[time-nuts] question Alan deviation measured with Timelab and counters

Stéphane Rey steph.rey at wanadoo.fr
Sat Jan 24 15:07:41 EST 2015


Hi guys.

After several experiments I could discover that the "bad" ADEV from the two GPSDO DUT are due to GPS lock losses. This is probably because the antenna is outside the windows but half the sky is hidden. We can see the on the frequency plot the sharp change of 0.5Hz and the locking. Good point.

I'm now trying to evaluate various architectures of 2-channels squarers and a DMDT. For that I'm designing a PCB with 4 squarers : simple 74ac04 gate biased at VCC/2, a LT1016 comparator, the transistor based differential amplifier from Winzel and the one from Charles. I will add two balanced mixers (minicircuits), IF filters and amplifiers. 
Does anyone has an idea of what I could add for this evaluation ? 

Cheers
Stephane


-----Message d'origine-----
De : time-nuts [mailto:time-nuts-bounces at febo.com] De la part de Stéphane Rey
Envoyé : mardi 20 janvier 2015 23:15
À : 'Discussion of precise time and frequency measurement'
Objet : Re: [time-nuts] question Alan deviation measured with Timelab and counters

Hi,

Following the tests results in the previous email, today I've performed additional measurements showing that the repeatability of the GPSDO DUT is not great but is coming from the design. I've tested several over sources and repeatability is correct.

I can already make some measurement. Good ! 

Now I'd like to improve. First I'm going to implement a squarer and then I will work on the DMTD... I'm thinking to make a setup on the table, and possibly make a small PCB then.

Any comment for the tests results of yesterday here under ? 

Cheers
Stephane


-----Message d'origine-----
De : time-nuts [mailto:time-nuts-bounces at febo.com] De la part de Stéphane Rey Envoyé : lundi 19 janvier 2015 22:32 À : 'Discussion of precise time and frequency measurement'
Objet : Re: [time-nuts] question Alan deviation measured with Timelab and counters

Hi

Here are the results of today's experiments. plots and TIM files attached to this email.


Setup #1 : dark blue
I've done again the floor measurement with same conditions : HP58503 for 10 MHz Standard, 1PPS for the EXT gating and the Rb on channel A. Same result (hopefully)

Setup #2 : Pink
Then I've made what Magnus has suggested, i.e. using the 1 PPS on Channel A, the Rb on channel B and internal gating.... The ADEV has increased by more than 1 order of magnitude. I guess this confirms the 1PPS stability is lower than the 10 MHz

Setup #3-6 : Dark Green, Red, Light blue and Dark yellow.
I've measured several times the GPSDO DUT with SEParate inputs. 1PPS on EXT, Rb on channel A and DUT on channel B. This gives 4 different plots... When starting the measurement the plots starts directly at different values... Mmmm very strange. Is it coming for the setup of the GPSDO ? To be investigated further with other sources. This is the plan for tomorrow. However the overal shape of the plot sounds relevant to me.

Setup #7-8 + #9 not showed here
I've tested the suggested splitted same signal on both inputs with 1m coax for channel B. I've discovered that when swaping the GPSDO on the standard input and the Rb on the channel A I have a slight difference. In order to confirm I've made two time each measurement and this confirms that having the Rb on channel A and GPSDO on the standard input gives the lowest ADEV. The setup #9 which is the same than the light green gives the superimposed plot on that one... So what does it mean ? One of the two sources is better than the other, but which one ? 


Some other comments :
- Swaping signals between channel A and B gives the same ADEV (setup #4 and 5, light blue and red)
- On some measurement on the GPSDO DUT, (not displayed here), I could see during the measurement suddenly an increase of one order of magnitude. The HP5370A do not show any difference (the time interval value continues to move with a beat but visually impossible to quantify if the value between two values has increased. No explanation for that. I'll redo the test with some other sources to check if it comes for the measurement system or the GPSDO DUT

In conclusion,
1. swaping the Rb and HP58503 doesn't give the same result. The GPSDO has standard seems the best (or the Rb measured) 2. the measurement on the GPSDO DUT gives different results with nearly one order of magnitude difference but shape is still the same.
3. the 1PPS must be connected on the EXT gating input

What do you think ? 

Cheers
Stephane


-----Message d'origine-----
De : time-nuts [mailto:time-nuts-bounces at febo.com] De la part de steph.rey Envoyé : lundi 19 janvier 2015 16:44 À : Discussion of precise time and frequency measurement Objet : Re: [time-nuts] question Alan deviation measured with Timelab and counters

 Actually I'm working in the RF department of a big lab, designing RF  electronics mainly in microwaves range. I'm luckilly having some tools  around to play with and a lot of components like  mixers/amplifiers/couplers/splitters/attenuators, ... almost whatever  the frequency is up to several tens of GHz.
 At home since the last 20 years I could as well get nice instruments. 
 The next two measuring tools really missing and for which I'm limited  are the phase noise and stability measurement and possibly a good  standard. My Effratom FRK Rb is old and probably not the best from a  phase noise and stability point of view but until now has never been  characterized. Otherwise I've almost everything I need up to 40 GHz I  guess.

