The time response of the receive filter has a great deal to do with how
a signal sounds to the ear. The classic mechanical and crystal lattice
filters tend to have a square shouldered flat topped frequency response
generally a small ripple Tchebychev frequency response. That means it
has a strong tendency to ring when hit by transients, like the beginning
and end of a CW dot or dash (or lightning or power line arcs). That WILL
change the sound of the signal. Even the termination of the filter can
affect the ripple. There's always a trade off in Tchebychev filter
design between steep skirts and pass band ripple. For any given number
of resonant circuits of a given Q, you achieve steeper skirts by
accepting more ripple. And with more ripple you accept more ringing.
Sometimes that ringing makes a signal rise and fall be slow. Sometimes
(in some Kenwood 270 Hz. filters) it converts noise to a tone at the
middle of the passband (which I find makes CW very tiring).
The ladder filters used by Tentec tend to have more rounded shoulders
and not as steep a skirt as the similar crystal lattice filters from the
other radio makers and Inrad. That lets the rest of the world claim
"better" frequency response specifications ignoring the impulse ringing.
It is easy to design a ladder crystal filter with a Gaussian response,
all rounded shoulders, no flat top, no infinitely steep skirts that will
still ignore adjacent channel signals, but won't ring nearly as much
when hit by a signal rise, fall, or impulse.
Line noise, when viewed with wide bandwidth, shows as a group of narrow
pulses (microsecond wide if the bandwidth is great enough) at the line
voltage peaks. 120 times a second for single phase lines, 360 times a
second for 3 phase power lines, 720 times a second for transmission and
distribution lines near a substation (because the transformer is
generally connected delta/Y and gives a 30 degree phase angle difference
between the primary phases and the secondary phases). My good old 75S3B
with mechanical filter rings on line noise so much there's no visible
amplitude modulation on the IF output (a connector I added), but if I
switch it to the AM "filter" made up of a pair of IF transformers, the
short pulses are clearly visible.
Its possible for a filter to enhance clicks or to reduce them. When I
listen to the supposed clicky W1AW with my Corsair II's 500 Hz filter,
it sounds beautiful. When I listen using the stock wide filters AND tune
to the edge of the passband, I might detect the tiniest of click. The
phase response of the filter changes most rapidly where the amplitude
response has the greatest curvature. When the Omni VI clicks on CW it
may be that much of the click is coming from the filter because the CW
carrier is just barely within the passband right at the corner of the
shoulder.
If is possible for a filter to create clicks from signals outside its
passband if the input to the filter or a wider filter in front of the
narrow filter is ringing from the rises and falls of the CW signal.
If you really want to preserve the signal's shaping, use only a Gaussian
filter, such as the under coupled IF's of a BC-453.
With the old S-line I had to quit making contacts on 75 meters from
thunderstorm static, but when I wired in the BC-453 + a converter to
work with the S-line transmitter, I could continue copying until the
lighting was visible out the basement window. The Tentec filters in my
Corsair II are good but not quite that good.
73, Jerry, K0CQ
--
Entire content copyright Dr. Gerald N. Johnson. Reproduction by
permission only.
|