# [time-nuts] Frequency counter questions

Bruce Griffiths bruce.griffiths at xtra.co.nz
Mon Apr 24 00:10:57 EDT 2017

Reciprocal counter where one measures the period of the output of a divide by N and then takes the reciprocal of this period multiplied by N as the frequency of the divider input. With a 100MHz clock one can achieve ~ 0.01ppm resolution  for a 1 second averaging time. if one uses interpolators with 100ps resolution (equivalent to a 10GHz clock)  the resolution improves to ~0.1ppb for a 1s averaging time.

Bruce

>
>     On 24 April 2017 at 13:25 Jerry Hancock <jerry at hanler.com> wrote:
>
>     For some reason I am not getting the individual emails so I apologize for not replying more promptly. I’ll have to check my profile.
>
>     As far as I can tell from the notes, and by the way, the number of notes was why I was trying to move this off list, to get to .001hz I need to measure over 1000 seconds. This is ok. Since I am looking for an average over time anyway, this is not a problem. By the way I am using a GPSDO and planned to use it divided down for the gate.
>
>     The only reason I mentioned a prescaler was that there was a 12 digit counter schematic on the web that looked pretty complete. This person used a prescaler and I was trying to wrap my head around how this helped with resolution and I guess from the replies, that is not a practical solution (using a prescaler) when you want high resolution unless I use the inverse operation which I can’t remember what it is called off the top of my head.
>
>     I’ve seen some HP 12 digit counters but since I have a GPSDO and who knows how many micro development boards around here, I thought I would take a run at it.
>
>     So to summarize, if I limit my high resolution to 99,999,999.999hz and use a gate of 1000 seconds, would that get me to .01hz? If not, then what would the possible resolution be?
>
>     Thanks for all the input, very helpful.
>
>     Jerry
>
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