On 5/5/2019 2:54 PM, Frank O'Donnell wrote:
On the K3S or K3, what filter(s) are people finding optimal for FT8?
IF selectivity is provided in DSP, not with plug-in filters as with older radios. Plug-in filters for these radios are ROOFING filters -- they protect the DSP from overload from strong signals outside the chosen IF bandwidth.
Nearly all WSJT modes are designed to work with the full bandwidth of an SSB channel -- about 2.7 kHz, which is the bandwidth of the stock roofing filter that comes with a K3 or K3S. In fact, Joe Taylor, K1JT has long advised that rigs should always be set for that wide bandwidth for his modes, and let WSJT-X do the required signal processing to separate stations.
There's a fundamental reason for that -- for most physical networks, and for digital simulation of physical networks and devices, any change in amplitude response has an associated change in the phase response. Decoding systems, including the human hearing system, are degraded by steeply changing phase response. We learned about this in pro audio world in the '70s, thanks to the work and teaching of the late Richard Heyser. THAT'S why Joe advises as he does. In the last few years, RTTY operators have learned this and taken it to heart. Gone are the recommendations for "double-humped" filters, with their associated phase distortion -- the top operators now use 400-500 Hz filters for RTTY.
A major shortcoming of the K2 is that when the multi-stage crystal filter is realigned for narrow SSB bandwidth, the frequency response looks like a cross-section of the rocky mountains, making SSB speech more difficult to copy. The radio sounds great when the TX filter is used for RX, which is at full SSB bandwidth.
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
