The easiest configuration (least mathematical) response curve for a
lattice filter is more or less square with notches adjacent to the band
for a tchebychev type response prone to ringing. The ladder filter with
nearly equal valued capacitors also has a nearly square response, but
without the close in notches characteristic of the lattice filter. The
ladder filter coupling coefficients can be chosen for a bessel amplitude
response which according to the gospel of shape factor gives a lousy
shape factor, but gives a superb time response and thus doesn't ring.
There are many compromises possible in between. Perfect symmetry is
harder to get in the ladder filter.
Its my view, that the Tchebyshev straight sided (small shape factor)
filters are a severe disservice to amateurs listening to weak signals in
the presence of noise, whether circuit noise, line noise or thunderstorm
static.
I've found in FD situations that the Kenwood 270 Hz filter takes so much
effort to discriminate between its narrow band of noise induced ringing
and CW signals that after a couple hours I can no longer copy. Brain's
worn out from that extra work. With the 400 Hz filter in my TS-120 and
now TS-130 I can go for 4 to 6 hours before the brain fails about sun
up. And that 400 Hz filter rings detectable on high speed CW.
The Tentec filters are ladder filters and seem to have a better time
response than mechanical or lattice filters. For the extreme noise
situation they could probably be improved further at the cost of less
adjacent channel rejection, which might be made up with more elements in
the filter, more loss, wouldn't fit the space allocated. Stacking
similar filters at the two IF's helps the adjacent channel problem.
Being ladder filters, the SSB filters approach the maximum conveniently
achievable bandwidth for their frequencies and I have no doubt are not
perfectly symmetrical, hence there might be a difference of impulse
response depending on where in the bandpass the carrier following the
impulse was tuned, hence a difference between USB and LSB.
If the CW mode sounds different than the FSK mode, there must be a
difference in key shaping between the two modes.
--
FAQ on WWW: http://www.contesting.com/tentecfaq.htm
Submissions: tentec@contesting.com
Administrative requests: tentec-REQUEST@contesting.com
Problems: owner-tentec@contesting.com
Search: http://www.contesting.com/km9p/search.htm
|