[time-nuts] Question about frequency counter testing

Bob kb8tq kb8tq at n1k.org
Fri Apr 27 12:08:11 EDT 2018


Hi

Since you are using averaging to get more bits (much like a CIC ) the idea that you
need noise to make it happen is actually pretty common. There are app notes 
coming at it from various directions on ADC’s and SDR going *way* back (like to
when I was in school …. yikes ….). 

What is a bit odd is working your head around it being needed in this case. 
It *is* sort of a 1 bit A/D. There is a very tenuous connection. 

Bottom line is still that you are doing signal processing. Since that is what is going 
on, you very much need to get into the grubby details to work out what the limitations
(and benefits) will be. It doesn’t *look* like a radio, but it has a lot of SDR-like issues.

Bob

> On Apr 27, 2018, at 11:58 AM, Tom Van Baak <tvb at LeapSecond.com> wrote:
> 
> Azelio, the problem with that approach is that the more stable and accurate your DUT & REF sources the less likely there will be transitions, even during millions of samples over one second.
> 
> A solution is to dither the clock, which is something many old hp frequency counters did. In other words, you deliberately introduce well-designed noise so that you cross clock edge transitions as *much* as possible. It seems counter-intuitive that adding noise can vastly improve your measurement, but in the case of oversampling counters like this, it does.
> 
> /tvb
> 
> ----- Original Message ----- 
> From: "Azelio Boriani" <azelio.boriani at gmail.com>
> To: "Discussion of precise time and frequency measurement" <time-nuts at febo.com>
> Sent: Friday, April 27, 2018 7:39 AM
> Subject: Re: [time-nuts] Question about frequency counter testing
> 
> 
> You can measure your clocks down to the ps averaged resolution you
> want only if they are worse than your one-shot base resolution one WRT
> the other. In a resonable time, that is how many transitions in your
> 2.5ns sampling interval you have in 1 second to have a n-digit/second
> counter.
> 
> On Fri, Apr 27, 2018 at 4:32 PM, Azelio Boriani
> <azelio.boriani at gmail.com> wrote:
>> Yes, this is the problem when trying to enhance the resolution from a
>> low one-shot resolution. Averaging 2.5ns resolution samples can give
>> data only if clocks move one with respect to the other and "cross the
>> boundary" of the 2.5ns sampling interval. You can measure your clocks
>> down to the ps averaged resolution you want only if they are worse
>> than your one-shot base resolution one WRT the other.
>> 
>> On Fri, Apr 27, 2018 at 3:38 PM, Bob kb8tq <kb8tq at n1k.org> wrote:
>>> Hi
>>> 
>>> Consider a case where the clocks and signals are all clean and stable:
>>> 
>>> Both are within 2.5 ppb of an integer relationship. ( let’s say one is 10
>>> MHz and the other is 400 MHz ). The amount of information in your
>>> data stream collapses. Over a 1 second period, you get a bit better than
>>> 9 digits per second.  Put another way, the data set is the same regardless
>>> of where you are in the 2.5 ppb “space”.
>>> 
>>> Bob
>>> 
>>> 
>>> 
>>>> On Apr 27, 2018, at 5:30 AM, Hal Murray <hmurray at megapathdsl.net> wrote:
>>>> 
>>>> 
>>>> olegskydan at gmail.com said:
>>>>> No, it is much simpler. The hardware saves time-stamps to the memory at each
>>>>> (event) rise of the input signal (let's consider we have digital logic input
>>>>> signal for simplicity). So after some time we have many pairs of {event
>>>>> number, time-stamp}. We can plot those pairs with event number on X-axis and
>>>>> time on Y-axis, now if we fit the line on that dataset the inverse slope of
>>>>> the line will correspond to the estimated frequency.
>>>> 
>>>> I like it.  Thanks.
>>>> 
>>>> If you flip the X-Y axis, then you don't have to invert the slope.
>>>> 
>>>> That might be an interesting way to analyze TICC data.  It would work
>>>> better/faster if you used a custom divider to trigger the TICC as fast as it
>>>> can print rather than using the typical PPS.
>>>> 
>>>> ------
>>>> 
>>>> Another way to look at things is that you have a fast 1 bit A/D.
>>>> 
>>>> If you need results in a second, FFTing that might fit into memory.  (Or you
>>>> could rent a big-memory cloud server.  A quick sample found 128GB for
>>>> $1/hour.)  That's with 1 second of data.  I don't know how long it would take
>>>> to process.
>>>> 
>>>> What's the clock frequency?  Handwave.  At 1 GHz, 1 second of samples fits
>>>> into a 4 byte integer even if all the energy ends up in one bin.  4 bytes, *2
>>>> for complex, *2 for input and output is 16 GB.
>>>> 
>>>> 
>>>> --
>>>> These are my opinions.  I hate spam.
>>>> 
>>>> 
>>>> 
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