On Wed, Aug 10, 2011 at 7:19 AM, Dan Kemppainen <[email protected]>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 _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
