> I see that from one way or the other, we always end up in a TimePod. OK, > then the TimePod has no comparator, no trigger but has A to D conversions. > Is the A/D conversion process supposed to be threshold-free?
Hey, everybody needs at least one or two TimePods. :) You can use a TimePod or TSC 512xA to measure additive jitter, or for that matter a mixer and a delay line. But these instruments will all do the job by making a phase noise measurement, then integrating the plot to find the equivalent RMS time jitter. This means that you'll have to decide what limits of integration you want to use. A counter, on the other hand, will give you the total jitter seen across its entire front-end bandwidth, so there is less thinking involved. The trouble is, any good shaper or ZCD will have very low jitter, perhaps too low for even a Wavecrest-class TIC to measure. This is what Wenzel's quick-and-dirty differential amp with a pair of 2N3906s looks like, when the splitter test mentioned by Bob is performed with a TimePod, TSC or other phase noise analyzer: http://www.wenzel.com/documents/waveform.html http://www.ke5fx.com/wenzel_shaper_resid_jitter.png That's about 100 fs of additive jitter, measured between 0.1 Hz and 100 kHz. Because the broadband floor is relatively high, a great deal of the total jitter comes from the higher decades. (The circuit's jitter contribution between 0.1 Hz and 100 Hz is only about 10 fs.) A counter will not be limited by the 100 kHz or 1 MHz integration range of a TimePod or TSC 5120, so you might see enough jitter to be noticeable on a Wavecrest in the 1 to 10-ps neighborhood. But maybe you only care about jitter at lower offsets... in which case the counter will make your shaper look a lot worse than it really is. For instance, if the reason you're investigating ZCDs is because you want to build a DMTD, then you may be more interested in a residual ADEV plot instead. The pair of bipolars contributes white and flicker PM noise, so its residual ADEV at t=1s isn't too different from the residual jitter in the ADEV measurement bandwidth, which was 500 Hz in this case: http://www.ke5fx.com/wenzel_shaper_resid_ADEV.png It's worth noting that I made these measurements on a TSC 5120A. The phase noise measurement could have been made on a TimePod, but the residual ADEV plot could not, as it's below the TimePod's ADEV floor. To me, this says that there are better ways to spend one's time than designing a fancy multistage ZCD. The important thing is to consider how much bandwidth is really required in your application, and whether/how it should be limited. -- john, KE5FX Miles Design LLC _______________________________________________ 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.
