[hpsdr] My PA & Alex

[David McQuate]

If the high PA power supply current is indicative of PA 
self-oscillation, and I'm guessing that is what's happening, it sounds 
like your PA is not unconditionally stable at all frequencies (in-band 
and out-of-band -- I'm not sure of the frequency of the spurious 
oscillation, perhaps VHF).  While Alex is designed to present a load 
close to 50 ohms, resistive, to the output of the PA *in band*, it most 
probably is highly reflective out of band (in the filter's stop-band).  
If the magnitude of the output reflection coefficient of the PA is 
sufficiently greater than one at any frequency at which the PA's load is 
also reflective, there will be oscillations.

  I'd say you need some sort of a diplexer, that lets the desired HF 
signals go to Alex unchanged, but absorbs power at other frequencies.  
It might be as simple as a series connection of a 50 ohm resistor and a 
small capacitor, connected in parallel with the input to Alex.  The 
reactance of the capacitor would be large at HF, but small at the 
frequency of the spurious oscillation.  If the latter is really at VHF, 
the frequency difference will be large, making the difference in 
capacitive reactance large.  If the oscillation frequency is closer to 
the PA normal operating frequency, the requirements on this diplexer 
become more difficult to realize.

Another solution would be to modify the PA to "neutralize" it, so that 
it's output reflection coefficient magnitude remains small (ie < 1).

73,
Dave
wa8ywq

On 9/10/2011 6:34 AM, Chris Smith wrote:
> ***** High Performance Software Defined Radio Discussion List *****
>
> Hi
>
> I managed to destroy a matched pair of MRF454s by over-driving them. My PA requires 4-5W in for 100W out and I'd pumped roughly 14W in and they had expired.
>
> So, having re-calibrated PowerSDR's PA settings using just Pennylane&  Pennywhistle into a dummy load for max 5W, I replaced the dead MRF454s and cautiously started again.
>
> I connected Pennylane ->  Pennywhistle ->  PA ->  Dummy load. Tried Tune at 1W - actually produced 3W but who's complaining. Tried various levels up to 10W. All OK. Tried 2-tone test at 1W: produced 1W. Wonderfull! Tried various levels up to 100W all produced roughly the expected o/p.
>
> Then I tried PA ->  Alex ->  Dummy load and here things fell apart. Tune at 1W produced about 3W as above into dummy load BUT when I turned Tune off the current drawn by the whole set-up jumped to 10A and wouldn't retrun to normal even when I dismissed PowerSDR. The only way I could get back to square 1 was to power down my set-up. I'm guessing that the combination is taking off at some frequency yet to be determined.
>
> I went back to PA direct to dummy load and was VERY relieved to see that the PA hadn't been damaged again. Tried all the above tests again to reassure myself. Tried through Alex again: same strange behaviour.
>
> So, everything is working fine EXCEPT I can't feed my PA o/p through Alex 'cos when I do the whole shebang takes off!
>
> I haven't yet tried toroids on the power lead to the PA as added decoupling etc.
>
> Does anyone have any ideas?
>
> Puzzled from Plaistow
>
> Chris G4NUX
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