Tchebychev filter approximations with steep skirts have a rotten time
response. They DO smear pulses into crashes. The Collins mechanical
filter in my 75S3B smears the 360 Hz repetition rate pulses of line
noise (three phase power lines) into continuos noise with no amplitude
modulation visible on the scope. Its a fault of the square topped filter
with steep skirts typical of mechanical and lattice crystal filters. Its
difficult to get as steep a set of skirts or square corners with the
ladder filter used in analog Tentec radios so those radios don't smear
the static nearly so much. The properly designed digital filter with
enough headroom to prevent clipping can do very well, even do a decent
job passing static as clicks, while having square corners to the pass
band at the cost of more time delay from using more filter coefficients.
The other factor of receiver quietness as perceived at the speaker or
headphones is the overall receiver gain. The direct conversion receiver
tends to have significantly less total gain than a superhet receiver.
That makes it sound quiet but on a quiet band (which can happen to any
band) it won't let the user comfortably hear the weakest signals because
there's just not enough gain to overcome the user's threshold of
detectability.
Many multiple conversion superhets do a poor job of maintaining signal
to noise ratio after the first mixer and first filter. Often there's IF
gain after the selectivity, LOTS of IF gain after the selectivity and no
more selectivity other than audio low pass filtering. Each mixer lowers
the S/N by 3 dB by including noise from the image if not tightly
filtered. The product detector is the last mixer in the chain. Then all
those images folded in to the audio stream can only be low pass filtered
to reduce hiss, though the audio states introduce some of their own.
I've found a passive speaker filter to be a nice addition to some radios
to remove that high frequency noise (hiss). Its a real pleasure to
listen to a CW beat note about 400 Hz coming through a 450 Hz low pass
filter at the speaker. Sounds like being in a room with a good code
practice oscillator and not through a receiver. I have a filter circuit
that I use (and have published locally) that has a set of cut off
frequencies and is made from readily available (Digi-Key and RS)
components.
73, Jerry, K0CQ
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Entire content copyright Dr. Gerald N. Johnson. Reproduction by
permission only.
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