What in the name of God am I adjusting?
The filter settings are badly named and only confuse. You are adjusting the shift of the 1:st IF filter. When you select Off there's no shift, the filter works as in any other Uniden scanner and pass a frequency range of 10MHz and the wanted signal are in the middle of that frequency range.
The IF filter are attenuating more the more you go from the middle of the pass band to the edges. In a non interfered location you get best performance from the Off setting.
If you select Normal or Invert the filters pass band will shift to the edge of the filters range. It will attenuate the signal more but will pass the whole 10MHz in one frequency direction but hardly anything in the other direction. In Normal it will pass frequencies 10MHz above the one you monitor and hardly nothing from frequencies below the one you monitor. Invert will do the opposite.
The Wide settings will only shift the filter half way to the edges.
If there where a filter that only passed the frequency you monitor you wouldn't need any filter settings at all but that filter would be super expensive, if it even was available to produce. A filter that's 10MHz wide doesn't cost much.
If you have a strong signal a couple of MHz away from the one you are monitoring it wouldn't make any difference to a non SDS scanner but the receiver chip in SDS scanners are sensitive to interferencies and you'll need to block that interfering signal by selecting a filter that will attenuate that signal the most. But if you select Normal or Invert it will block that signal but also open up a 10MHz wide frequency range in the other direction where it could be another signal that interfere. Then you have the Wide selections that only open up half that frequency range and will hopefully block both those interfering signals.
There are two IF filters in SDS scanner, one at about 250MHz and another at about 350MHz. There could be some performance difference between them and usually you shift to the other filter when you select to use IFX to a frequency. Sometimes there isn't any filter setting that will help and neither will IFX. Then enabling Attenuator might be an option, if the wanted signal are strong enough, as the interfering signal will also be attenuated.
The receiver in the SDS scanner models use a very mediocre performing receiver chip that's not at all intended for narrow band scanner use. Its main use are for terestrial TV and satellite reception that use 5-10MHz wide channels and where it is no stronger signals nearby in frequency. Uniden use this $0.85 chip to reduce production costs. It isn't sensitive enough so there's a pre-amplifier used to boost the antenna signal. That will also make it even more sensitive to interferencies and overload issues. The receiver chip has its own amplifier that are self serving and cannot be controlled and will try and reduce interferencies by reducing its gain. The sensor for that detects signals up to 7Mhz away so that a strong signal that wouldn't affect another scanner will in a SDS scanner reduce the overall sensitivity so that you might loose the signal you are monitoring. The sensor and amplifier sits before the filters, to the front end of the receiver, so any setting of filters will not affect that phenomenon. It's one of the results of using a receiver chip designed for 5-10MHz wide channels.
But knowing all the details will probably not help. It is almost impossible to know what filter or IFX to use so it's down to trial and error testing.
/Ubbe
