[hpsdr] My PA & Alex

[Chris Smith]

Hi John/Graham

I tried earthing the two boards to the casing of Alex as per the documentation without any noticeable difference. Having done a few experiments disconnecting possible feed-back paths I have eliminated the power supply path via Pennywhistle (removed supply to PW - doesn't stop the take-off). The only thing which stops the self oscillation once it has started, is to remove the Pennywhistle to PA signal path. It's almost as if there is an earth loop causing feed-back. I'm having a few hours away from the problem as I don't fancy spending quite a few pounds and a lot of time replacing the MRF454s again!  I think my next move will be to add some L & C to the PA supply.

Many thanks for your response.

Cheers

Chris G4NUX


On 10 Sep 2011, at 14:40, John Ackermann N8UR wrote:

> Do you have the Alex boards grounded directly to each other and to the enclosure?  If not, you're relying on the various coax shields to tie the system together and you could end up with reactances at VHF that might set up parasitics.
> 
> John
> ----
> 
> Chris Smith said the following on 09/10/2011 09:34 AM:
>> ***** 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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