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LF: Filters

To: [email protected]
Subject: LF: Filters
From: "hamilton mal" <[email protected]>
Date: Sat, 18 Jan 2003 11:46:19 -0000
Reply-to: [email protected]
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I have been watching the discussion about Filters on here with interest. With DCF39 you have two modes to cope with, the carrier and the data bursts. A notch filter does not handle this condition very well, because of the sudden bandwidth extension when the data burst occurs. If you cater for this then you also desense/attenuate adjacent frequencies of interest around 137.7 khz a popular frequency for QRS.
The preferred approach would be a tight Bandpass filter at the front end of the RX and good IF selectivity as narrow as you  can manage. This way you will be able to hear/see even the weakest of signals. Some of the  weaker signals on the band at my qth are perfectly readable but only in a very narrow bandwidth, giving a good signal over noise. These same signals are barely detectable with a wide bandwidth, either aural or visual. To ensure that nothing of interest is missed on 73 or 136 khz it is preferable to tune the band slowly with very narrow selectivity. 
To try and monitor the whole band visually with the appropriate bandwidth selected does not work for weak signal reception, these weak signals would be buried in the noise and not detectable.
Most LF operators suffer from poor reception on LF because of environmental noise pickup and the last thing needed is to attenuate signals even more with unnecessary notch filters. The best approach is to use a good sensitive RX with good IF selectivity plus a DSP audio filter (or IF) and if necessary a well designed tight bandpass filter (tuned would be an advantage) at the front end of the RX.
73 de Mal/G3KEV   
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