The filter is 10MHz wide and having the setting on Normal or Invert will use the filter at its edges, where ripple and phase alteration might happen. But the SDS scanners excels on problematic digital signals so probably no bigger issue.
It's the exact same filter as in Unidens other scanners and there they use the filter symetrical, as corresponds to the Off setting in SDS scanners. During beta testing most people had problems from cellular towers in frequencies that where higher than the one monitored. So the low pass filter became the default Normal setting.
The receiver in SDS scanners use an automatic gain control, that unfortunatly due to the receivers design for television and satellite signals that are widemodulated 5MHz-10MHz, will sample the whole 10MHz width for RF signals and reduce the gain if the sum of the signals are too high. So the filter settings has to be used to try and block the strongest signals from interfering with the AGC sensor. Without a spectrum analyzer it's a trial and error excersise to find the best filter setting.
There's a computer program that can be used to study the spectrum that the filter passes, by using the optitional feature USB connector to hopefully see if there are any offending signals that could interfere with the receiver and then manually adjust the filter setting appropriatly.
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