Are you referring to the recomendation by Upman to switch from NFM to FM if experiencing issue with LSM systems?
I will also post my IFX list when I get a chance...there are a lot of 700/800MHz frequencies that are messed up by cell phone towers or whatnot. I only have a list for the West site because the IFX filtering helps most when signals can be knocked off the air by interference. The other sites when testing were like -35dBm so basically, couldn't interfere with that in ytesting.
Paul
It's related - when using a narrower filter on analog signals you might experience some distortion if the signal deviation is more than the passband of the filter, etc. But with digital modulated signals the problems can be more severe. Something called "group delay" in the filter characteristics comes into play and can mess up a digital signal's phase and amplitude characteristics. The more narrow the IF filter the more important that becomes relative to the bandwidth of the signal. The cheap and dirty way around that which is what most consumer scanners fall back on is to use a wider filter to avoid the affects of the non-ideal filter characteristics which usually become worse at the filter edges.
So what is a relatively minor annoyance for analog FM signals becomes a big hairy deal for complex digitally modulated signals. Since the 436 and 536, unlike the 396 through 996 models, really DO have two IF filters with the NFM one being the narrower of the two, since those filters were of marginal quality for digital signals but fine for analog, using the wider filter for P25 might improve the decode overall including on tricky simulcast variants. You sacrifice some adjacent channel selctivity for better digital decode - again, cheap and dirty compromise.
If the filters were of much higher quality this would not be as severe an issue but it's a whole lot of stuff, really, in the design, including LO accuracy and phase noise, etc., all of which are a compromise in lower cost consumer scanners, of course.
Again, this is a seperate issue from the "SDS Filter" choices. Different concept and animal. That method involves basically moving the IF signal around in the passband of one of the IF filters, I think Ubbe's description is best and it sounds like it is the second IF filter being 10 MHz wide or so. What you're doing here is trying to move the IF signal away from the center of the filter where it normally is suppose to be so that you get closer to one of the "filter walls" in the hopes that you can move the interfering signal outside the passband of the filter and reduce its negative affects on the desired signal. But, again, when you do this, you start bringing in the negative effects of the non-ideal filter edges affecting your signal. This is why using it can sometimes reduce some signal amplitude of the desired signal but the hope is that you gain more in reducing the interferer level. Again, always a compromise.
And the IFX ("IF Exchange") is another thing alltogether. In the pure superhet models pre-SDS series it basically swapped the LO feed into one or more of the IF mixers from high side to low side injection or vice-versa. What that is supposed to do is relieve problems stemming from IF image issues. It is an effective and clever method that can work for that kind of issue. But IF images are just one superhet receiver bugaboo. There are many others and many of which will not be affected by a high side/low side LO mixer injection swap. I am unsure how this works in the SDS receivers. I assume they are hybrid designs using some superhet up/down conversion at the front end before the sdr portion comes into play. If so, then the IFX LO high side vs. low side mixer injection can work the same way as in pure non-sdr superhet receivers.
-Mike
