[time-nuts] Oscillator stability was (New NTBW50AA)

Bob Stewart bob at evoria.net
Sat Sep 14 18:59:08 EDT 2013


Hi Warren,

There's a lot of good information in this post, and I've discovered a lot of it while writing my GPSDO PLL.  In point 5, you mention that to correct for frequency it has to be off frequency.  I've seen this on both sides of it.  From time to time, I see extremely long periods of no DAC activity.  And at other times, it's moving back and forth; apparently because the DAC is about 1 bit too small.  In my PLL, I've just decided to accept the phase oscillation when it happens and blame it on two main factors.  One is the size in bits of the DAC, about which there's little I can do.  (Well, Jacques Audet came up with something to get one more bit out of it.)  

The other is the Oncore sawtooth, which I'll start working on sometime this week.  The idea of keeping the phase a bit off from the PPS is just a bit too much to bother with for me, I think.  All I have to work with is an 18F2220 PIC running at 40MHz, and I'd like to keep it as simple as possible.  It's already a lot more accurate than I had expected to be able to get.  And it's not like the slope detector on this thing is all that reliable down in the single digit nanoseconds.

Sorry for the thread steal.  Thanks for the review.

Bob - AE6RV





>________________________________
> From: WarrenS <warrensjmail-one at yahoo.com>
>To: Discussion of precise time and frequency measurement <time-nuts at febo.com> 
>Sent: Saturday, September 14, 2013 5:21 PM
>Subject: Re: [time-nuts] Oscillator stability  was (New NTBW50AA)
> 
>
>OT for the current heading, so I renamed it "Oscillator stability"
>
>as Tom says, "it's a complicated subject".
>And to complicate things even further, here are a few of advanced subtleties that I've observed from the TBolt when using LadyHeather.
>
>
>1) The TBolt uses the received GPS signal as the reference for the calculated OSC freq offset and the Phase offset, over a measurement time of 1 sec.
>Unfortunately, from second to second the GPS signal is very noisy.
>Fortunately, LadyHeather can display plots of the data with several helpful user options, such as gain, offset, and most important length of time averaged.
>
>2) The Osc freq display plot of LH is so noisy that I find it mostly useless,
>unless I turn on LH's display filter to average the data over more than 1 second.
>LH allows you to manually turn-on and set the display filter to any time period desired,
>doing so this will greatly reduce the noise of the Osc plot.
>(10 samples is the display filter default, but I find 100sec to be a better place to start, for most things)
>A problem with the LH Osc freq plot even after filtering, is that the frequency difference data can not be counted on to be more accurate than a few parts in 1e-12 due to some offset rounding problems that often occurs.
>
>
>3) On the other hand, the filtered Phase plot has no known offset error.
>The Tbolt accurately shows time/phase different between the 1PPS and the received GPS signal,
>and when disciplined, it assumes if there is a difference, then the GPS is always right.
>This is why the antenna placement and setup is so important. Gunk in, Gunk out.
>
>
>4) The GPS signal, even on a 'perfect antenna', tends to wonders around ~10 ns PP independently of the time period averaged.
>So if LH is showing less than ~ 10 ns of GPS noise in the phase plot,
>it is because the control loop is set too fast and therefore forcing the Osc's freq to change a little so that it's phase will wonder around with the GPS's noise.
>Tbolt's max useable time constant is only 1000 sec, which is not nearly long enough to avoid this problem, when using a stable external osc like a good RB.
>To avoid tracking the noisy GPS data, the extended TC method must be used to set the Tbolt's osc to values >> 1000 seconds,
>and/or you can reduce the speed of  the phase tracking using the damping setting..
>
>4) How well and how fast the Tbolt minimizes the PPS phase and freq offset compared to the received GPS signal all depends on tuning.
>The Tbolt's TC determines what the Frequency  tracking time constant will be, and the TBolt's damping setting determines what the Phase tracking time multiplier is.
>If you set the damping factor to a large value like 100, then the Phase tracking will pretty much be turned off,
>making the disciplined loop more of a freq lock loop instead of a phase lock loop. This is done by lowering the gain of the loop's PID integrator.
>This is a way to set the phase's tracking time constant to be much slower than the freq TC setting, and if desired the phase tracking TC can be made several days long.
>Turning off the phase tracking has it's own set of pros and cons, but in most cases it is generally not desirerable in GPSDO.
>A damping setting of 0.7 to 1 will give the best overall compromise between the trade-off of not adding extra freq noise but still allowing good phase tracking.
>
>
>5) What some do not realize is to correct for any phase drift error, the Oscillator must be set off frequency.
>When the frequency is correct there is no further change in the present phase, whatever the present phase may be or wherever it may of come from.
>The faster you want to correct or change the present phase difference, no mater how it got there, the larger that the present frequency error must be made. (this causes freq noise)
>The trade off is, if you do not correct the present phase error then the past average frequency will be in error.
>
>6) So it is all a matter of what is more important to the application, present frequency error and noise or the average of all past frequency errors?
>The goal of most GPSDO is to keep the average past frequency error to zero. (a Phase Lock Loop)
>Where as for many transmitter things such as used by Hams,  it is the present errors and noise that is more important.
>So no need to cause a present freq error just to fix something that happened in the past.
>The past is gone and what happened before does not matter anymore. (a Freq Lock Loop)
>
>ws
>
>***********************
>
>----- Original Message ----- From: "Tom Van Baak"
>To: "Discussion of precise time and frequency measurement" <time-nuts at febo.com>
>Subject: Re: [time-nuts] New NTBW50AA
>
>
>> I get a spread of around 300ppt, that means I'm always within
>> 300x10^-9 Hz of 10MHz or .0003Hz at 1GHz?
>
>Note "300 ppt" is a "fractional frequency", unit-less value, so
>at 10 MHz, 300e-12 * 1e7 Hz =  0.003 Hz
>at 1 GHz, 300e-12 * 1e9 Hz = 0.3 Hz
>
>> I suppose the ppt spread is pretty much a function of how stable the osc is once
>> other factors like temp, antenna position, sat acquisition, etc, are optimized?
>
>Yes and no; it's a complicated subject. For now, just two points:
>
>A measure of frequency implies some measurement duration. A given TBolt may be off by more than 1e-11 in frequency over seconds or minutes even though if you measure over a day it is accurate to less than 1e-13. This is one problem with interpreting the "OSC" value.
>
>A second problem is that a TBolt can't really know its own accuracy; that requires an external frequency reference, so take the PPS and OSC with a grain of salt. However, in general, the TBolt will steer its 10 MHz oscillator so as to minimize the PPS and OSC values. As such they can be used as a rough idea of how well the unit is performing.
>
>/tvb
>
>
>
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