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

[Daniele Tincani <danieletincani@yahoo.com>]

Hello Stefan, LF, VLF,

 

SS> So just look at 9 kHz without the generator and then increase the noise gernerator level so that the noise increases say 20 dB.

 

DT> OK I started with the minimum adjustable level on the Agilent 33120A (50mVpp) and got about -86dB on SpecLab at 9KHz. Then I increased the output from the instrument up to 510mVpp and got about -66dB on SpecLab. This sound OK to me (20log(510/50) = 20.2dB). The response curve is still very similar to the simulated one.

At 9KHz without the generator, with the BBB-4 switched off and max attenuation on soundcard input (I have a potentiometer on the isolation transformer to the PC) I read about -148dB on SpecLab at 9KHz.

SS> What i would worry about is the high gain arround 2 kHz, where the mains hum is dominant, even if the frequency response (without an input signal) would be flat. Maybe this could become a problem if the levels get so high that the amp stages become nonlinear and/or the soundcard input.

DT> Yes, the BBB-4 was explicitly designed by McGreevy for reception of natural radio emissions in locations far away from power lines, buildings, trees, etc. (basically, in the middle of a desert :-)).

SS> But just try what happend in reality!! In my experience it is a good indicator to see the diurnal noise levels having a minimum at arround 8...10 UTC. If the level difference is about 10...15 dB (depending on the WX of course) it is a good first step to assume a sensitive RX.

 

DT> Good test, I will try probably next saturday. I could start say at 6.00 UTC and try to collect as many hours of a broadband spectrum as allowed by residual battery life.

 

Cheers

Daniele

 

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From: Stefan Schäfer <schaefer@iup.uni-heidelberg.de>
To: rsgb_lf_group@blacksheep.org
Sent: Mon, November 8, 2010 7:10:42 PM
Subject: Re: LF: Re: BBB-4-like receiver ready for first tests

Hi Daniele, Jim, LF,

Am 08.11.2010 16:47, schrieb Daniele Tincani:

Also consider that when I created a short circuit on the antenna input of the rx (see Q2 in my e-mail), I got a response curve on SpecLab similar to that I had with the Agilent, but with a peak level about 30dB lower (about -107dB around f=2KHz on SpecLab).

This means that your generated noise level was to low and you have observed the soundcards noise, at least outside the region of 2 kHz. So just look at 9 kHz without the generator and then increase the noise gernerator level so that the noise increases say 20 dB.

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From: James Moritz <james.moritz@btopenworld.com>
To: rsgb_lf_group@blacksheep.org
Sent: Mon, November 8, 2010 3:22:59 PM
Subject: LF: Re: BBB-4-like receiver ready for first tests


The filter rolls the gain off rapidly below about 1kHz and above about 10kHz. So this would be OK for whistlers and 9kHz reception, but would attenuate VLF utilities at higher frequencies.

Normally the MSKs are so strong that an attenuation of say 20 dB (compared to 9 kHz) will probably no problem. 20 dB gain reduction would not even mean 20 dB S/N reduction...

What i would worry about is the high gain arround 2 kHz, where the mains hum is dominant, even if the frequency response (without an input signal) would be flat. Maybe this could become a problem if the levels get so high that the amp stages become nonlinear and/or the soundcard input.

But just try what happend in reality!! In my experience it is a good indicator to see the diurnal noise levels having a minimum at arround 8...10 UTC. If the level difference is about 10...15 dB (depending on the WX of course) it is a good first step to assume a sensitive RX. The rest can be seen in tests where a far field signal is generated on the Dreamers Band ;-)


73, Stefan/DK7FC