Chris, What you are suggesting is different from what I'm doing. Sampling is different from time stamping. I'm looking for events. You are looking for "data". You want a PC. I want a small micro (specifically the LPC1114). You want to overpower the problem. I want to finesse it. Nothing wrong with either. It just depends on what you are trying to accomplish.
My desired accuracy is based on what I have seen displayed on line frequency monitoring sites on the 'net. Numbers like 60.053 etc. That says (roughly) 1E-5 accuracy. Now it may be that they show more precision than is warranted. Or they are averaging over a number of cycles. Having thought about this for quite a while I think synchronizing a free running clock to the line may be the best that can be done. And the accuracy per cycle is probably no better that 1E-4 - maybe worse. Line noise and varying line voltage make this a wicked problem. But then again any time you try for a lot of accuracy the problems get wicked. My backup plan is the H bridge suggested earlier in the discussion. The size of the capacitor can be adjusted for the amount of noise rejection desired. And I must say that it is an elegant solution and the parts count is small. With a 10 mV offset op amp (comparator) and an input voltage of 20 V peak (roughly 14VAC) and a 10 us RC, the timing should be quite consistent. I will report back once I have some results. Which may take a while. Simon Engineering is the art of making what you want from what you can get at a profit. On Monday, February 10, 2014 4:02 AM, Chris Albertson <[email protected]> wrote: > > > > >On Sun, Feb 9, 2014 at 4:27 PM, M. Simon <[email protected]> wrote: > >Tom, >> >>I was hoping for 1E-5 precision or better. My time stamping counter will >>have a 30 MHz clock (for convenience). > > >I think noise will not allow that level of accuracy with only one time stamp >per cycle. I think you'd be better off making many samples per cycle. >Taking 48,000 samples per second give 800 samples per cycle. 48K is the >"standard" sample rate for audio used with video. Likely your computer >already has the means to sample two audio signals at 48K samples per second. >It is not taxing on any modern computer or even for a smart phone. > > >Doing this on my iMac is so trivial I don't bother to save the setup. I have >an audio interface to samples 24 bits at 96K SPS. I placed a 1 volt peak to >peak signal on it and then brought up a spectra display. I can log the >spectra to a file. This kind of software is available for free. >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.
