Topband: Keying waveform

[Tom Rauch]

Hi Mark,
It's pretty tough to look at a waveform and tell anything, because even
(what we think is) a small sharp corner anywhere in the envelope from zero
to full power can be a problem. The ideal shape would be like a half sine
wave starting on what we would equate to the negative peak (at zero level),
rising to maximum level at what would be a turn over point at the top that
we could consider the positive peak of a sine wave.

Same way on SSB. A flat top doesn't mean a thing. It is the sharp or
fast-turn corners that require bandwidth. It's a slope direction change over
a short time problem in every case.

The Handbook shape is not an ideal shape. It is a shape similar to a simple
R/C filter from the 1950's, before cutting edge devices like op-amps came
along and before narrow crystal filters became popular. It also tends to
re-enforce the idea that changing speed changes bandwidth. That's as silly
as saying if we talk faster a SSB or FM signal is wider.

An ideal shape would come from filtering the key's square wave by a lowpass
filter with sharp cutoff and linear modulating the rig, and using linear
amplifiers with long hang ALC or no ALC. That would cost about $0.50-$1.00
in a new design, so that probably is what rules it out. Another way would be
to just send the nasty CW through a narrow crystal filter and use linear
amps beyond that point.

Even if we have an ideal shape, it does not mean the rig would be narrow.
Some rigs transmit while the VCO's are shifting frequency, and others have
VCO leakage.

73, Tom W8JI

> The accompanying text claims it has 5ms rise time, and you can plainly see
> the rounded corners, er, I think?  Tom, am I misinterpreting this photo?