Dear Daniele, LF Group,
You wrote:
...>The spectrum taken at the Agilent output appeared to be pretty flat
at -114dB on
SpecLab from DC to about 22KHz
The info missing from your test is what noise output level the Agilent AWG
is set to - I imagine this would be volts RMS over some noise bandwidth.
The -114dB level you quote is very low, and possibly just due to the
internal noise of the sound card itself (my PC shows a level of
about -118dB). Does the noise floor fall when the sound card input is
disconnected from the generator? If the generator level is very low, this
would account for there being little change in output level with the
generator disconnected. Try increasing the generator output level - does the
output from the circuit increase by the same amount? If you want to check
the circuit gain, it would probably be better to use a sine wave input
signal.
...>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.
Yes, that looks reasonable - so the circuit is probably mostly working,
although without positively measuring the gain, it is still possible there
is some fault around the input. I would expect an overall voltage gain
roughly around 60dB.
...>Q1: could such a response curve be suitable for reception of sub-9KHz
transmissions in the dreamers band or for chasing whistlers only?
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. This could be a problem if
you wanted to use a VLF utility for frequency-locking purposes. Reducing L1
to 22mH increases the upper cut-off frequency to over 20kHz - it also
increases the in-band gain slightly too. C3, R4 and D1, D2 also will reduce
the gain at higher frequencies, so you may wish to reduce R4.
Cheers, Jim Moritz
73 de M0BMU
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