SSB Overshoot

[Jack Smith-6]

I've made a quick series of measurements at 7 MHz, 50 watts output and
varying audio drive levels and posted the result in a PDF file that may
be viewed at
http://www.cliftonlaboratories.com/Documents/SSB%20Burst%20Analysis%20Elecraft%20K2.pdf

The most recent activities at Clifton Laboratories can be viewed at
http://www.cliftonlaboratories.com/Updates.htm. I put up a panadapter
shot of the 20 meter band 14.000-14.100 showing the SS activity at the
Updates page.

Jack K8ZOA
K2: 5689
www.cliftonlaboratories.com
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[Don Wilhelm-3]

Jack,

Those displays look quite similar to what I would have expected - overshoot
will occur if the audio is increased sufficiently to activate the ALC.

The displays do point out the necessity of not driving the K2 into ALC if
overshoot is to be avoided - for digital modes, that means to increase the
requested power to well above the intended operating power and limit the
drive using the mic input level.

Of course, the same thing applies to most any other transceiver if the IMD
is to be kept at a minimum - the thing unique about the K2 is that the
control of the power output level is also involved.

I always have bad feelings about attempts to use ALC as a power output
limit - expecially on SSB because it only leads to distortion.  It is a form
of compression, but audio compression is much better at increasing the
average power of a signal.  One could design a compressor that works at RF,
but it would be quite different than the ALC found in most transceivers.

73,
Don W3FPR

> -----Original Message-----
>
> I've made a quick series of measurements at 7 MHz, 50 watts output and
> varying audio drive levels and posted the result in a PDF file that may
> be viewed at
> http://www.cliftonlaboratories.com/Documents/SSB%20Burst%20Analysi
> s%20Elecraft%20K2.pdf
>
> Jack K8ZOA
> K2: 5689
> www.cliftonlaboratories.com
>

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[Jack Smith-6]

Yes indeed. One should not depend on ALC to set output power.


Jack

Don Wilhelm wrote:

> Jack,
>
> Those displays look quite similar to what I would have expected - overshoot
> will occur if the audio is increased sufficiently to activate the ALC.
>
> The displays do point out the necessity of not driving the K2 into ALC if
> overshoot is to be avoided - for digital modes, that means to increase the
> requested power to well above the intended operating power and limit the
> drive using the mic input level.
>
> Of course, the same thing applies to most any other transceiver if the IMD
> is to be kept at a minimum - the thing unique about the K2 is that the
> control of the power output level is also involved.
>
> I always have bad feelings about attempts to use ALC as a power output
> limit - expecially on SSB because it only leads to distortion.  It is a form
> of compression, but audio compression is much better at increasing the
> average power of a signal.  One could design a compressor that works at RF,
> but it would be quite different than the ALC found in most transceivers.
>
> 73,
> Don W3FPR
>
>  
>> -----Original Message-----
>>
>> I've made a quick series of measurements at 7 MHz, 50 watts output and
>> varying audio drive levels and posted the result in a PDF file that may
>> be viewed at
>> http://www.cliftonlaboratories.com/Documents/SSB%20Burst%20Analysi
>> s%20Elecraft%20K2.pdf
>>
>> Jack K8ZOA
>> K2: 5689
>> www.cliftonlaboratories.com
>>
>>    
>
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>
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>
>
>  

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[Geoffrey Mackenzie-Kennedy-2]

Don W3FPR wrote:

 <snip>

> I always have bad feelings about attempts to use ALC as a power output
> limit - expecially on SSB because it only leads to distortion.  It is a
> form
> of compression, but audio compression is much better at increasing the
> average power of a signal.  One could design a compressor that works at
> RF,
> but it would be quite different than the ALC found in most transceivers.

------------------------------------------------------------------------

'Forward gain control' can be very effective on SSB when done within the low
level RF stages of a transmitter. IMHO the usual type of ALC which is a
'Backward gain control' system should only be used as a safety or protective
system set to limit abnormal power output, not to increase 'Talk Power' for
the reason you mention.

73,

Geoff
GM4ESD



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