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List:       gnuradio-discuss
Subject:    Re: [Discuss-gnuradio] RFX900 Failure
From:       Matt Ettus <matt () ettus ! com>
Date:       2010-02-26 19:45:42
Message-ID: 4B8824E6.2020608 () ettus ! com
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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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>> Discuss-gnuradio@gnu.org
>> http://lists.gnu.org/mailman/listinfo/discuss-gnuradio
>>
>>
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