Hi Pick a couple of local broadcast stations and record them. That will give you a baseline for each of the parameters you are after in real time. They *will* drift.
Past that, I’d go with a sweep of each node before installation. That will give you the frequency response and (to some degree) a guess for noise and spurs. Bob > On Oct 9, 2017, at 10:20 AM, jimlux <jim...@earthlink.net> wrote: > > I'm trying to come up with a relatively simple scheme to calibrate an HF > antenna array - I've got a bunch of RTL-SDRs operating as a distributed array > spread over a few 10s of meters. > > The things I want to do are: > a) determine the phase/time offset between stations relative to other nodes > b) determine the sample rate (clock rate) variation relative to other nodes > c) determine the amplitude calibration (of each node). > > One of the schemes I cam up with was to take the output from the sample clock > oscillator, divide it down to around 500 kHz or 1 MHz, and then use that to > generate short pulses by switching a precision voltage reference. > > That pulse train (the spectrum of which is a comb with lots of harmonics) > would be connected to the antenna of the node. > > So, I get a precise amplitude pulse train into my own node receiver - so I > can calibrate my receiver gain. And, I radiate a low power pulse train to > the other nodes. By looking at the digitized signal on the other nodes, I > can figure out relative clock rate (and, to a lesser extent, whether the > antenna has changed) > > This scheme seems to hang together, but a lot depends on that switch that > turns my internal clock derived pulse train into a precise amplitude and > edges. > > Off hand, it seems that almost any sort of transistor (BJT or FET) would work > as long as the rise/fall time is fast enough to get the harmonic comb up high > enough (I'm only interested up to, say, 50 MHz - yeah, the RTL-SDR will tune > higher, but for this project I'm not so worried about that). > > I suppose too, that I could do a "bench calibration" of each unit the first > time (to take out component/component variations in the switch), as long as > the switch properties are stable, or at least vary in a known way. > > In terms of amplitude, the amplitude of the fundamental should be pretty > stable, but I can see the relative amplitude of the high harmonics falling in > precision - small changes in switch rise/fall time will affect that more. > > In terms of frequency, I think it should work fairly well - I tune the RTL's > front end, look for my calibration combs that are "in band" and fit an > appropriate function to the signal (there should be a well defined phase > relationship between the harmonics) > > So, one remaining issue is how to get "time" out of this. Since the > individual nodes are battery powered and not connected to a network in real > time, I would assume that their internal clock is good to maybe 1 second. I > was thinking I could try and encode a standard time code (Irig) on the pulses > from my comb generator, either by changing the pulse rate, or by changing the > pulse width? > > Any other clever ideas? > _______________________________________________ > time-nuts mailing list -- time-nuts@febo.com > To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > and follow the instructions there. _______________________________________________ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.