Re: [Discuss-gnuradio] RFX900 Failure

[David Evans]

Hi,

Thanks for the fast response. Yes the SAW filter is broken and putting a 
capacitor in as you suggested has brought the output power back up to 
normal. :-)

So, a couple of questions please,

   * Can the SAW be damaged by too much power from the PA? (I could
     find no info on the maximum power limits for this device, just the
     bandwidth and attenuation).
   * Can the SAW be damaged by a mismatched load (i.e no load!). I've
     asked around about this and get differing opinions
         o Yes, because the reflected power will be additively doubled,
           and enhanced due to the high Q of the filter
         o No, because the filter is a passive device, and the power
           will just pass through.
   * Also, apparently, SAW filters can easily be damaged due to
     physical shock, damaging the piezo electric material, so maybe
     this was just a one-off. I'll have to replace the chip anyway

Thanks again guys,
David


Matt Ettus wrote:
> 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
>
>
>
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