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

[Daniele Tincani <danieletincani@yahoo.com>]

 

Hello LF, VLF,

 

I don't want to annoy you with my homebrewing stories and questions, however I would greatly appreciate any comment/advice on the following.

 

I have just completed construction of a BBB-4-like "whistlers receiver" and started some very simple tests.

Basically I connected a noise source (Agilent 33120A) to the antenna input of the receiver by means of a short 50-ohms coax cable, then I connected the output from the receiver to the line input of my PC soundcard, using a small 600 ohms 1:1 transformer.

 

I used SpecLab as my spectrum analyser, with its "factory" settings (apart from sample rate and bits/sample, plus adjustments to the spectrogram size, colors and scales).

 

The spectrum taken at the Agilent output appeared to be pretty flat at -114dB on SpecLab from DC to about 22KHz (where a rolloff was visible, I suppose due to the limited bandwidth actually available on the soundcard at 48KHz nominal sample rate).

The resulting signal at the output of the receiver looked in good accordance with the response curve I had obtained from circuit simulation in LTSpice. Please see the attached JPEG's. The small peaks at 4800Hz and 9600Hz were present also in the spectrum taken at the Agilent output.

 

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

 

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

 

Response_from_BBB-4.jpg
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LTSpice_model.jpg
Description: JPEG image