[time-nuts] Fwd: Homebrew frequency counter, need help

Bob Camp kb8tq at n1k.org
Sun Dec 14 20:22:24 EST 2014


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 <magnus at rubidium.se>
> To: Bob Camp <kb8tq at n1k.org>
> Cc: magnus at 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



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