Hi About all I’d say is that if Jim Barnes said that’s the way to do it. then that’s the way to do it. There are only a very few people who I’d say that about.
Bob > On Nov 23, 2014, at 9:05 AM, Jim Lux <[email protected]> wrote: > > I'm writing a short simulation program to generate samples from a analog > system with some op amps, etc., and I'm wondering if anyone has some > practical experience on picking parameters for the generator. > > I'm generating minutes worth of data sampled at 1 kHz, and my opamps have > their flicker/white knee at around 3-4 Hz (at least that's what the LT1679 > data sheet claims.. we shall see if the model matches the data sheet matches > what I measure on the actual hardware) > > I'm using a Barnes-Jarvis (or Barnes-Greenhall) type generator for the > flicker noise, which basically sums up a bunch of stages to create an > arbitrarily smooth representation. See threads: > https://www.febo.com/pipermail/time-nuts/2010-April/046926.html > https://www.febo.com/pipermail/time-nuts/2013-November/081534.html > > The actual PTTI paper is > > http://tycho.usno.navy.mil/ptti/1987papers/Vol%2019_19.pdf has the details > http://tycho.usno.navy.mil/ptti/1992papers/Vol 24_44.pdf has some > corrections, but is a partial page.. > > You need to pick a few parameters: how many stages to cover your frequency > band of interest, how big the frequency steps are (e.g. octaves), and where's > the "top band" filter cutoff (typically 0.3 to 0.5 relative to the sample > rate) > > If you picked 4 stages, with a starting frequency of 0.4, and octaves(R=2), > then the individual filter cutoffs would be > 0.4 > 0.2 > 0.1 > 0.05 > > I'm interested in the behavior down in the 1 Hz and below range, say, to 0.01 > Hz. So to cover 0.01 Hz to 1000Hz, one would need about 16-17 octaves which > is an enormous number of stages and I've got to believe you'd have all sorts > of numerical problems > > And I think I don't need to do this > I can add white noise to establish the noise floor to match lab measurements > (there's sources other than the op amps) for higher frequencies, say in the > 20-1000 Hz area. > > It would seem, then, that I can start the first filter at around 5 Hz and go > down from there, if my assumption that most of the flicker noise is coming > from the opamp and it's flicker noise comes above the thermal noise at 3-4 Hz. > > Then, going in, say, octave jumps, I can get down to 0.01 Hz in about 8 > steps. (this seems to match Figure 2 in the paper.. they used a 8 stages > with a frequency ratio of 2.4, and the spectrum looks pretty flat for a good > 5 decades. > > I suppose I could just write it and see what comes out, but if someone out > there has worked with this kind of thing before, a bit of practical guidance > would be useful. > > > _______________________________________________ > 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. _______________________________________________ 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.
