[time-nuts] Cable length calibration

[Attila Kinali]

On Wed, 29 Jun 2016 20:18:49 +0000
"Poul-Henning Kamp" <phk at phk.freebsd.dk> wrote:

> If it wasn't behind a choke, the inrush current to the antenna
> preamp power filtering capacitor could be measured, but the choke
> ruins that.
> 
> The trouble is how to do it without frying the antenna preamp...

That's rather "simple". The impedance of the LNA is anything but
constant over frequency, said capacitor with the choke are one of
those things that make it different than 50 Ohm. You only need to
find one frequency at which the impedance is close to a short
or to an open and then send down a sine at that frequency and
measure the reflection. This is probably easier than doing TDR,
but unfortunately it is still quite involved.

Another way would be to send down a sine at a known frequency,
couple it out at the LNA and inject sharp pulses into the
antenna, at the rate of the sine. This way the whole path
through the antenna and the LNA down to the receiver can
be measured. It even makes it possible to measure the phase
differences between different frequencies (think L1, L2).
But this would require a custom receiver.

			Attila Kinali

-- 
Malek's Law:
        Any simple idea will be worded in the most complicated way.