[Elecraft] CW rise time mod

Tue Apr 1 11:37:43 EST 2008

While the impact on a receiver might cause "clicks" to be heard, that's a
failing of the receiver, not the transmitter.

The real issue about the CW keying waveform is the production of sidebands
around the CW signal at the transmitter. All amplitude-modulated signals,
which CW is one type, have sidebands. The only way to prevent them entirely
is to not modulate the signal. Since the rate of the modulation is much less
with CW than it is with a spectrum of voice covering, say, 300 to 3000 Hz,
the sidebands produced by CW keying are much, much smaller than those
produced by voice modulation. 

But that doesn't mean the sidebands produced by CW keying can be ignored,
especially in today's world of very selective receivers that allow signals
to be much closer to each other than in the past. 

Without the sidebands, the CW would be unreadable. It's a question of how
wide the sidebands must be and how the energy is distributed in them to
produce an easy-to-copy signal that is not wider than necessary. 

"Easy-to-copy" is a value judgment. There are no absolute values.

Exotic computer-controlled keying circuits with linear RF amplifiers have
given designers the ability to control the keying waveform and the energy
distribution in the sidebands to a degree never contemplated only a few
years ago. 

But the underlying question is unchanged: what is the best tradeoff between
bandwidth and readability of a CW signal? 

It's still a judgment call. 

Ron AC7AC

-----Original Message-----
From: elecraft-bounces at mailman.qth.net
[mailto:elecraft-bounces at mailman.qth.net] On Behalf Of AJSOENKE at aol.com
Sent: Monday, March 31, 2008 9:16 PM
To: reiserj at optonline.net; elecraft at mailman.qth.net
Subject: Re: [Elecraft] CW rise time mod

It's not that 3ms is a lot of time in terms of human scale. But, it is the  
rise time of an electronic pulse. This can have a lot of impact on the 
transient  waveform that results in an audio demodulator - i.e. receiver.
The 
difference is  noticeable enough to make the difference in a crowded band
weak signal 
situation  when the receiving station is differentiating what he hears.
That's 
why a banjo  sounds different than a guitar  or violin.

Al WA6VNN

++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
+

In a message dated 3/29/2008 5:34:22 P.M. Pacific Daylight Time,  
reiserj at optonline.net writes:
People,

8 ms.- 5 ms.= 3 ms., does it  matter in any practical sense?  I would really

like to know who cares,  and why?  Can anyone hear the difference? 
Three-thousanths of a  second?  Not my old brain.

73,
John, W2GW
K3  #384

----- Original Message ----- 
From: "Lyle Johnson"  <kk7p at wavecable.com>
To: "Elecraft Reflector"  <elecraft at mailman.qth.net>
Sent: Saturday, March 29, 2008 5:21  PM
Subject: Re: [Elecraft] CW rise time mod

>> Or do Rev A  RF boards have a lower value for C222 than that shown on 
>> the
>>  schematic?
>
> Early production K3 RF boards have a 1 uF capacitor  instead of a 0.1 
> uF
> capacitor installed at C222.  Yes, the  published schematics show a 0.1 uF

> capacitor.  The effect of the  larger capacitor is to increase the TX 
> waveform rise time to about 8 ms  instead of 5 ms.
>
> Rev B RF boards have the correct 0.1 uF value  installed.  Sometime 
> during
> Rev A RF board production, the value  installed on the board was changed 
> from 1 uF to 0.1 uF.
>
>  Surface mount ceramic capacitors are not marked with a value, so you
>  cannot tell which you have by visual inspection.
>
> You can  determine if you have a 1 uF rather than a 0.1 uF by:
>
> 1)  Measuring the capacitance if you have a capacitance meter.
>
> 2)  Looking at the Tx output RF envelope on an oscilloscope or 
> "station
>  monitor" scope.  If the fall time and the rise time look very similar in

> duration, you have the 0.1 uF cap. If the rise time is about 50% longer  
> than the fall time, you have the 1 uF capacitor.  You don't need  an 
> oscilloscope with an accurate time base to make this comparative  
> measurement.  If your oscilloscope has a low bandwidth (2 to 10  MHz), use

> the 160 meter band.
>
> 3) If you are concerned  that your unit may have the 1 uF capacitor 
> and you
> have no way to  determine it otherwise, you can just replace it with the 
> 0.1 uF part  and sleep better at night :-)
>
> If you don't change it, you will  not damage anything.  Your K3 will 
> just
> have slightly softer  keying and an upcoming firmware adjustment of the 
> keying time will be  less accurate.
>
> 73,
>
> Lyle KK7P
>
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