Hello Daniele!
I can't comment on the BBB4-RX because I don't know about it's design.
In my experience, a MiniWhip (PA0RDT, tnx Roelof)-like design of
antenna, is the best and most convenient way to receive VLF-signals
transmitted by amateurs. Just bring it some (the more, the better!)
meters away from noise-sources and it will work fine.
To match it a little bit better to VLF, one could increase the
capacitors and coils used in the original design. I also did some tests
with different transistors (BF1005 and BFY90 as 2nd stage) but I found
out that the difference is marginal.
I know this is a bit "off-topic", cause you asked for the BBB4...just
keep it in mind in case you are not happy with your (maybe too
complicated) VLF-setup.
73
OE3GHB
Gerhard
Am Sonntag, den 07.11.2010, 10:56 -0800 schrieb Daniele Tincani:
>
> 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
>
>
>
>
>
>
>
>
>
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