[Elecraft] K3S filter for FT8?

[Jim Brown]

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