[time-nuts] Frequency counter questions

Bob kb8tq kb8tq at n1k.org
Mon Apr 24 07:53:27 EDT 2017


Hi

Let’s back off a bit. 

To most of us, a 12 digit counter displays 12 digits regardless of the frequency 
you put into it. That’s been the way good bench counters have done it since 
the early 1970’s. Typically the number of digits is specified at a one second 
gate time. An HP 5335 is one example. It is called a 9 digit (or more properly 
9 digit per second) counter. Over the last 30 years, the last “per second” part
has commonly been dropped. 

In the world of computers, the number of digits displayed is not the big deal
it was back in the days of nixie tubes. As Chris points out, comping up a display
is what things like the F7 are intended to do. The Discovery boards are a 
cheap way to get this running. 

Thus the confusion.

If you prescale you divide down the input frequency. The counter is “reading”
the divided down frequency. That’s not a big deal on a modern counter. It is
a big deal on an old style counter On an old style counter, you don’t get a 
constant number of digits at a given gate time. If you put in 1,000 cycles (also
called Hertz ..) and count them for one second, there will be four digits displayed. 
If you prescale the signal by 100, your counter will display 10 (two digits displayed).
In general prescalers are something to avoid on an old style counter.

With a modern 9 digit counter, if you put in 1,000 Hz and ran for a second you would
get 1,000.123456 on the display. You might or might not get good accuracy in 
all those digits. That’s roughly what the display would look like. 

So now back to the real questions: 

What is your input signal? How high is it in frequency? Is there a range of frequencies? 
Is it modulated? …..

What kind of accuracy are you after? (resolution and accuracy are two different things).

Is this a learning experience? Would a purchased counter for < $100 be an equally good
answer? 

Is the count it’s self significant? There are *many* different ways to measure frequency 
other than a straight counter. 

Lots of fun.

Bob


> On Apr 23, 2017, at 9:25 PM, 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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