Hi > On Oct 23, 2014, at 2:01 AM, Poul-Henning Kamp <[email protected]> wrote: > > -------- > In message <9A96CAA5BA7B467D9A106EC858EA0DCE@pc52>, "Tom Van Baak" writes: >>>> 3) Every instant on a sine wave is actually a data point, not just >>>> the zero crossing(s). So in reality there is near infinite information >>>> available. >>> >>> Sorry, but no. >>> >>> If you tell me it is a sine and give me the time of two zero crossings >>> I can tell you everything there has or ever will be to know about any >>> point on that sine-wave. >> >> What about phase noise measurements? > > Yes, what about it ? > > The CE mark requires that you meet various EMI/EMC rules, one of which > is in essence a phase-noise requirement for a 0Hz carrier. > > But it didn't take long for vendors of clock generator chips to solve > that issue: The FM modulate the clock so that if you follow the prescribed > measurement procedure there is no spikes above the red line. > > However, if you measure it without the heavy averaging usually required > for phase noise measurements, you find that it is as noisy as always, > but at varying frequencies which average out. > > All statistical treatments of signals are subject to this kind of > effect because the entire point of using a statistical treatment is to > reduce the amount of information to something we can cope with. > > ADEV throws a lot of information away (otherwise we wouldn't need > phase noise), phase noise throws a lot of information away (otherwise > we wouldn't need ADEV) etc. > > There's no way around thinking critically about what information > you throw away and why in each specific application. > > In the context where this issue came up, the trowing away was about > how much of a signal you should feed into a PLL that steers an *XO. > > There we want to throw as much noise away from the input signal as > possible while still keeping the *XO at frequency. > > Therefore you only want to feed the minimum amount of information > about the input frequency necessary for the frequency steering into > the PLL, anything above that just adds noise. > > In this application any information excess to the frequency of the > input signal will be noise, and that includes any spurs, harmonic > or not. > > That's why we usually throw all information about amplitude away > and focus on one direction of zero crossing, which is (or with a > trivial capacitor can be made) well defined for any signal. > > And to return to the original question: > > The only reason to look at both zero crossings would be to double > the frequency of the input signal to the loop (ie: 2Hz from a 1PPS > instead of a 1Hz), at the cost of adding a whole lot of noise > in the process. Don't do it. > > -- > Poul-Henning Kamp | UNIX since Zilog Zeus 3.20 > [email protected] | TCP/IP since RFC 956 > FreeBSD committer | BSD since 4.3-tahoe > Never attribute to malice what can adequately be explained by incompetence. > _______________________________________________ > 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.
Looking at both zero crossings would give you a lot of information about the duty cycle of the input waveforms. If that’s what you are after - there are easier ways to do it. If that’s not what you are after, it’s just going to mess up the readings. Bob _______________________________________________ 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.
