[Ronja] News from the RX development

[Karel Kulhavy]

I have improved the frontend even more so the range increase should be even more than 35%.

The trick is that the working resistor of the photodiode creates most of the receiver
noise. The lower the value, the more noise. Low value is needed when Ronja runs in sunlight, where the noise doesn't matter because there is much more noise from the sun
anyway. But in the darkness the value could be increased dramatically and the noise dramatically reduced.

For this I made a photocoupler from a photoresistor and green superbright LED. It can
regulate the resistance from 90 Ohm to beyound 40 Megaohm (where my meter doesn't measure
anymore). The current standard resistance is 120 kOhm and the one that would improve range by 35% is 390k.

Now I made a PI controller http://en.wikipedia.org/wiki/PID_controller that tries to keep the photodiode voltage at a constant.

I have check the signal shape and it's OK. I don't have enough stages built yet to
truly measure the range.

Karel