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

[Daniele Tincani <danieletincani@yahoo.com>]

Hello Stefan, LF, VLF,

 

now next questions: :-)

1) is the noise of the active antenna well below the typical band noise?

2) is the dynamic range large enough to manage the strongest hum levels at my listening position?

 

Ideally I would like to get some information about the MDS and the SFDR (at least) of the active antenna, for example at 2KHz and 9KHz, by injecting test signals at increasing levels.

 

I'm trying to figure out what the simplest and cheapest testbench could be for this purpose. Probably I will need to build at least two good LPF's at 3KHz and 10KHz (to get rid of harmonics from the generator) and a variable attenuator (may be a simple potentiometer in series with a high-value resistor could do the job?). Then I would place an oscilloscope on the rx input and a spectrum analyser on the rx output and try to get some (very rough!!) information about the MDS and the SFDR. Does the above look sensible?

 

To capture several hours of reception (including the daily QRN minimum) at my usual listening position would be OK for attempting a direct performance comparison with my loop antenna, but I'm afraid that local hum could hide real figures. I think I should at least perform a similar data collection in a different location, far away from powerlines, in order to get an approximate assessment of the rx sensitivity.

 

Best regards

Daniele

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From: Stefan Schäfer <Stefan.Schaefer@iup.uni-heidelberg.de>
To: rsgb_lf_group@blacksheep.org
Sent: Tue, November 9, 2010 3:02:47 PM
Subject: Re: LF: Re: BBB-4-like receiver ready for first tests

Hi Daniele,

OK now. So you have done it right. The generated noise is well above the noise of the soundcard+the noise of the antenna. And the noise of the soundcard+the noise of the antenna is well above the noise of the soundcard (you mentioned -148 dB). So, you should see the actual frequency response of the active antenna in Figure_1 :-) As far as i understand it (am not an expert i think ;-) ).

And now, what do you want to do now? :-)

73, Stefan

BTW: You are in a good position to my earth antenna, almost perfect! :-)


Am 08.11.2010 23:34, schrieb Daniele Tincani:

Stefan,

 

the soundcard is a M-Audio Audiophile 2496 PCI. The sample rate is 48KHz at 24 bits/sample (as configured on SpecLab, but the same is reported by both Windows and M-Audio control panels). The FFT settings are those I get by selecting the dafault "factory" settings on SpecLab, I attach a JPEG of the configuration panel for clarity.

 

So, I have the generator at its minimum amplitude, giving -86dB (at the output of the BBB-4) measured on SpecLab at 9KHz. Attached here as Figure_1.

Now I switch the generator OFF (power switch OFF) and leave all the rest untouched. The result is showed in Figure_2.

 

Best regards

Daniele

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From: Stefan Schäfer <schaefer@iup.uni-heidelberg.de>
To: rsgb_lf_group@blacksheep.org
Sent: Mon, November 8, 2010 11:08:14 PM
Subject: Re: LF: Re: BBB-4-like receiver ready for first tests

Hi Daniele,

-148 dB is a very low noise level for a soundcard! But what is the sample rate and FFT settings? I remember you have a good one, not just a PC internal or cheep USB card...

Again, what happens when you switch the noise generator OFF (after generating -86 dB) and let the active antenna and PC settings as is?

73, Stefan

Am 08.11.2010 22:53, schrieb Daniele Tincani:

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