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Re: [Discuss-gnuradio] RFX900 Failure

From: Matt Ettus
Subject: Re: [Discuss-gnuradio] RFX900 Failure
Date: Fri, 26 Feb 2010 11:45:42 -0800
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On 02/26/2010 10:05 AM, David Evans 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
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.


Thanks again guys,

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
- 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

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.


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