Hi (yes, this is a bit confusing … it’s my replies to a forward from Magnus who got a bounce on submittal)
> Begin forwarded message: > > Date: December 14, 2014 at 7:57:39 PM EST > From: Magnus Danielson <mag...@rubidium.se> > To: Bob Camp <kb...@n1k.org> > Cc: mag...@rubidium.se > Subject: Re: [time-nuts] Homebrew frequency counter, need help > > Hi Bob, > > Repost my email as I accidentally posted it with wrong from address. > > On 12/14/2014 08:26 PM, Bob Camp wrote: >>>>>>> Since I really want to reduce the noise, what is the best test set you >>>>>>> suggest? All the frequency source I have: FE5650 Rb , PRS10 Rb , MV89a*2 >>>>>>> OCXO, >>>>>> >>>>>> If the MV89’s are in good working condition, they are the best thing to >>>>>> compare.The have the best ADEV of the group you have available I would >>>>>> check them for output level and stability before I trusted them. There >>>>>> are a lot of defective parts on the market. People get some, sort them >>>>>> and sell the bad ones. The bad ones just keep getting re-sold again and >>>>>> again … My guess is that they were good parts at one time and they got >>>>>> damaged when pulled off boards. If you use them, keep them on power at >>>>>> all times. Any OCXO will do better if you run it that way. >>>>> >>>>> In order to test if systematics is messing badly with you, measure the >>>>> ADEV of the oscillator as it is steered (and stabilized) to a number of >>>>> different frequencies. For larger offsets to the counter reference, >>>>> multiple beatings occurs within the regression interval. You want that >>>>> number to be an even number of beats, or the beat count to be so large >>>>> that the phase of the last beat does not care. Linear regression helps >>>>> out, as it weighs out the outermost measures compared to the central one, >>>>> making the beating at the beginning and end not care as much. >>>>> >>>>> These are *systematic* noise effects, and as you play around with >>>>> systematics and processing, you might have the systematics works for or >>>>> against you, but at the same time, the random noise you try to measure >>>>> will suffer the processing filtering, and you need to recall that. If you >>>>> balance these properly, you can make good and correct measurements, it's >>>>> just that few do. >>>>> >>>>> Oh, and only use ADEV, MDEV and TDEV to estimate random noises, system >>>>> noises as they show up there should be estimated separately and removed >>>>> from the random noise estimates. They have *way* different behaviors. >>>> >>>> … and this is where it gets complicated. I would toss in the Hadamard >>>> deviation into that mix as well. >>> >>> The Hadamard deviation is a great tool as it is not sensitive to linear >>> frequency drift as Allan deviation is. This would help to remove the >>> systematic effect, just as a quadratic curve-fitting of the raw-data and >>> ADEV of the residual. >> >> I like the Hadamard because it’s a bit better for mapping to the frequency >> domain. It’s what HP used to get phase noise from phase error data. I find >> that it gives a bit better detail on some types of problems. > > I use if regularly, but TimeLab unfortunatly does not have the MHDEV. The whole process of getting *correct* versions of things into a program is (unfortunately) much harder than simply tossing it in there. I’m glad that the stuff in TimeLab works correctly. > >>> >>> Modified Hadamard deviation (MHDEV) is a good replacement for MDEV, with >>> the same properties for drift. Similarly will Time Hadarmard Deviation >>> (THDEV) replace TDEV. However, for longer taus you want better processing, >>> so therefore you want to consider the TOTAL set of deviations, such that >>> confidence intervals is better. >>> >>>> If I had to only use three, I would include it with modified ADEV (MDEV) >>>> and TDEV. All three are available in TimeLab with the click of a button. >>>> If you start getting lots of data (9,000 points per second) I would toss >>>> in a frequency domain (FFT) analysis as well. FFT on phase data is not (as >>>> far as I know) a feature of TimeLab. >>> >>> FFT on phase-data is only available in TimeLab when doing phase-noise >>> measurements. FFT is the way to analyse systematic noise rather than random >>> noise where ADEV and friends is being used. You need to separate them, and >>> the ADEV plot is not good for both. >>> >>> There is a set of FFT based ADEV-style measures, which uses FFT, filtering >>> of the various ADEV styles. There is a nice set of articles covering that >>> approach, and actually the only style of ADEV processing that I haven't yet >>> implemented, even if I have done most others. >> >> Stable-32 will take phase data and convert it to the frequency domain. > > Depending on what processing you are going to do, phase or frequency may be > optimum. > Phase is better for normal deviations. > Frequency is better for modified deviations. Stable 32 is nice in that it will convert one to the other with the click of a button. > >>> >>>> To start with, on all of these measures, you are looking for bumps and >>>> spikes. They are telling you that something is wrong. If you flip over to >>>> the phase plot in TimeLab, spikes and abrupt steps in it also are telling >>>> you the same sort of thing. Exactly what this or that bump is telling you >>>> may not be obvious at first. Posting plots to the list is a great way to >>>> get things sorted out. >>> >>> Bumps, spikes and slopes... ADEV isn't the only tool one should be using, >>> FFT might be much better for systematic noises. >> >> Right, so when you see them, alarm bells should go off. Something is indeed >> wrong and further investigation is required. > > Maybe, ADEV is good at smoothing out things, so spikes in spectrum-analysis > might not be as easy to spot in the ADEV form. A good reason to look at multiple data sets and analysis approaches > >>> In the end of the day, there is an overbeleife in ADEV both as a scale as >>> well as a processing tool, to analyze deviations, without considering the >>> separation of various systmeatic effect and systematic noises, while ADEV >>> and friends is there to analyze random noise types, it does not handle >>> systematics good. Seems like we have to kill ADEV as the universal measure. >>> Ah well. >> >> It’s been around much longer than some of the others. It also has some nice >> convergence properties. That’s made it the spec of choice when describing >> the performance of a wide range of products. You could buy a box that had a >> “measure ADEV” button on it a very long time ago …. like back when I started >> doing this … Having a piece of gear to point at for a spec measurement is a >> real good thing. It eliminates a wide range of discussions. That goes at >> least double if it has the logo of a well known test gear outfit on it. > > That may be, but ADEV is often misused to be the only plot. Ok, so what gear with a major label on it would you use in the 1980’s and 1990’s to measure spec performance on a few million OCXO’s ….. > > I actually got an old Timing Solutions At least up to the end of the 1990’s that name would get you a Timing Who? response from > 90% of the customers of OCXO’s. > test-set that does ADEV at 0.1, 1, 10 and 100 s using an offset rubidium at > 5.000055 MHz. I've never been able to get the serial port to do anything > useful. Love to pull the data out of that one and into TimeLab. They are good boxes. They just are from a very specialized outfit. That also makes getting them repaired a bit tough. Bob > >>> >>>>> Cheers, >>>>> Magnus - considering what beer will be best to start the evening with >>>> >>>> That means it’s 5 o’clock somewhere in the world …hmmm …. choice of beers >>>> … It’s winter over here, so the dark stuff is slowly taking over the >>>> inventory. I have a nasty suspicion that it’s winter in Sweden as well :). >>>> Probably something with stout in it’s name …. >>> >>> Hibernation Ale from Great Divide Brewing in Denver, Colorado, USA was the >>> choice for the evening. Good beer for handling the winter. >> >> I’ve got a keg of Troeg’s Mad Elf that’s been “aging” in the cooler for >> about 14 months. I’m considering the need to tap it vs letting it age a bit >> more … life is so full of difficult decisions … > > Decisions, decisions. Which beer to pick. :) > > Gotta check what's in the secret box. :) > > Cheers, > Magnus _______________________________________________ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
