Hi Jim
Thanks for that, I knew we could depend on an analysis from you.
You confirm my thoughts and findings, I went for a step tuned, low loss
loop, I get 20dB gain around >1kHz wide on tuning to 9kHz and can't see
a need for a pre amp. With a self resonance around 600kHz it should be
quite useful.
My sound card is clean flat and quiet, I do appreciate the need for a
pre amp to overcome a noisy one. I am trying out a 4 turn coupling loop
I don't know what impedance this is giving me but it is a better match
for a 4 to1 isolating transformer to the sound card than a 1 to 1
transformer and seems to work well.
Using this method there is no need for any filtering of lower or higher
frequencies, one disadvantage being the rejection of MSK transmissions
for frequency locking when the loop orientation nulls out the one you
are using.
Given some fine WX I will do some more test outside, essential to get
away from the computers and crud from mains appliances I find.
73 Eddie G3ZJO
On 05/03/2011 22:47, James Moritz wrote:
Dear Eddie, Gary, LF Group,
The loop is tuned, but is very low Q, and has more of a low-pass
response really. It was done this way partly in order to obtain a wide
bandwidth around 9kHz, which allows effective use of noise blanking or
clipping (implemented using the DSP facilities of Spec Lab) to reduce
the impact of QRN impulses. It also avoids the need for re-tuning,
which is very convenient. At the same time, it provides a decent
amount of attenuation of signals above the VLF range, which would
otherwise tend to overload either the preamp or the sound card input.
The frequency response is defined by the loop inductance, the shunt
capacitor at the input, and the input resistance of the preamp stage
(about 300ohms, due to the series/shunt negative feedback). The low Q
means the loop voltage is small, so a fairly low-noise preamp is
needed. The band noise has always been >10dB above the internal preamp
noise in practice, so this is satisfactory. The overall gain of the
preamp is about 60dB at 9kHz, and this was chosen as a compromise - it
is large enough so that the band noise swamps the internal noise and
QRM in the sound card input, but small enough so that the utilities
and QRN do not overload the sound card.
You can get much higher signal voltage out of the same loop antenna by
changing it to a high Q resonant design. I guess with the relatively
high impedance load of the sound card input (several kohm), you might
get a Q between 10 and 100. This would give a voltage gain of 20 -
40dB, more if you made a really low-loss loop and optimised a tapping
point for the sound card input connection. But the bandwidth would
also be greatly reduced, probably to only a few hundred Hz or less,
and this is a bad thing. The narrow bandwidth "stretches" the QRN
impulses making clipping / blanking less effective. Also, accurate
tuning is then required.
I suppose one motive for having such a high Q loop would be to
eliminate the preamp. This might work, depending on how noisy the
sound card codec chip and the PC it is attached to are. My experience
has been that this varies a lot between different PCs and frequencies,
and which audio input you use. E.g. the laptop I usually use has a
"Mic" input with considerably higher gain than the "line" input. But
for some reason, the SNR is worse even though the signal level is
higher when using this mic input. Also, the input noise level may vary
depending on the source impedance of the antenna connected to the
input. Since the preamp components only cost pennies, and allow you to
get ample SNR as well as several dB improvement through the use of
noise clipping and "no-tune" operation, this seems a better and
simpler solution.
When comparing noise levels, it is important to check what the noise
actually is, especially since PCs and associated stuff are potentially
powerful noise sources. Listening to the output around 9kHz using the
SpecLab VLF SDR function is useful - you are hoping to hear the band
QRN and probably assorted mains-borne noises, rather than a lot of
digital hash from the PC.
Cheers, Jim Moritz
73 de M0BMU
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