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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