Message-ID: <4BF7051B.7010309@iup.uni-heidelberg.de>
Date: Sat, 22 May 2010 00:11:39 +0200
From: =?ISO-8859-1?Q?Stefan_Sch=E4fer?= <Stefan.Schaefer@iup.uni-heidelberg.de>
User-Agent: Mozilla/5.0 (Windows; U; Windows NT 6.1; de; rv:1.9.1.8) Gecko/20100227 Thunderbird/3.0.3
MIME-Version: 1.0
To: rsgb_lf_group@blacksheep.org
References: <008f01caf784$779a11d0$0301a8c0@your91hoehfy9g> <Pine.LNX.4.64.1005192058370.18919@paranoid.lipkowski.org> <4BF4FBF4.2020208@iup.uni-heidelberg.de> <4BF53399.4010403@iup.uni-heidelberg.de> <1C7BCD9DEB29472F9BB467C4EE390840@JimPC>
In-Reply-To: <1C7BCD9DEB29472F9BB467C4EE390840@JimPC>
Subject: Re: LF: Re: Clipping or blanking/9kHz intermods
Content-Type: text/plain; charset=ISO-8859-1; format=flowed
Content-Transfer-Encoding: quoted-printable
Sender: owner-rsgb_lf_group@blacksheep.org
Precedence: bulk
Reply-To: rsgb_lf_group@blacksheep.org


Hello Jim and all others,

Tnx for your ideas!
I could answer in a long mail, as usual ;-) but today i try to keep it=20
shorter ;-)
I have done some modifications to the active antenna. The main change is=
=20
a source resistor of 100 Ohm, of course without the parallel C. This=20
reduces the voltage gain of the first stage but anyway. It also improves=
=20
the linearity. Additionally i meant that the audio input level was to=20
high for the soundcard (the software audio level control was just a=20
little above zero!). Thus i reduced the circuit from 3 amp stages to 2=20
stages (2x BF862, the 2nd is the LED driver as well). Now the volume=20
control is set in the middle, fine. But the overall level is abt 20dB=20
less and so one can see spikes in the ragnge of 5 to 15 kHz that comes=20
from the PC ( i supply the optical/electrical back converter out of the=20
12V of the PC). By adding a common mode choke i reduced those spikes=20
arround 15 dB but they are still visible, anyway. Mostly i want to get a=
=20
clear 8,96...8,98 window, anything else is not really interesting for me.
The IM is still available but its influence on the desired frequency=20
decreased strongly. There is such a strong decrease of QRN on the 600=20
window, i almost cannot belive it. I am still not sure about that...=20
This happend from 18 to 20 UTC (still visible on the 2nd grabber). First=
=20
i thought the battery gets empty but later some VLF transmitters came up=
=20
again so i hope this is really the noise decrease that occurrs each=20
evening. If this is true, the new arrangement could be much more=20
sensitive, i hope so.
BTW, isolating the ground from the lightning arrester changed nothing (i=
=20
assume still abt 20 pF coupling capacity).
To see if something is wrong with the circuit or if the QRN in the 600=20
window is still caused by IM (this would be wondering because the noise=20
appears very homogen now, not like yesterday) i do transmit now a signal=
=20
out of my VFO that is connected to a 10m long wire that is in the near=20
of the main (1st) grabber in the 2nd floor, so the distance is abt 25m=20
while the antenna voltage is 6,7 Vp. I am interested how the 600=20
window(s) will look tomorrow in the morning :-) Just for fun i do change=
=20
the QRG as if i would transmit DFCW-600 but without switching out the=20
signal during the letters (or e.g. between 2 dots), just to see a=20
difference to lines that could be caused by IM (like yesterday). The=20
color scale is not optimal in the moment but first i want to see if the=20
QRN comes back in the morning. All pics are staved separately...

73!!

