[time-nuts] Frequency Comparator Ideas needed.

Dan Kemppainen dan at irtelemetrics.com
Wed Aug 10 14:19:53 UTC 2011

Hi All,

Been lurking here for a while, some neat traffic going around. I'm 
thinking this may be a question for this list...

I'm wondering if there are any ways to compare two frequencies and 
tell if one is lower or higher than the other, at a very high rate of 
speed with a digital output. I would like to know how each cycle of 
the signals compare in terms of frequency, basically is one higher 
than the other or not. Ideally this returned signal would be a digital 
logic signal (high or low) and be glitch free.

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.

I've currently been looking at digital Phase/Frequency type detectors, 
and am considering building a discrete unit based on high speed 74AUP 
or ECL/PECL type logic.

Just wondering if I'm missing something obvious, that you guys may 
know about!

Dan Kemppainen

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