On 3/3/2014 12:24 PM, 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.
Whoever suggested that does not know enough about RTTY to be credible or you have severely misinterpreted what was said. The 170 Hz shift, 45.45 baud RTTY signal has a true bandwidth of about 370 Hz. The "400 Hz" filters are 400 Hz at -3dB and about 440 Hz at -6 dB. Their -1 dB bandwidth (area of generally flat phase response/minimum group delay) is a very close match to the required 370 Hz bandwidth which provides the nearly optimum trade off of AGC protection vs. bandwidth. A 700 Hz or 1000 Hz roofing filter will allow a strong adjacent channel signal that would otherwise provide no interference to capture and pump the hardware AGC (or DSP AGC) in the K3 resulting in potentially severe blocking effects. Using a 700 Hz or 1000 Hz roofing filter for RTTY is no better in terms of blocking than using an 1800 or 2700 Hz filter - the most damaging interference is always the closest in frequency. 73, ... Joe, W4TV ______________________________________________________________ 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
