Re: VLF: BBB-4-like receiver ready for first tests

[Stefan Schäfer <schaefer@iup.uni-heidelberg.de>]

Hello Daniele,

Q1: could such a response curve be suitable for reception of sub-9KHz transmissions in the dreamers band or for chasing whistlers only?

The frequency response appears a little suboptimal to me since the mains hum would be even more amplified instead of attenuated. But i think it is rather useful to check the actual antenna configuration at its planned location. There you will see if and how much QRM bothered the system is. The final frequency response is not only a function of the amplifier but also to the location of the antenna. The lower frequencies are more attenuated by a lossy environment (trees, buildings). This effect can even be seen within the 0...24 kHz range. So maybe this amplifier frequency response can linearise the final frequency response so that the curve appears rather flat.

In my opinion it is useful to apply some band limiting. This can easily be done by a (or some) simple RC LPF and HPF. It is useful to exclude the strong LF/MF stations from the soundcard input and extremely strong mains hum components.

Recently i have been going arround on the roof of our institute (abt 30 m above GND) with headphones, a netbook and a battery fed active E field antenna. There is a big difference if one is 2m or 4m or 6m distant of the buildings wall. Imagine the E field lines and/or the local field strength of that arrangement. As i was going to maximum distance (on the edge of the floor), the signal levels got so strong that the S/N of DHO38 decreased from 50 dB to 30 dB and i could listen to AFN (US BCD stn).

73 and good luck!

Stefan/DK7FC

 

Then I created an external short-circuit on the BNC input connector and looked at the output. I hoped to get some information about the internal noise of the receiver. The resulting noise curve was basically the same as the response curve when white noise from the Agilent was applied at the antenna connector. I deduce that an internal noise source exists in the receiver, close to the input stage.

 

Q2: am I wrong? Is this normal (I mean: expected)? Should/could it be improved by means of modifications in the circuit?

 

Best regards

Daniele