[hpsdr] Clean RF linear amplifier for <100mW to 100W

[Tom Cathey]

Hans,

Thanks for the interesting info, OM.

Yes, we talked via email a few months ago about your development of a super
clean SS linear for the HPSDR rigs. Looks like you've made some good
progress.

So, as a pre-driver stage, just by idling PennyWhistle up to 3A in class A
and demanding only 8W out, you are able to see -50db 3rd. That is
impressive!

I ran into the same heat roadblock as you at the 100W level. I finally had a
pair of 4CX-250's running in class A, generating much heat - but making at
least -50db 3rd. But it required tuning each band, a noisy blower for air,
etc. Your CPU-water-cooled solution is unique and I would certainly like to
take a look at it when the paper comes out in TAPR Quarterly.  That's sounds
like a cooling system used by some of the newer high performance computers.
400 watts of heat is a lot to dissipate, but you are seeing stellar IMD
numbers as a result!

BTW, in the meantime, a friend is going to give me a 200W PA Unit and LP
filter board from a junk FT-1000D. I have a 1000D myself and the 3rd IMD is
about -40db at 40 watts out, so this will get me by for now. I will remote
switch the 5V LP filter relays with Penny.

I consider it a worthwhile goal to get a transmitter system working to at
least -45db 3rd or higher. Once we hear (with our ears) the difference in
side channel products between -30db and -45db, we never go back... :-)

73,

Tom, K1JJ

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Tom,
I am using behind Penelope a CCI kit with two MRF433 as a driver stage to
maximal 8 W in class A (bias current 3 A) with IMD3 better than -50 dB
overall at 7 MHz. 
I am just measuring in detail a next stage following with two MRF429 (50 V),
that's a CCI kit as well (AN-758). 
If run in class A with 8 A quiescent current it delivers up to 100 W with
IMD3 a little less than 50 dB below one of the test tones (clearly better
than 50 dB at your envisaged 30 W). 

Since 400 W of steady state power must be dissipated in this mode of
operation I have water cooled the transistors by using a computer processor
cooling system.

I am just busy measuring all specifications in detail, that's IMD3 as
function of power at all bands. 
I will send a paper on it to Stan for publication in the next PSR, the TAPR
quarterly newsletter.

Best 73, Hans, DL2MDQ.




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