[USRP-users] Safety Calculation

Radio User radiogeek381 at gmail.com
Fri Oct 13 21:23:18 EDT 2017


Yes this has been addressed before.  But it is worth going through an
example of where the number might come from.   Note that I do not work for
Ettus.  I have Ettus radios on my bench here. (And like them a whole lot.)
I am not advocating for a different limit in the datasheet.  However, I
encourage my fellow Ettus users to look at the schematics.  There's a lot
to be learned from them.

The B2xx has a lineup between the antenna input and the first really
sensitive part that looks like this:

ANT ---> Switch ---> Switch --> Balun --> expensive A/D with integrated
preamp

According to their datasheets, the two switches are practially
bullet-proof.  The absolute maximum rated input power is about a watt for
both of them.

The Balun is similarly robust, with an absolute maximum rating of about 2
watts.

But the AD9361 has lots of magic smoke inside that is just waiting to be
liberated.  That chip as an absolute max rating of just 2.5 dBm.

The Ettus knowledge base webpage at
https://kb.ettus.com/B200/B210/B200mini/B205mini#Input_Power_Levels indicates
that the max input power is 0dBm.  Why wouldn't they say 2.5dBm?  I can
only surmise, but I've been an engineer for a long time and I'd never tell
a customer to push it to the limit.  The risk is that the customer will
wreck something that I'll have to replace or repair, or risk disappointing
a customer. The prudent and responsible thing to do is to allow a little
margin between the system absolute maximum rating and the component
absolute maximum rating.

(And yes, I know I ignored the losses through the switches... but that only
gives a little more margin.  A good thing.)

-----------------------

Additionally, be careful about path loss assumptions for antennas that are
close to each other.  Proximity may mean that the two antennas are in each
other's near fields.  Strange things happen in that region.  Wave fronts
aren't planar. Sources are distributed.  Apertures are not what you think
they are.

----------------------------

I hope this is helpful.
73,
matt
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