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Message-ID: <006b01cb8026$57e29210$4001a8c0@lark>
From: "Alan Melia" <alan.melia@btinternet.com>
To: "RSGB LF Gp" <rsgb_lf_group@blacksheep.org>
References: <798586.46009.qm@web111901.mail.gq1.yahoo.com> <6EEAA299A2084FACB780B4F9A675C31C@JimPC> <304036.93066.qm@web111919.mail.gq1.yahoo.com> <4CD83D22.50202@iup.uni-heidelberg.de> <011101cb7fae$2383fa40$4001a8c0@lark> <412787.5028.qm@web111908.mail.gq1.yahoo.com>
Date: Tue, 9 Nov 2010 15:42:34 -0000
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Subject: Re: LF: Re: BBB-4-like receiver ready for first tests
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Hi Daniele that looks like what I would expect...it looks like you are
"doing it right", I am not sure but 50mV (probable +3dB for open circu=
it
might be a high level to hit the BBB-4 it might be worth and attenuato=
r on
the BBB-4 input.

Alan
G3NYK


----- Original Message -----=20
From: "Daniele Tincani" <danieletincani@yahoo.com>
To: <rsgb_lf_group@blacksheep.org>
Sent: Tuesday, November 09, 2010 6:18 AM
Subject: Re: LF: Re: BBB-4-like receiver ready for first tests


Hello Alan, LF, VLF,

thank you very much for your suggestions about how to perform more sen=
sible
measurements.

The Agilent 33120A is a general-purpose function generator. Among othe=
r
functions, it is specified to produce a noise signal (gaussian) with=
 10MHz
bandwidth. Amplitude can be set between 50mVpp and 10Vpp into 50ohms,=
 on
open
circuit (probably on the BBB-4 hi-Z input) the amplitude levels become
100mVpp
to 20Vpp.

At 50mVpp (I admit: with no particular precautions about case leakage,
coupling,
etc.), it looks pretty flat on SpecLab at about -114dB (long-term aver=
age
value), at least up to 22KHz with 48KHz sample rate.

The souncard alone shows a noise level which is much lower. Please see=
 my
attachments.

Best regards
Daniele



________________________________
From: Alan Melia <alan.melia@btinternet.com>
To: rsgb_lf_group@blacksheep.org
Sent: Tue, November 9, 2010 2:33:36 AM
Subject: Re: LF: Re: BBB-4-like receiver ready for first tests

I worry that Daniele may be confusing himself here. I dont know that=
 noise
source but is it specified for producing audio white noise?? If not th=
en it
cannot be relied upon to be flat at audio. I made a noise source with=
 a
zenner diode an a couple of transistor stages like the standard noise=
 bridge
source, but bigger coupling caps. This gave a genuine flat response on=
 spec
lab or Argo from low frequencies to 48kHz (96ksampling Delta 44)
If the agilent is intended to work on a 50ohm system then the level wi=
ll be
approximately twice that on the scales. Most audio generators would be
expected to operate around 600 ohms. However there is another problem=
 case
leakage makes most audio generators unreliable for "radio" type work=
 below
1mVrms. The solution maybe to move the generator away from the receive=
r put
an potential divider direct on the receiver input and use screened bal=
anced
output preferably with a centre tapped ground from the generator, not
connected at the receiver.

I would not have thought a whistler receiver should have a peak respon=
se at
2kHz?? Most whistler plots I've seen show best responses 4 to 9kHz.

The BBB-4 seems to have a filter in it and I think it would be best to
measure the response with a calibrated audio generator with a terminat=
ion
resistor across the BBB-4 input. Paul Nicholson has done a lot work an=
d
published the result here on the noise in various front end configurat=
ions.
Also measuring low levels of noise with a PC is fraught with difficult=
y
because they produce so much themselves. You must be very careful that=
 you
are not measuring PC noise. I prefere an oscilloscope or even an audio
mVmeter, You must also remember there may be unexpected effects from=
 you
modification. That headphone lead in use, with the extra gain, can act=
 as a
transmitting aerial!! So you really need to use screened twin cables=
 here.

Alan G3NYK

----- Original Message -----=20
From: "Stefan Sch=E4fer" <schaefer@iup.uni-heidelberg.de>
To: <rsgb_lf_group@blacksheep.org>
Sent: Monday, November 08, 2010 6:10 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 abou=
t
> > 30dB lower (about -107dB around f=3D2KHz 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 <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 leve=
ls
> get so high that the amp stages become nonlinear and/or the soundcar=
d
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 arroun=
d
> 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
>