SOLVED: Identifying interference with oscilloscopes and software-defined radio

[djhuft]

First of all, "our" frequencies of interest are filled with noise, 3-30 (100) KHz, and most of it comes from lightning discharges.

There is ALWAYS natural noise, and most in areas with lots of lightning.

If we want high accuracy, we need signals with short rise time (high frequency) - It is achieved only close to the flash discharge (10-20 to 100-200km)
When we get closer, we are disturbed by the "precharges", and are we farther away, the higher frequencies are attenuated, and finally there are only signals around 10-12KHz
Dedicated receivers for long distance have a narrow range around 11KHz 

Most spikes you see in an FFT analysis mean so little that there is no reason to dampen them further

I can not see you need extra filters

Finally I show pictures of what different filters do.
12KHz is very hard, but here it works well on middle and long distance.

/Richo

Thanks a lot, Richo! It is very interesting to know this information about the physics and various frequencies.

I just captured the following screenshot with my oscilloscope:

The large peak in the FFT plot is actually centered at 25.2 kHz, and its amplitude is about 10 mVrms.
In comparison, my thresholds are set at +/-200 mV, so you are correct, the interference is indeed making only a very small difference. I should have considered this sooner.

It is interesting to consider the effect of filtering on timing too. I can understand why it is important to balance signal rise time requirements with filtering in order to get accurate timing information. After all, radio signals travel 1 km in 3 microseconds. .

Very interesting! Thank you.