Re: [Discuss-gnuradio] RFX900 Failure

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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>

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