[Elecraft] KPA500 Power In Digital Modes

[Joe Subich, W4TV]

Jim,

> Right -- my test conditions for SSB are MORE demanding than either
> PSK31 or JT65A, and were intended to approximate the response of the
> K3 (and amp) to a speech signal.

That SSB is *more* demanding than PSK31 is incorrect.  SSB audio can
be compressed/clipped significantly without IMD if the compression is
done correctly.  PSK31 on the other hand *can not* withstand *any*
compression without significant close-in IMD.  See the work by W7AY
for that information.  JT65A (and other JT modes) on the other hand
is a saturated single tone mode akin to FSK that has *no issue* with
compression and is, in fact, not compressible.

> When I have time (I'm currently hot and heavy with antenna work), I
> will, however, repeat these measurements with a PSK31 test signal,
> and with JT65A. I don't expect the results to be a lot different.

If you do the measurements correctly, they will be a LOT different.
JT65A will show little or no change in IMD from 25 to 100 W average
(and PEP) output on the K3 (or 100W to 600W average and PEP output
on the KPA-500.  PSK31, however, will go to pot in a hand basket if
the average power output on the K3 goes above 30 watts or so or if
the average power output of the KPA500 goes above 150 watts or so.
If you are measuring *TRUE* peak power, those are equivalent to
100-110 W PEP from the K3 or 600W PEP on the KPA500 given the 6 dB
crest ratio in the PSK31 waveform.

JT65 and JT9 (all sub-modes) have a 0 dB crest factor, PSK31 has a
6 dB crest factor - other modern data modes have various crest factors
- you need to understand the crest factor of the input waveform.

> Did you miss the CW measurements, where the modulation was a shaped
> square wave?

No, I did not "miss" the CW measurements - they have no bearing on
the issues here.

73,

   ... Joe, W4TV


On 2014-11-11 4:07 PM, Jim Brown wrote:
> On Tue,11/11/2014 7:17 AM, Joe Subich, W4TV wrote:
>> Your measurements are not of the conditions I describe.  You are using
>> *filtered and band limited pink noise* (slide #3 - EQ settings) not the
>> specific digital signals I mention *and* your measurements are at PEP
>> not average power (slide #2 - LP-100A set to peak).
>
> Right -- my test conditions for SSB are MORE demanding than either PSK31
> or JT65A, and were intended to approximate the response of the K3 (and
> amp) to a speech signal. I also made one measurement of the K3 at high
> power without compression. Occupied bandwidth was nearly identical to
> the 10dB compression, so I did the rest with compression.
>
> Did you miss the CW measurements, where the modulation was a shaped
> square wave?
>
> When I have time (I'm currently hot and heavy with antenna work), I
> will, however, repeat these measurements with a PSK31 test signal, and
> with JT65A. I don't expect the results to be a lot different.
>
> 73, Jim K9YC
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