Too narrow a filter results in "smearing" one bit into the next - equivalent to severe multi-path - which causes a degraded signal to noise ratio. While a narrow IF filter may reduce interference from AGC capture, there is a fine line between the improvement in sensitivity due to keeping the strong adjacent frequency signal from "turning down the RF gain" and the loss of SNR due to self- induced multi-path (inter symbol interference).
In general one wants a filter that is at least 375-400 Hz wide at the -1 dB points with flat phase delay across the passband. Filters less than 370 Hz (@ -1dB), especially multi-pole crystal lattice/ladder filters, have significant phase non-linearities particularly in the "knee" region that create inter-symbol interference. 73, ... Joe, W4TV On 4/13/2016 9:56 AM, [email protected] wrote:
The optimal band width may depend on how crowded the band is. Wide filtering may work well for a DXer trying to copy a super weak signal on a quiet band. I only use RTTY in contests when the bands are packed and I have had great results using very narrow roofing filters. John KK9A from: Dave Hachadorian Tue Apr 12 23:54:57 EDT 2016 Current thinking is to use a fairly wide filter for RTTY. The software filters that are applied in the sound card are much narrower, about 50 Hz wide. Lately I've been using the 2.7 KHz roofing filter, with DSP bandwidth set to about 600 Hz. Dave Hachadorian, K6LL Yuma, AZ ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:[email protected] This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html Message delivered to [email protected]
______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:[email protected] This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html Message delivered to [email protected]
