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

Mon Oct 9 10:20:12 EDT 2017

```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?
```