[time-nuts] Frequency counter questions

Chris Albertson albertson.chris at gmail.com
Mon Apr 24 12:45:12 EDT 2017

On Sun, Apr 23, 2017 at 6:25 PM, Jerry Hancock <jerry at hanler.com> wrote:

> 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?

It is best to keep this kind of discussion on the list.  That way if some
one like me sys something dumb, it will get noticed and called out.

OK, so it seems you

(1) are interested in measuring frequency up to 100 MHz but no higher.
(2) want to use an STM32 ARM uP to
(3) Are using the older (and simpler) design that simply counts cycles and
divides by the gate time to measure frequency

If the above assumptions are wrong.  STOP reading now and post corrections
or additions

OK so we are good to go....

I assume you know about and intend to use the built in counter/timmer on
the STM32.   You first step is to find out the maximum pule rate you can
place on that pin.  Lets guess it is 48 MHz. But you are a conservative
engineer an want to de-rate it to 24 MHz.

This means to measure up to 100 MHz you will need a prescaler that can
divide by 4.    This will transform you maximum 100 MHz to a maximum of 25
MHz.   It will force you into using 4X longer gate time.   This is the only
reason to pre-scale, because you can't otherwise process the signal.

Now it is very simple.   You write software that starts the counters on the
desired pin and then your GPS sends a signal every one second (You do NOT
need to divide down the 10MHZ from a GPSDO because the GPS already has a
one pulse per second output that feeds the DO.)   SO every one second you
read the number of counts and keep doing this for as many seconds as you
need   After the gate time which might be one second or 4,000 seconds you
add up all the one second readings and compute the answer.

Actually you can do better given the excess computer power you have:  You
can output a frequency estimate after the first send and then each second
you can output a more accurate value until finally after 4,000 seconds out
have 0.001 Hz.   So in the first second you put up a display that is good
for +/- 4 hZ and then four seconds later you can be a 1.0 Hz and after an
hour 0.001 Hz

As I wrote before.   I use the "trigger" output of my old Tektronix scope
as input to my counter because the scopes trigger is MUCH better.   If you
own a 'scope this will save you a LOT of work as all you would need to
build is a voltage converter to take the trigger output to 3v3 volts for
the STM32.  The STM already can do rescaling and counting.    Connect an
I2C display and you are done with the hardware.  For a better and cheaper
display, the STM32 can serve a web page and then you see the frequency on
your phone.

Chris Albertson
Redondo Beach, California

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