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
From:
Stefan Schäfer <[email protected]>
To:
[email protected]
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
From:
Stefan Schäfer <[email protected]>
To: [email protected]
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.
From:
James Moritz <[email protected]>
To: [email protected]
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
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