[Elecraft] Product Idea - 25 Watt PA for the K3

[ab2tc]

Hi,

Very good points indeed. I am not surprised at all that the KPA3 is much
cleaner at 30W than the K3/10 is at 10W. At 25/30W it's loafing along close
to its optimum IMD level and the "driver" (the KPA3/10) is loafing along at
3W or less. So let's face it, neither of these amplifiers are very linear at
their rated full power levels. So please don't let KPA500 users worsen their
IMD levels by skimping on its driving source.

AB2TC - Knut 


Matthew Cook wrote
> To make this idea work you need to make a 25Wpep amplifier that is kept in
> it's linear region so that the IMD from this amplifier does not cause
> problems in the output of the KPA500.
> 
> To make a class A-B 25W amplifier such as this with enough linearity will
> result in you designing an amplifier capable of 100Wpep at full smoke. 
> The
> non-linear nature of HF PA's is easily found on the net.  Having designed
> PA's for HF manpacks a number of years ago we simply turned down our
> 100Wpep design to 25Wpep and pocketed the performance increase at the
> expense of cost.
> 
> The K3/100 at 25-35W has excellent linearity (highly likely by design, pat
> designers on back for this one) which allows a more aggressive approach
> within the KPA500 for grunt at the expense of "some" linearity.   You're
> not increasing the cost of the KPA500 trying to eeek out every last bit of
> linearity to get around the rubbish coming from the transceiver.
> 
> I'm keen to do some measurements in the near future of the SPE 1.3k-fa
> amplifier being driven by a K3/10.  Having measured the IMD of my the
> K3/100 at 10W (KPA3 disabled) recently I was a little surprised at the
> result, it was not as clean as when driven at 30W with the KPA3 enabled.
> I'd not be surprised to see some output degradation in the SPE from the
> K3/10 c.f K3/100 driven at 20W with a -3dB inline attenuator.   This would
> certainly explain some of the "wide" local signals that I've been seeing
> in
> my SDR recently.
> 
> Engineering is always about compromise, and cascaded amplifiers are one of
> the more trickier things to strike the right balance with.
> 
> 73
> 
> Matthew
> VK5ZM
> <snip>





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