Hi Should drink coffee first and *then* type messages..
On Sep 15, 2013, at 9:12 AM, Bob Camp <[email protected]> wrote: > Hi > > The OCXO in the TBolt is quite low phase noise as a stand alone oscillator, > It's not all that spur free in the TBolt. There's not a lot you can do about > that with any settings. The OCXO in the NTBW50AA (and similar) is not as lot how about LOW phase noise > phase noise as the TBolt. It's also got issues "as installed". The whole > settings thing does not apply to the NTBW50's since you can't change the loop > settings on them with LH. > > Bob Your best bet for low phase noise at microwaves is still to lock up a 100 MHz VCXO and don't sweat the noise & spurs at lower frequencies. Bob > > On Sep 15, 2013, at 7:14 AM, Jim Sanford <[email protected]> wrote: > >> 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 >>> >>> >>> >>> _______________________________________________ >>> time-nuts mailing list -- [email protected] >>> To unsubscribe, go to >>> https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts >>> and follow the instructions there. >>> >> >> _______________________________________________ >> time-nuts mailing list -- [email protected] >> To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts >> and follow the instructions there. > > _______________________________________________ > time-nuts mailing list -- [email protected] > To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > and follow the instructions there. _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
