On 02/26/2010 10:05 AM, David E. wrote:
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
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.