[hpsdr] My PA & Alex

[Chris Smith]

Hi Graham

Ok, I spent all afternoon adding grounding wires from all the modules you suggested,  including the PA, to a single point in Pandora and it made not one iota of difference. If I apply enough drive to give me more than approx 5W out of the PA then, when the drive is removed, the whole thing takes off. However when drive is applied again the expected o/p is seen. But I have to cycle power to the PA to stop the self oscillation. If I bypass Alex everything behaves impeccably.

I'm getting very despondent and not a little frustrated. It's beginning to look as if I can't use my PA with Alex. If I leave the PA out of the loop (Pennywhistle -> Alex) everything works fine though because I have a 6dB pad on the i/p of PW I can only get just over 10W out.

When I worked on an airfield for British Aerospace I had a prototype wireman working for me who used to regularly give badly behaved equipment what he called "the soak test" i.e.  he would open the workshop window and lob the kit into the static water tank a few feet away. When the fire service came to flush the static tank each year there would be a large amount of electronic gear in there and the firemen would scratch their heads and cart it away for disposal. Just now, if there was a static tank handy I would be sorely tempted to lob all my HPSDR kit in it!!!

73

Chris G4NUX

On 10 Sep 2011, at 21:09, Graham / KE9H wrote:

> ***** High Performance Software Defined Radio Discussion List *****
> 
> We are not talking about earthing the boards inside of ALEX, although that
> is also recommended.
> 
> You want a solid ground connecting the Penelope ground, to the PennyWhistle
> ground to the amplifier ground, to the ALEX case ground, short and low inductance,
> independent of the coax shields.  Also good RF isolation in the DC feeds to the different modules.
> 
> --- Graham
> 
> ==
> 
> On 9/10/2011 2:47 PM, Chris Smith wrote:
>> ***** High Performance Software Defined Radio Discussion List *****
>> 
>> 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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