Nobody seems to talk about Group Delay Time.
The steeper the LC or Xtal filter is, the higher are Group Delay variations.
One filter side up, the other side down.
In narrow filters these are very close.
This is at least one reason for "ringing".
8 Xtal filters are clearly steeper than 5 Xtal types, and have higher
variation of Group Delay.
DSP makes very steep filters WIHOUT group delays.
K3 concentrates best selectivity here. Correct!
Roofing filters just protect the rest of RX from too big signals.
We tend to think that roofing means final selectivity. Wrong!
It would be interseting to measure Group Delays between different
Xtal roofing filters with and without DSP.
Let us imagine an ideal 9MHz SSB-filter plus/minus 1,5kHz. 10 Xtal
filter has
shape factor of 1,33. Now -60dB points are plus/minus 2kHz.
Now, the Group Delay difficulties are on both side from 1,3 to 2,2kHz.
Now the best way of using DSP is to delete these variations by
choosing (IF) DSP bandpass of plus/minus 1,2kHz. Still OK for contest,
but you do not agree. A 3kHz good Xtal filter became only 2,2kHz final
selectivity.
I would rather use 5 xtal roofing and all bandpass DSP.
Group Delay was importand in analog TV-transmitters (ancient times).
It was found the hard way. Once understood, it was controlled
and measured every time. Final solution was to use transmitters with
normalized IF signal processing. Centralized block for pass-band filtering
with standard Group Delay was the answer.
Benny OH9NB
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