[time-nuts] TimeLab

Magnus Danielson magnus at rubidium.dyndns.org
Sun Oct 9 12:37:39 EDT 2016


Which removes the real-time processing benefit of using TimeLab in the 
first place.

What I propose is not too complex to do.

Cheers,
Magnus

On 10/09/2016 06:19 PM, Bob Stewart wrote:
> Don't forget the possibility of saving the data to a file and pre-processing the file before sending it to Timelab.
> Bob
>  -----------------------------------------------------------------
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>
> GFS GPSDO list:
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>
>       From: Magnus Danielson <magnus at rubidium.dyndns.org>
>  To: time-nuts at febo.com
> Cc: magnus at rubidium.se
>  Sent: Sunday, October 9, 2016 10:48 AM
>  Subject: Re: [time-nuts] TimeLab
>
> Hi,
>
> Well, yes. You can do some fancy stuff with additional hardware, but I
> think with already a handful of relatively simple software fixes and
> some basic setup conditions, a sufficiently robust method emerges.
>
> I could not sign-swap the measurements in TimeLab when I tried.
> I don't seem to be able to force the unwrapped phase to be +/- half cycle.
> I don't seem to be able to offset my readings. I have two sources of
> offset, one is the additional delay of cables, and the other is the
> offset due to wrong cycle (I hope this one can be baked into alternative
> phase-unwrapping mode). I would prefer if I could hit calibration to
> establish the zero-level. Typically I use a BNC barrel and well, it does
> add smoe more delay
>
> What I propose should be doable with a simple counter like 5335A,
> 53131/2A or similar. If you have a locked say 100 Hz or 1 kHz signal
> (TADD-2 can be useful if the GPSDO does not output proper signal), you
> can do the picket fence and resolve things, it is just that there is a
> few things to aid in the post-processing to make values useful.
>
> I further hint about a few things which makes easier to analyze is the
> improved support for zooming.
>
> Oh, I do care about phase variations and absolute phase measures. I do
> such measures a lot. ADEV and TDEV is not all the things I measure,
> especially when considering systematic effects.
>
> Cheers,
> Magnus
>
> On 10/09/2016 03:42 PM, Bob Camp wrote:
>> Hi
>>
>> Given that *some* of us have more than errr … one counter :)
>>
>> There are several setups that involve two or three counters to resolve some of these issues. Having
>> multiple serial ports or multiple devices on a GPIB isn’t that big a problem. Addressing multiple devices
>> (setting up the addresses in TimeLab) is an added step. Coming up with standard setups would be the
>> first step. Getting them documented to the degree that they could be run without a lot of hassle would be
>> the next step.
>>
>> Another fairly simple addition (rather than a full blown counter) would be some sort of MCU to time tag
>> the input(s). It’s a function that is well within the capabilities of a multitude of cheap demo cards. Rather than
>> defining a specific card, it is probably better to just define a standard message (115200 K baud, 8N1, starts
>> with “$timenuts$,1,”, next is the channel number, after that the (32 bit?) seconds count.The final data field is
>> a time in nanoseconds within the second, *two byte check sum is last, cr/lf). If there is a next generation version that is
>> incompatible, the 1 after timeouts changes to a 2.) Yes, even 10 seconds after typing that definition I can see
>> a few problems with it. Any structural similarity to NMEA is purely intentional. That’s why it needs a bit of
>> thought and work before you standardize on it. It still would be a cheap solution and maybe easier to integrate
>> into the software than multiple counters. You do indeed have all the same setup and documentation issues.
>>
>> In any of the above cases, the only intent of the added hardware is to get a number that is good to 10’s of ns.
>> Anything past that is great. Once you know where all the edges really are, sorting out the phase data becomes
>> much easier.
>>
>> Bob
>>
>>> On Oct 9, 2016, at 7:32 AM, Magnus Danielson <magnus at rubidium.dyndns.org> wrote:
>>>
>>> Fellow time-nuts,
>>>
>>> I don't know if it is me who is lazy to not figure TimeLab out better or if it is room for improvements. I was considering writing this directly to John, but I gather that it might be of general concern for many, so I thought it be a good topic for the list.
>>>
>>> In one setup I have, I need to measure the offset of the PPS as I upset the system under test. The counter I'm using is a HP53131A, and I use the time-interval measure. I have a reference GPS (several actually) which can output PPS, 10 MHz, IRIG-B004 etc. In itself nothing strange.
>>>
>>> In the ideal world of things, I would hook the DUT PPS to the Start (Ch1) and the reference PPS to the Stop (Ch2) channels. This would give me the propper Time Error (DUT - Ref) so a positive number tells me the DUT is ahead of the reference and a negative number tells me that the DUT is behind the reference.
>>>
>>> Now, as I do that, depending on their relative timing I might skip samples, since the counter expects trigger conditions. While TimeLab can correct for the period offset, it can't reproduce missed samples.
>>> I always get suspicious when the time in the program and the time in real world does not match up.
>>>
>>> I could intentionally shift the PPS output of my DUT to any suitable number, which would be one way to solve this, if I would tell TimeLab to withdraw say 100 ms. I might want to do that easily afterhand rather than in the setup window.
>>>
>>> To overcome this, I use the IRIG-B004 output, which is a 100 Hz signal with a stable rising edge aligned to the PPS to within about 2 ns. Good enough for my purpose. However, for the trigger to only produce meaningful results, I will need to swap inputs, so that the PPS from DUT is on Start/Ch1 and the IRIG-B is on Stop/Ch2. This way I get my triggers right. However, my readings have opposite sign. I might have forgotten about the way to correct for it.
>>>
>>> However, TimeLab seems unable to unwrap the phase properly, so if I have the condition where I would get a negative value of say -100 ns then the counter will measure 9,999,900 ns, so I have to force a positive value as I start the measurement and then have it trace into the negative. I would very much like to see that TimeLab would phase-unwrap into +/- period/2 from first sample. That would be much more useful.
>>>
>>> I would also like to have the ability to set an offset from which the current zoom window use as 0, really a form variant of the 0-base but letting me either set the value or it be the first value of the zoom. I have use for both of these. I often find myself fighting the offset issues. In a similar fashion, I have been unable to change the vertical zoom, if I don't care about clipping the signal then it forces me to zoom in further than I like to. The autoscale fights me many times in a fashion I don't like.
>>>
>>> OK, so there is a brain-dump of the last couple of weeks on and off measurement experiences. While a few things might be fixed in the usage, I wonder if there is not room for improvements in the tool. I thought it better to describe what I do and why, so that the context is given.
>>>
>>> Cheers,
>>> Magnus
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