So, what settings would you recommend for an application where frequency
accuracy and low phase noise of the 10 MHz output are the objectives?
In particular, I'm not sure I understand how to establish the "extended
TC method."
Thanks!
Jim
[email protected]
On 9/14/2013 6:21 PM, WarrenS wrote:
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"
<[email protected]>
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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