Re: [Discuss-gnuradio] RFX900 Failure

[Matt Ettus]

On 02/26/2010 10:05 AM, David Evans wrote:
> 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. :-)

Great!  If you can survive without the filtering, you're actually better 
off now because you'll have 1.5 to 2 dB less loss.

> 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).

It can be.  I had been told by Sawtek that the part was supposed to be 
good to half a watt.  While the RFX900 won't put out that much power, 
some of them will come within 1-2 dB of that.  The longer you operate 
near any parts maximums, the shorter its life will be.  This is only the 
second time in many many boards and over 4 years that we've seen this 
problem, so you just might be on the unlucky tail of the distribution.

> * 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

Not enhanced by the Q, but a high SWR can result in higher peak voltage 
which can stress a device.

> o No, because the filter is a passive device, and the power
> will just pass through.

Each time the power passes through it is subject to insertion loss which 
is dissipated, so reflected power will add a little to the dissipation 
if you have a really bad reflection.

> * 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

My best guess is that this is due to sustained high power over a long 
period heating and aging the device.

Matt


> 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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