[time-nuts] Frequency Comparator Ideas needed.

Chris Albertson albertson.chris at gmail.com
Wed Aug 10 18:45:03 UTC 2011


On Wed, Aug 10, 2011 at 7:19 AM, Dan Kemppainen <dan at irtelemetrics.com>wrote:

>
> Basically I have a relatively high frequency signal, say around 50Mhz +/-
> 150Khz that I would like to compare to a reference frequency (50Mhz) on a
> cycle by cycle basis. The two signals are not locked together, and the
> signals should never be exactly the same with the exception of transitions
> of the input signal across the reference frequency. I'd like to know when
> the transition happens to within a few tens of nanoseconds if possible.


If you measure the PERIODS of each signal, not the frequency you problem is
much easier.

You need to have two Time Interval Counters, one for each signal.  Each
counter first "squares" the signal then measures the timefrom raising edge
to raising edge and then outputs length of the period.  Basically the
counter is copied to the buffer by each raising edge of the signal and this
also resets the counter.

When the periods are the same, obviously the frequency is the same.

Fortunately counters that work at the sub-nanosecond level are available.
 You can build a "pictic" for about $50 each or buy an HP "Univesal Counter"
from ebay for a couple hundred dollars.   These "Univesal Counters"
typically have two input channels and can be set to measure the ratio of the
frequencies and output the ratio on a computer interface.  There is however
some latency.    PicTic like devices have one function: They measure the
time from a "start" pulse to a "stop" pulse with about 250 pS resolution.
 Good enough for you use, I think but you'd need to supply some signal
condidtioning and so on.  The HP counters are self contained

IF you need to know when the "crossover" happens with near zero latency then
you are into a DIY build

One more thing. The couters, like the HP units or the PicTic are actually
more sophisticated then just "counteing" they can interpolate between counts
so the time resolution is much better than the counter frequency/1 and you
really can get below one nS.
-- 

Chris Albertson
Redondo Beach, California



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