From: "John Miles" <[EMAIL PROTECTED]> Subject: Re: [time-nuts] Fast frequency counting question Date: Sun, 4 May 2008 16:55:29 -0700 Message-ID: <[EMAIL PROTECTED]>
> > The quickest and easiest way to achieve the required resolution is to > > buy a high speed (~100MHz) sampling ADC evaluation kit from Analog > > Devices, Linear Technology etc, use a low phase noise crystal oscillator > > (eg Wenzel ULN or equivalent performance device) or equivalent bandpass > > filter the output and use as the sampling clock source. Then either: > > > > 1) Sample the 40MHz signal directly (possibly after some bandpass > > filtering) then post process the raw data. > > > > 2) Use a mixer to produce a ~1MHz output bandpass filter it and sample > > this signal (amplifying if necessary) with the ADC then post process > > the raw data. This will achieve lower noise. However the mixer LO has to > > have very low phase noise. > > > > High speed ADC evaluation kits are readily available (at least from > > Linear) and considerably cheaper than a timer counter with equivalent > > resolution and noise f such an instrument is available at all. > > The LTC evaluation kits include a board with local sample storage and a > > USB interface to a PC. > > The last time I looked at those eval boards, though, they were pretty > adamant about not releasing API specs, so you couldn't use them as platforms > for your own DAQ applications. Irritating, but I can see why, because > they're probably just breaking even on the boards. > > Would be nice if that's no longer the case. Failing that, one of the GNU > Radio USRP boards might be the most economical way to go. > > Also: the effect to be studied must be coming from some type of circuit > built with some standard semiconductor process, which in turn is driven by > some sort of clock. If you go nuts with sampling precision, aren't you just > going to get more information than the underlying process is capable of > generating? (I'm probably not phrasing that very well, but you get the > idea, hopefully.) Oh... what can happend the first 40000 cycles in an oscillators power on time? Caps gets biasing, silicon heating up and biases moves, transient behaviours time out and it also takes a few cycles to start cleaning out noise. The AGC takes time to bias... Not to speak about the actual transient of power-on... Then, the initial drift of the crystal itself? The first few cycles will not be pretty. That's for sure. Interesting that they need to measure it. The heat-up time should be specified in the datasheets. There are different levels to heat up in this aspect... digital circuits usually needs a low jitter enought clock for the initial operation. Only for full operation the stability come into play. Just don't release the reset before you approach something of a usefull clock. It is to be expected. If you depend on the first cycles to be clean, I guess you have set yourself up for trouble. I always assume the oscillator will be dirty. I have never dreamt of assuming anything else. Cheers, Magnus _______________________________________________ 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.
