.com/files/tow-time2011.pdf
> > John Ackermann shows graphs that compare the results in "GPS
> > Pulse-per-Second Comparative Noise", at
> > https://www.febo.com/pages/gps_pps/
> > It appears the implementation of the sawtooth error correction
> > severely degrades the performance of the system. There could be many
> > reasons, which is why it is important to nail down as many of the
> > error sources as possible.
> >>> /tvb
> >> ----- Original Message -----
> >> From: "Bob Camp" <lists at rtty.us>
> >> To: "Discussion of precise time and frequency measurement" <
> time-nuts at febo.com>
> >> Sent: Wednesday, March 05, 2014 3:03 PM
> >> Subject: Re: [time-nuts] Another "atomic" clock question
> >> Hi
> >> If you are going to decode and use the sawtooth data out of the
> >> receiver, there?s no need to eliminate the hanging bridges. The
> >> sawtooth data does that for you already. Put another way, heating
> >> the receiver is *harder* than just using the decoded data?.
> >> Bob
> > Thanks, Bob. I am not planning on heating the crystal. I want to
> > replace it with a precision DDS.
> > The sawtooth data is truncated at 1ns. I want to do much better.
> > Again, this is not intended as a quick-and-dirty fix. I would like
> > to separate out the error sources in a GPSDO and see what can be
> > done to improve the results.
> > Thanks,
> > Mike
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