[time-nuts] Oscillator stability was (New NTBW50AA)
WarrenS
warrensjmail-one at yahoo.com
Sat Sep 14 22:21:13 UTC 2013
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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