Those low-pass filters can only stop interference from signals generated above it's cut-off frequency. That cut-off frequency is usually something like 40-50 Mhz. If your transmitter is producing those frequencies then a low-pass filter is very nice to have.
The problem is that all interference isn't produced by those unintended and unwanted frequencies, but by the intended and desired frequencies of use. It's the device that's being interfered with's fault, not the transmitter's. The interfered with device is just too sensitive, isn't protected correctly by the manufacturer, or isn't protected enough. In cases like that, a low-pass filter, or any kind of filter, is useless.
[Once upon a time, less than one generation ago, 27 Mhz was used as an 'IF' stage frequency in TVs. Talk about getting interfered with! In both directions! Get to close to one of those TV and the CB radio was 'wiped out' by interference.]
About the only 'cure' for RFI with some devices is distance, or a high-pass filter on that device being interfered with. (Try convincing some new owner of a 'wiz-bang' super great humongu$ TV that his TV is at fault, not your transmitter.)
You also get in to the 'gray' area of working on someone's equipment and the resulting liability/responsibility of doing so. Being sued certainly happens, don't think that it doesn't! (Give -THEM- the filter and let them put it on their stuff!) The 'Interference Hand Book' from the FCC goes a long way toward explaining all this, and is a very handy thing to have! It's free, go download it. Been there, done that, believe me...
- 'Doc
The problem is that all interference isn't produced by those unintended and unwanted frequencies, but by the intended and desired frequencies of use. It's the device that's being interfered with's fault, not the transmitter's. The interfered with device is just too sensitive, isn't protected correctly by the manufacturer, or isn't protected enough. In cases like that, a low-pass filter, or any kind of filter, is useless.
[Once upon a time, less than one generation ago, 27 Mhz was used as an 'IF' stage frequency in TVs. Talk about getting interfered with! In both directions! Get to close to one of those TV and the CB radio was 'wiped out' by interference.]
About the only 'cure' for RFI with some devices is distance, or a high-pass filter on that device being interfered with. (Try convincing some new owner of a 'wiz-bang' super great humongu$ TV that his TV is at fault, not your transmitter.)
You also get in to the 'gray' area of working on someone's equipment and the resulting liability/responsibility of doing so. Being sued certainly happens, don't think that it doesn't! (Give -THEM- the filter and let them put it on their stuff!) The 'Interference Hand Book' from the FCC goes a long way toward explaining all this, and is a very handy thing to have! It's free, go download it. Been there, done that, believe me...
- 'Doc