Stefan/DK7FC


Am 21.05.2010 03:10, schrieb James Moritz:
> Dear Stefan, LF Group,
>
> ----- Original Message ----- From: "Stefan Sch=E4fer"=20
> <Stefan.Schaefer@iup.uni-heidelberg.de>
> ...>
> 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=20
> similar improvement in apparent SNR, and both will introduce some=20
> intermodulation, since they are inherently non-linear techniques. Your=
=20
> spectrograms seem to confirm there is not a great difference... BTW,=20
> the Spectrum Lab clipper, where the clipping threshold tracks the=20
> average signal level, does seem to give better results than simple=20
> fixed-threshold clipping, especially when other signals or narrow-band=
=20
> 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=20
> peak FS of 10s of volts per metre, so I get these problems a lot... I=20
> find it is important to keep an open mind about possible sources of=20
> this kind of QRM.
>
> The first thing is - are you sure the intermodulation actually occurs=20
> in your active whip? Any length of wire acts as an antenna, and will=20
> often be connected to some non-linear device (e.g. mains cable=20
> connected to rectifier diodes), so there are an almost infinite number=
=20
> of places outside the antenna circuit that the 9kHz IM product could=20
> be generated.
>
> A prime candidate is the earth connection - if your antenna is high up=
=20
> on a building, the earth connection will be carrying considerable=20
> broadcast-frequency current - I have found this often results in=20
> intermodulation, perhaps due to some kind of electrolytic effects=20
> occuring at the actual ground electrodes. This effect makes my TX=20
> antenna unusable for reception at many frequencies. Any mains power=20
> supply connected to the antenna preamp can cause a similar effect if=20
> the internal diodes, regulators, etc. are not well decoupled at=20
> broadcast frequencies.
>
> Another place where intermods can occur is the PC, which will=20
> typically have long cables connected to it (mains, network, sound card=
=20
> etc...) again acting as antennas and carrying MF broadcast currents.=20
> Any intermod generated in something connected to the PC can be coupled=
=20
> into the sound card inputs by the ground loops that tend to exist=20
> inside the machine. For example, I found using my 9kHz antenna/preamp=20
> at home with a laptop with AC adaptor gave a similar 9kHz line to the=20
> one you see. I eventually found this was still there with the preamp=20
> output disconnected from the sound card, but with the earth connection=
=20
> present between the computer and the preamp. I assume the cables=20
> connected to the preamp were acting as an antenna, and flowing to=20
> ground through the PC and adaptor. The cure was to wind several turns=20
> of the AC adaptor lead through an EMC ferrite core to reduce the MF=20
> current flowing in it. For the DK7FC grabber, you could easily test if=
=20
> the 9kHz comes from the antenna or PC end of the system by un-plugging=
=20
> the optical fibre.
>
> Then there are usually many other cables, or large pieces of metal,=20
> near the antenna, via which intermods can be occuring. The resulting=20
> 9kHz voltages/currents will be picked up by your antenna. Moving the=20
> antenna is the easiest way to check this, or also to solve the=20
> problem, usually.
>
> But then, if the FS is high, it may be that the active antenna is the=20
> problem after all. In your circuit, it is unlikely that IM is occuring=
=20
> after the input stage due to the presence of the bandpass filter. You=20
> could try adding a source degeneration resistor to reduce the gain and=
=20
> improve the linearity of the input stage (say 100R, but you would also=
=20
> probably need to change the gate bias voltage to get a sensible bias=20
> current). I would suggest putting about 47R resistor in series with=20
> the gate, to reduce the likelihood of VHF oscillation occuring, which=20
> can cause strange problems, including intermodulation at low=20
> frequencies. Probing the drain with an oscilloscope would reveal what=20
> large signals are present. Also, the drain resistor in the second=20
> stage needs looking at - 1k is rather too high a value for a 12V=20
> supply and a FET with zero gate bias and Idss of 10 - 25mA.
>
> BTW, problems with high broadcast station field strength were some of=20
> the reasons leading me to favour a loop antenna design, for 9kHz and=20
> other frequencies as well. It is much easier to include filtering=20
> directly at the antenna terminals, since the loop is much lower=20
> impedance. Also, ground connections are less of an issue.
>
> Cheers, Jim Moritz
> 73 de M0BMU
>
>