Re: [Discuss-gnuradio] RFX900 Failure

[Matt Ettus]

On 02/24/2010 09:42 AM, David Evans wrote:
> Hi all,
>
> Power output has significantly dropped, initially by 8dB, now much more.
>
> My first thoughts are that the PA has failed, so is it possible to break
> the transmitter...
> - by prolonged transmitting at high power (i.e. setting it to/near
> maximum)?
> - using a mismatched antenna?
> - mismatching resulting in VSWR effects? (err, without a load)?
>
> I'm obviously going to have to test now, where to start, any
> suggestions, like what voltage swing before and after the 3315 should I
> expect?


I have seen this once before with someone who was transmitting at max 
power continuously.  The problem may be in the SAW filter, which would 
make it easy to fix.  You can just put a cap of anywhere between 50 and 
1000 pF, size 0603 in the empty capacitor location which is in parallel 
with the filter.

In order to tell if that really is the problem, you would probably need 
to probe with an RF probe for your spectrum analyzer or vary fast 
oscilloscope.  You could probe at the antenna port and immediately 
before the SAW filter, and if there is a big loss in the filter you know 
that is bad.

If you don't have the equipment to test this, it may be easier to just 
put the cap in there and try it.

Matt