On 3/3/2014 9:24 AM, Walter Underwood wrote:
It has been suggested that a using a 400Hz filter for RTTY (or other digital modes) puts the edges of the signal in the part of the passband with the worst phase distortion. A 700Hz filter puts the signal in a better-behaved part of the passband. The DSP filters have no phase distortion, of course.
The recommendation I've seen was for 400-450 Hz bandwidth. And yes, the phase distortion created by the filter slopes is the reason. But this not a case of "if a little is good, more is better." As you open up the IF bandwidth, you also let in more noise and QRM, which is basis of the 400 Hz recommendation. Good engineering (and life in general) is about compromise between multiple factors, and the guys who know the most about decoders made the 400-450 Hz recommendation.
Further, it is quite easy to set the K3 to switch in the roofing filters at any frequency you want. You do this in the setup menu for each filter by setting the filter frequency. You could, for example, set the 400 Hz filter to switch at 350 Hz, so that at 400 Hz DSP IF you would have only the DSP filter. Or you could set it to switch at 450 Hz, so that at 500 Hz DSP filter setting you would have a 500 Hz DSP IF cascading with the 400 Hz filter.
73, Jim K9YC ______________________________________________________________ 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
