[Elecraft] K3 SSB - Is the 5 pole filter good enough

[Stewart Baker]

Having run a quick test switching between USB and LSB on a quiet 
band 10m, I noticed an appreciable difference in the tone of the 
noise spectrum.

I dug out my noise generator and examined the audio response using 
Spectrum Lab. I set the DSP B/W to the maximum as what I wanted to 
see was the 2.7KHz SSB roofing filter response.

On USB the trace was substantially flat with the -3dB points at 
430Hz and 2.77KHz. Not too bad and could probably be improved by 
moving the carrier offset frequency.

On LSB, however the trace was far from flat having a slope from 
audio LF to HF of about 6dB. Not very good at all...

I tried adjusting the FL FRQ, but all this did was shift the 
overall position of the sloping trace.

Although the crystal filters are spoken about as being roofing 
filters designed to keep strong signals away from the DSP 
filtering they do affect the wanted passband.

On the K2 there used to be a big difference between the USB and 
LSB response until a modification to the output of the filter was 
added. The KSB2 uses a 7 pole filter, giving a more or less 
symmetrical response. I wonder if 5 poles is just too few ?

Or maybe I have a duff (defective) 2.7KHz filter ?

I wonder if others have plotted their 5 pole filters.

73
Stewart G3RXQ