 I'm doing further measurement right now which sounds much much more  consistent. I will share tonight.
 Cheers
 Stephane



 On Mon, 19 Jan 2015 08:59:58 -0500, Bob Camp <kb8tq at n1k.org> wrote:
> Hi
>
>> On Jan 18, 2015, at 5:12 PM, Stéphane Rey <steph.rey at wanadoo.fr>
>> wrote:
>>
>> Bonsoir Magnus (Are you in Sweeden ?)
>>
>> Being able to measure high stability and low phase noise is 
>> definitely a need for me as I'm trying to design low noise 
>> synthesizers and I'm already reaching the limits of my current tools 
>> for phase noise and I can't afford an E5052 for my own. At work I've 
>> one but I will probably not stay after august. And anyway I need such 
>> tools in my lab at home…
>
> If you have tools at work, the best possible thing to do is to get 
> some oscillators / standards characterized. If you *know* what this or 
> that oscillator is doing in terms of ADEV or phase noise at this Tau 
> or frequency offset, it’s much easier to figure a lot of this out.
>
> The most basic way to do phase noise in the basement is with a single 
> mixer setup running into some sort of audio FFT device. A sound card 
> can be used or an audio spectrum analyzer. Parts are < $100 to get one 
> setup once you can do the audio measurements.
>
> For ADEV, a DMTD or it’s cousin, the single mixer is the easy way to 
> go. The single mixer does not get a lot of discussion these days. It 
> is much easier to set up than a DMTD. It does require an offset 
> oscillator. Once you have a single mixer phase noise setup, you are 
> about half way to a single mixer ADEV setup. Cost for one is < $100 in 
> parts. You already have a counter to collect the data out of it.
>
> In both cases you are running a comparison device. Having a 
> characterized OCXO to compare to is a really nice thing.
>
> Bob
>
>
>
>> As low-noise and stable synthetizers depends on the standard used, I 
>> need as well to measure them as well...
>>
>> Let's start with this simple experiments and once I will understand 
>> the ins and outs I will try to improve. I know techniques of 
>> cross-correlations and you've already talked about DMTD that for sure 
>> I will have to come to...
>>
>> Good night
>> Stephane
>>
>> -----Message d'origine-----
>> De : Magnus Danielson [mailto:magnus at rubidium.se] Envoyé : dimanche
>> 18 janvier 2015 22:46 À : Stéphane Rey; 'Discussion of precise time 
>> and frequency measurement'
>> Cc : magnus at rubidium.se
>> Objet : Re: [time-nuts] question Alan deviation measured with Timelab 
>> and counters
>>
>> Bonsoir Stéphane,
>>
>> On 01/18/2015 10:34 PM, Stéphane Rey wrote:
>>> Thanks a lot Bob and Magnus for your very helpful comments.
>>>
>>> The HP5370a was indeed in TI mode. By the way what is the difference 
>>> with +/-TI, the button just aside...
>>>
>>> But I guess I understand where I've missed something : I've tried to 
>>> put the Rb on channel A and the DUT on channel B but result was 
>>> always the same but I do understand now that there is indeed a 
>>> switch to change from COMmon to SEParate and it was always on COM 
>>> meaning I believe that channel B wasn't used. This explains a lot of 
>>> things I did not understand. I'm sorry for these so basic issues 
>>> that might have been solved if I had read carefully the HP5370a manual first.
>>
>> Good. This confirmation makes sense to be and Bob, now we can relax 
>> as the mystery is solved.
>>
>>> So possible conclusions until now are that I have actually measured 
>>> the ADEV floor of the system rather than my DUT... which is already 
>>> nice. The second conclusion from these oscillations seen with the 
>>> GPSDO under test is that there is very likely in this GPSDO design a 
>>> systemic noise added to the 10 MHz output (power supply, PCB 
>>> coupling, ... I'll make further investigations on it later on).
>>
>> It's a great opportunity to learn the tools, and once you have the 
>> tools, you can see if you can't improve things.
>>
>>> I will experiment all the suggestions you made and will come back. 
>>> For information the 1PPS from the HP58503b has a positive pulse 
>>> width that is only few us length.
>>
>> This only makes it hard to view on a scope, but long enough to 
>> reliably trigger your counter and scope.
>>
>>> Now, when considering that the method is to compare the DUT to an 
>>> other source, I assume then that the other source shall be at least
>>> 1 order of magnitude better than the DUT. Otherwise this will be 
>>> impossible to distinguish who is the instability contributor between 
>>> the source and DUT, right ?
>>
>> For a simple setup, yes. But then we are the time-nuts, we have ways 
>> of handling these things. :) Let's get you started with the basic 
>> measurement, it will be a good start.
>>
>>> Then the second question is what kind of very stable source can be 
>>> used to measure DUT which could be Rb or GPSDO which are already in 
>>> the range of 10E-10 to 10E-12 < 100s ?
>>
>> Time-nuts tend to spend their time and money getting even more stable 
>> clocks and tools. If you have the right tool, you can measure near 
>> and
>> *under* the noise-level of your reference, but not without running 
>> into issues. One such trick is called cross-correlation, while 
>> another is to use three-corner hat techniques.
>>
>> Cheers,
>> Magnus
>>
>>
>>
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