VLF baseband receiver: FAQ

 
 

Are the transmitter call signs reliable?

The call signs assigned to frequencies originate from the ITU BR IFIC listing (2018).

 
 

Oh wow, the whole waterfall is covered by horizontal lines!

The VLF band is the home of the sferics, i. e. effects of lightning discharges. Atmospheric discharges cover the whole frequency range, so you observe them as horizontal lines.

This is the impact of one medium-sized thunderstorm cell in a distance of 400 km:

 
 

 

You talk about an electrically quiet rural location. What is this noise fence all over the band?

There is a robotic lawn mower working on the property. His name is "Rasenrobi" and he is on duty between 08:00 UTC and 14:00 UTC from monday to friday. The problem arises from his guiding loop which is supplied with a signal in the medium wave frequency range. The steep impulses generate noise down into the VLF band.

According to the lawn mower's duty cycle, the RRI (Rasenrobi Interference) lasts for about 80 minutes, while he is mowing the lawn. Then he has to return to his base station to load his batteries. During this time the guiding system is shut down. Just be patient or return after his working hours.

 
 

Rasenrobi

Rasenrobi

 

What is this >etherprobe<? Something before Einstein?

The etherprobe is my version of an e-field probe. Impedance transformation is carried out with a JFET op-amp instead of single transistors as applied in the PA0RDT MiniWhip. The functionality is absolutely the same with the probe picking up the electric field. It is even powered by the nice PA0RDT remote power supply.

I added a 3-pole active low-pass filter as an anti-alias filter to prepare the operation of the ADC following. It is provided with a Chebyshev response for a characteristic frequency of 47 kHz. With 0.5 dB maximum passband ripple, it achieves a roll-off steep enough to suppress signals from beyond the Nyquist frequency.

The Flatmax Audioinjector audio adapter used as ADC seems to lack a sufficient low-pass filter. Before introducing the Chebyshev filter, there appeared aliased signals from MSF and other strong transmitters in the passband.