W8JI:
> The ideal shape is a raised sine wave shape, not the "perfect"
> shape they show.
I suppose it depends on your definition of the perfect wave shape. If what
you see is the same as my reference on p 15.6 of the 1998 ARRL Handbook, I
would gladly trade my eye teeth for the waveform as represented in figure
15.8(a). This represents the classic grid-block keying waveform as used
with simple single-pole R/C timing circuits. Here, the leading edge rises
quickly and gradually decelerates until the final envelope is formed. The
trailing edge falls quickly but then trails asymptotically to zero. It's
this waveform that carries that plosive, bell-like sound that's so
unmistakable through selective fading. I'm sure others will disagree:
Ten-Tec believes that the perfect waveform is perfectly symmetrical. If one
views the keyed waveform of the Omni V or early Omni Six transceivers,
you'll see a gorgeous, symmetrical wave-form. Later, in an attempt to
reduce leading edge power spike, the ALC time was shortened which
successfully cured the power spike, but left a sharp leading edge rise time
of 1 mS or less. Additionally, the integral CW wave-form adjustment control
became a useless feature as it had no control over the leading edge.
In 1998, I spent a considerable amount of time in developing a two-speed ALC
circuit which resulted in zero power overshoot, yet left the leading edge
dynamic characteristics controllable. The newer genre Omni Six Pluses
sound quite hard on the air and in fact, Ten-Tec obviously realized the
failed impact of the CW wave-form control and later substituted the 10K
control pot with a fixed resistor in its place. On the other hand, the
keying on a stock Yaesu FT-1000D sounds much too soft (yes, yours too Tom).
But the FT-1000MP is just the opposite. It makes me believe that little
thought is given to the keyed wave-form in today's transceivers when they
gladly accept any result even if it varies wildly from one generation of
transceiver to the next...especially within the same manufacturer. The
mind-set appears to be "why bother with CW keying improvements when that
cost can be spent on adding more useless features."
It's true that CW bandwidth is a function of the envelope rise and decay
time, but more than one type of wave-shape can produce an equal level of
occupied bandwidth. The "perfect" wave shape is purely subjective.
Provided that both rise and decay time is greater than 2 mS or so, occupied
bandwidth should be reasonable...reasonable in that the signal is easy to
copy during QRM and selective fading, and reasonable in that the minimum
amount of bandwidth is required for copy at the receiving end. A few
manufacturers allow easy manipulation of the keyed wave-form from the front
panel. But one adjustment affects both the rise and trailing edges. I
believe Kenwood has been offering this as well as Kachina. Personally, I
like the concept as presented by George Grammer in the November, 1966
edition of QST. (See pp. 11-15, 166). George developed a unique keying
circuit in that two controls are brought out from the transmitter to
*separately* adjust the wave-form rise and decay time. Keying could then be
adjusted to conform to band conditions. Limits placed on both controls
prevent accidental and unnecessary hard keying. Additionally, the amp key
line could be controlled through an envelope comparitor such that no matter
what the rise/decay time, the amp only switches at a zero voltage point with
a fixed amount of hang-time to compensate for any additionally RF delay at
the antenna jack.
My next step is to modify my Omni Six's keying to resemble exactly what I
see in the ARRL Handbook. For me, this is the perfect wave-form.
-Paul, W9AC
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