[time-nuts] Question about frequency counter testing

Azelio Boriani azelio.boriani at gmail.com
Fri Apr 27 10:32:28 EDT 2018


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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