[Elecraft] Filter Recommendations

[Jim Brown]

On 8/26/2018 6:50 AM, Don Wilhelm wrote:
> The 2.8kHz filter has a flatter passband and steeper skirts than the 
> 2.7kHz.

Yes. I use 2.8 kHz filters in all three of my main receivers, and 2.7 
kHz in my Sub Receivers. The better flatness of the 2.8 kHz filter 
reduces incidental AM when transmitting RTTY. I don't bother matching 
the 2.8 and 2.7 filters because I don't use them when set for diversity 
reception -- rather, I'm trying to dig weak CW signals out of the noise.

I do SSB contesting, so first bought 1.8 kHz filters, which I find too 
narrow. I've settled on 2.1 kHz.

For RTTY and some CW I use 400 Hz filters, and 250 Hz for CW most of the 
time.

As Don as observed, it's important to understand that these are roofing 
filters, and that the K3 IF is in DSP, and is variable from 50 Hz to >4 
kHZ. The settings of the DSP IF are what mainly set the RX bandwidth. In 
addition to protecting the DSP IF from overload and the AGC from signals 
outside the set bandwidth, the roofing filters also CASCADE with the DSP 
IF. That is, when the DSP IF is set to about the same bandwidth as the 
selected roofing filter, the two filters combine to provide much steeper 
"skirts," so that very close signals are more deeply rejected. This is 
what helps N0NB copy his net with QRM only 2 kHz below them.

73, Jim K9YC