[Elecraft] K3, Elecraft Transverter K144XV-K

[Joe Subich, W4TV]

 > My observations on most RF power amplifiers is that gain is higher at
 > lower drive levels so proportionally there is more output.  e.g. full
 > drive =10w, half drive = 7w, quarter drive = 5w.

You are describing classic compression in the amplifier [chain].  Where
you have cascaded amplifiers, measure the Po/Pi slope *for each device*
and adjust the relative gains/drive levels so that each device has a
similar curve - or at least all begin to compress at the same level -
and operate below the onset of compression.

 > Yet these amplifiers sound fine in SSB so must have good linearity.

You can't say that.  Non-linearity will show up first in IMD products
*outside the SSB bandwidth* - your neighbors will hear the distortion
before your QSO partner does or you hear it in your monitor.

Now, there is no issue using these amplifiers in CW/RTTY/JT-mode as all
of those modes are "one tone at a time" and do not generate significant
IMD when feeding even a class C amplifier (transitions will become a
bit sharper/more "clicky").  However in general, one should keep any
*amplifier chain* used for "linear" modes (SSB, PSK31/63/125, etc.)
below the 1 dB compression point.  In other words, if 10W drive gives
750W output you should need any more than 25W for 1500W output.

The 1 dB compression point is generally associated with IMD in the -33
to -35 dB range provided other issues (e.g. crossover distortion, bias
stability, etc.) are well controlled.  Ideally one would keep any gain
compression well below 1 dB (20%) but that is often difficult with
solid stage devices at high power levels - particularly since solid
state devices tend to be rated for saturated mode (class C, pulse)
outputs without regard for compression.

73,

    ... Joe, W4TV


On 2015-01-08 12:03 PM, Edward R Cole wrote:
> David and Lexa,
>
> My observations on most RF power amplifiers is that gain is higher at
> lower drive levels so proportionally there is more output.  e.g. full
> drive =10w, half drive = 7w, quarter drive = 5w.  So one finds that an
> amplifier hits a minimum output with lowered drive that is not linear
> with change in drive level.  Yet these amplifiers sound fine in SSB so
> must have good linearity.
>
> My 50w 2m transverter actually will produce nearly 60w with 1.5mw drive
> and 35w with 0.73 mw, but minimum output with 0.1mw is about 15w.  I
> insert a 3-dB attenuator to get down to 7w output to drive my 150w amp
> to 55w which is the drive required for 1400w with my 2m-8877.  I prefer
> this arrangement vs running the transverter near max at 55w when running
> digital modes (less heat stress on the transverter amp which also
> translates as less thermal frequency drift).  My 25w 1296 transverter
> runs 20w at full output and about 8w with minimum drive.  I've seen this
> consistently with RF power amplifiers (my HF amps operate similarly)
>
> 73, Ed - KL7UW
> http://www.kl7uw.com
>      "Kits made by KL7UW"
> Dubus Mag business:
>      dubususa at gmail.com
>
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