[time-nuts] precision frequency/time/amplitude reference

Bob kb8tq kb8tq at n1k.org
Mon Oct 9 11:02:16 EDT 2017


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. 


> On Oct 9, 2017, at 10:20 AM, jimlux <jimlux at 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?
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