[Elecraft] K1 carrier rise/fall times unequal?

Mon Apr 21 15:03:00 2003

Vic is entirely correct on this one, guys. I know that there are all sorts
of formulas for bandwidth that seem to involve code speed only, but they
make a lot of assumptions and do not really give correct answers most of the
time.

In addition to the rise and fall time aspects of keyed bandwidth, there is
another more subtle cause involved. That is the manner of the transition
from carrier off to the leading edge; from leading edge to full amplitude;
from full amplitude to trailing edge; and finally from trailing edge to full
off.

Those four transitions can be anything from very abrupt to very gentle. It
has been found that waveforms with abrupt transitions have wider bandwidths
than the rise and fall times would predict, even at a fixed speed. I know
from measuring the waveforms on my IC-765, two PROs and the K2 that there is
a wide variation in how these four transitions are accomplished. Each radio
does the four transitions differently.

My K2 has fairly short rise and fall times - about 3 msec for the rise time
and about 1.5 msec for the fall time. But the off-to-risetime is abrupt
while the risetime-to-on is very smooth and gentle. Similarly the
on-to-falltime is very abrupt while the falltime-to-off is less so.

So, the keying is pretty hard on my K2, but careful receiver monitoring does
not reveal that excessive bandwidth is involved with clicks.

My PRO2 has adjustable rise and fall times from 2 to 8 msec. Two msec is
very hard and clicky while 8 is rather  soft and narrow. I use either 4 or 6
and like the sound.

I mentioned this to Wayne and sent him a photo of the K2 envelope of a dot
and he agreed that it was pretty sharp.

I think that the final arbiter is performance. If the keying is not
offensive to your band neighbors and sounds good to you and is easily read
when the going gets tough, then it is doing its job.

But first principles tell us that the time-domain characteristics of each
keyed element determine its spectrum, not the repetition rate of a string of
elements, as Vic reminds us

73/72, George
Amateur Radio W5YR -  the Yellow Rose of Texas
Fairview, TX 30 mi NE of Dallas in Collin county EM13QE
"In the 57th year and it just keeps getting better!"
<mailto:[email protected]>

----- Original Message -----
From: "Vic Rosenthal" <[email protected]>
To: "Don Wilhelm" <[email protected]>
Cc: "David Steere" <[email protected]>; <[email protected]>
Sent: Monday, April 21, 2003 1:28 PM
Subject: Re: [Elecraft] K1 carrier rise/fall times unequal?

> Don Wilhelm wrote:
>
> > As far as bandwidth of the signal goes - the waveshape alone does not
tell
> > the whole story because the keying speed is also a primary factor - a 10
wpm
> > CW signal is more narrow than a 30 wpm CW signal and so on.
>
> Actually, this is incorrect.  It is true that the faster you send, the
> harder the keying needs to be (otherwise the elements start to run into
> each other), so faster CW requires harder keying, which makes the signal
> wider.  But speeding up or slowing down does not change the actual
> bandwidth that the signal takes.  The bandwidth is dependent on the rise
> and fall times of the elements.
>
> To the original poster:
> Having said that, I suspect that it's a little more complicated.  You
> can have different shaped leading and trailing edges with the same
> overall rise/fall time, so just looking at a scope is not sufficient to
> determine whether your signal is clean.  I suggest using a second
> receiver with a sharp IF filter.  Adjust it so the signal from your K1
> is S9 and carefully tune across the signal.  If you hear clicks for more
> than about 500 Hz above or below the signal itself, then perhaps it is
> too wide.  Note that you cannot determine if a signal is too hard by
> listening ON the frequency of the signal, because the receiver's filter
> will soften what you hear.  What you are listening for is clicks on make
> and break when you do NOT hear the tone of the signal.
>
> By the way, if you do have excessive clicks, you may be interfering with
> other band users even if you are well within the band, so if there's a
> problem you have to fix it!
>
> 73
> Vic K2VCO
>
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