Dear Stefan, LF Group,
----- Original Message -----
From: "Stefan Schäfer" <[email protected]>
...>
In Spectrum Lab there are different methods for noise reduction (e.g. of
lightnings). Those were discussed here in the last weeks. While
Jim/M0BMU preferes the clipper (using 0 dB above AVG if i remember
correctly), Markus/DF6NM prefers the noise blanker.
...>
I don't really have a particular preference; both seem to give a similar
improvement in apparent SNR, and both will introduce some intermodulation,
since they are inherently non-linear techniques. Your spectrograms seem to
confirm there is not a great difference... BTW, the Spectrum Lab clipper,
where the clipping threshold tracks the average signal level, does seem to
give better results than simple fixed-threshold clipping, especially when
other signals or narrow-band noise are present.
...>
I still have to reduce the IM in the first stage (2nd order low pass
filter at the output of the first stage does not better the situation),
either by reducing the wire length or adding a series resistor or...hm,
will see (any ideas?).
...>
At my QTH, there are 4 MF broadcasters within 1km, giving combined peak FS
of 10s of volts per metre, so I get these problems a lot... I find it is
important to keep an open mind about possible sources of this kind of QRM.
The first thing is - are you sure the intermodulation actually occurs in
your active whip? Any length of wire acts as an antenna, and will often be
connected to some non-linear device (e.g. mains cable connected to rectifier
diodes), so there are an almost infinite number of places outside the
antenna circuit that the 9kHz IM product could be generated.
A prime candidate is the earth connection - if your antenna is high up on a
building, the earth connection will be carrying considerable
broadcast-frequency current - I have found this often results in
intermodulation, perhaps due to some kind of electrolytic effects occuring
at the actual ground electrodes. This effect makes my TX antenna unusable
for reception at many frequencies. Any mains power supply connected to the
antenna preamp can cause a similar effect if the internal diodes,
regulators, etc. are not well decoupled at broadcast frequencies.
Another place where intermods can occur is the PC, which will typically have
long cables connected to it (mains, network, sound card etc...) again acting
as antennas and carrying MF broadcast currents. Any intermod generated in
something connected to the PC can be coupled into the sound card inputs by
the ground loops that tend to exist inside the machine. For example, I found
using my 9kHz antenna/preamp at home with a laptop with AC adaptor gave a
similar 9kHz line to the one you see. I eventually found this was still
there with the preamp output disconnected from the sound card, but with the
earth connection present between the computer and the preamp. I assume the
cables connected to the preamp were acting as an antenna, and flowing to
ground through the PC and adaptor. The cure was to wind several turns of the
AC adaptor lead through an EMC ferrite core to reduce the MF current flowing
in it. For the DK7FC grabber, you could easily test if the 9kHz comes from
the antenna or PC end of the system by un-plugging the optical fibre.
Then there are usually many other cables, or large pieces of metal, near the
antenna, via which intermods can be occuring. The resulting 9kHz
voltages/currents will be picked up by your antenna. Moving the antenna is
the easiest way to check this, or also to solve the problem, usually.
But then, if the FS is high, it may be that the active antenna is the
problem after all. In your circuit, it is unlikely that IM is occuring after
the input stage due to the presence of the bandpass filter. You could try
adding a source degeneration resistor to reduce the gain and improve the
linearity of the input stage (say 100R, but you would also probably need to
change the gate bias voltage to get a sensible bias current). I would
suggest putting about 47R resistor in series with the gate, to reduce the
likelihood of VHF oscillation occuring, which can cause strange problems,
including intermodulation at low frequencies. Probing the drain with an
oscilloscope would reveal what large signals are present. Also, the drain
resistor in the second stage needs looking at - 1k is rather too high a
value for a 12V supply and a FET with zero gate bias and Idss of 10 - 25mA.
BTW, problems with high broadcast station field strength were some of the
reasons leading me to favour a loop antenna design, for 9kHz and other
frequencies as well. It is much easier to include filtering directly at the
antenna terminals, since the loop is much lower impedance. Also, ground
connections are less of an issue.
Cheers, Jim Moritz
73 de M0BMU
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