[time-nuts] Another "atomic" clock question

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
patents.

Thanks,
John Westmoreland

On Thu, Mar 6, 2014 at 2:40 PM, Bob Camp <lists at rtty.us> wrote:

> Hi
>
> 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.
>
> Bob
>
> 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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