[Elecraft] KPA500 Power In Digital Modes

[Jim Brown]

On Wed,11/19/2014 11:45 AM, Joe Subich, W4TV wrote:
>> Joe made this statement assuming that the KPA500 produced FAR more
>> distortion than it actually does. A few days later, I showed
>> measurements with a K3 driving a KPA500 to 500W with PSK31 showing
>> that the distortion was EXTREMELY low.
>
> Jim, is misconstruing what I wrote.  I specifically said "*average*
> power from the KPA-500 should be kept 6-10 dB below the maximum".
> Jim made his measurements at PEP which *includes* the crest factor
> (PEP = average power X Crest factor or alternately Average Power =
> PEP/Crest Factor).  If his measurements were made at average power,
> the results would have been considerably different. 

Joe, you must misunderstand my measurements. They are ACCUMULATED peaks 
for a fairly long measuring period, so they show transient clicks and 
other forms of distortion far better than an averaged measurement. 
Indeed, I showed measurements of the KPA500 running at 500W out that 
have very low distortion (as indicated by occupied bandwidth). Yes, 
bandwidth (and distortion) increases at bit if the KPA500 is driven harder.

As to peak and average power -- the measurements are of Peak Envelope 
Power, which for CW, RTTY, PSK31, and the WSJT modes is keydown power, 
and for all but CW, is the same as the Average Power. The average power 
is less on CW because of the spaces between elements.

Crest Factor is relevant only on SSB. For normal speech, crest factors 
range between 10 and 20 dB. We often use compression to reduce that 
Crest Factor, and the K3 sounds very good when it is set for about 10 dB 
of compression on peaks. I haven't measured the resulting Crest factors, 
but long experience in pro audio suggests values of 6-10 dB would be the 
result.

The KPA500 is quite well protected against conditions of overload. It 
will add resistive attenuation if overdriven by as little as a dB and/or 
into a poorly matched load, and will Fault if overdriven by a lot. Thus, 
it is probably not possible to drive a KPA500 deep into saturation. This 
is true regardless of the type of modulation in use (CW is modulation by 
a square wave). The highest power measurements I made of the KPA500 were 
at the level which more drive would have caused that resistive 
attenuation to be added.

Indeed, the only way to reduce the Crest Factor with a KPA500 is to 
apply more compression to the driving signal.

Note also that I showed measurements of the K3 driving a very good legal 
limit tube amp to its rated power, and the waveforms were nearly 
identical. That tube amp, a Titan 425, has no protection circuitry, only 
a red LED indicating excessive grid current.

So -- the ONLY way in which my measurements show "better" numbers than 
an averaged measurement is that they use peak power as a reference, and 
applies ONLY to the SSB measurements.

73, Jim K9YC