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

[John A. McCabe]

Hi Stewart,

Although I have not ran all the tests you did, I also noticed that there 
is a significant difference in tone response between LSB and USB. I also 
tried adjusting  FL FRQ with about the same results as you got. My 5 
pole 2.7  filter does not seem to have a very flat response. I have 
thought about switching to the 8 pole filter because of that.

73,

John, KD8K

Stewart Baker wrote:
> 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
>
>
>
>
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