[hpsdr] My PA & Alex

[Al K0VM]

Chris,
     It is likely that your PA is not "unconditionally stable"..  That 
is to say oscillation free into loads of any impedance....
Yes it is stable into a 50 ohm load ( a wide band load) and yes the Alex 
is nearly 50 ohm at your operating frequency.. But like all LC filter 
devices, Alex  is far from 50 ohms at is reject frequencies.   Usually 
there is very little that can be done external to the PA to resolve the 
problem.   I have no ideas for what might have to be done internal to 
the PA to fix the problem.

    My limited experience with this issue has been with VHF PA and 
duplexers where changing the length of coax between the PA and the 
filter resolved the issue (rotated the phase of the off frequency 
reflections).  This might be beneficial for one HF band  but is likely 
to require different coax lengths for each band.

    If you have spare drive available on the input to your PA,  there is 
a slim chance that a resistive pad on the input to the PA might be helpful.

AL, K0VM



On 9/11/2011 4:05 PM, hpsdr-request at lists.openhpsdr.org wrote:
> Date: Sun, 11 Sep 2011 18:28:17 +0100
> From: Chris Smith<chris at vspl.co.uk>
> To: Graham / KE9H<KE9H at austin.rr.com>
> Cc:hpsdr at lists.openhpsdr.org
> Subject: Re: [hpsdr] My PA&  Alex
> Message-ID:<20110911172822.5516148002 at diego.dreamhost.com>
> Content-Type: text/plain; charset=us-ascii
>
> 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


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