USRP + case = electrical short?


I have a USRP with an RFX2400 mounted on the A side and a FLEX900
mounted on the B side. Everything works great UNTIL I connect the SMA
jumpers from a TX/RX port on either board to the case. When the
connection between the TX/RX ground and the case is made, a short
occurs, the regulator gets very hot, and the voltage at the input jack
drops to about 2V. Of course I disconnected the power as soon as I
discovered what was happening. The board seems happy out of the case, or
even in the case if the antenna port jumpers are not installed. I have
to other units just like this one running with no problems, though one
of them has a FLEX400 instead of a FLEX900.

So… any ideas where the short is? I inspected the case and USRP but
didn’t see anything telling, but there’s a lot of tiny components and



Mike W. * 703-668-4516
Network Engineer, BAE Systems


I experienced a similar issue with my BasicRX and
other daughterboard. Here is what I found. When all
is working fine, the motherboards green LED blinks.
Otherwise, it does not blink or come on when there is
a short. When I loosened the nut/washer that holds
the SMA jumper cable to the case, the short went away.
It appears that when this nut it tightened too much,
the locking washer will bite through the black pant on
the enclosure and make contact with the enclosure
metal which causes a short. My solution was to loosen
this nut a bit.

A better solution is to find plastic thin washers and
use this instead of the locking washers supplied with
the enclosure. However, I was not able to find a
plastic washer in the right size. Someone on this
mailing list used a sheet protector with an
exacto-knife to cut out a shape of a washer.

Does anyone have a better/easier solution to this


— “Weber, Michael J. (US SSA)”
[email protected] wrote:

occurs, the regulator gets very hot, and the voltage

Mike W. * 703-668-4516
Network Engineer, BAE Systems

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George B.
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