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[Discuss-gnuradio] RE: USRP + case = electrical short?? EXPLAINED

From: Weber, Michael J. \(US SSA\)
Subject: [Discuss-gnuradio] RE: USRP + case = electrical short?? EXPLAINED
Date: Tue, 6 Mar 2007 17:31:14 -0500

I have identified the cause of the shorting issue I was experiencing and
thought I would share. The bottom-left corner mounting hole of the USRP
passes very close to the internal heat sink pad for the 3.3V regulator;
this pad is at 3.3V due to the regulator design (tab is hot). Some of
the green board mask had scraped off when the mounting post for the Side
B daughtercards was installed, which brought that mounting post, and by
extension the enclosure, to the 3.3V potential. Photos of the board site
are at http://www.likesgadgets.com/gr/  When the SMA jumpers connected
the ground to the case, a short occurred.

I was also able to reproduce the short out of the enclosure just by
tightening down the mounting screw for a daughterboard into that same
mounting post; the lock washer bit through the daughterboard mask and
shorted the daughterboard ground plane through the mounting post to the
3.3V heatsink pad.

Even though there is no lock washer between the mounting post and the
3.3V regulator heatsink pad, the raised edge of the pad caused the mask
to rub off when the mounting post was tightened (or perhaps earlier, who

The short term solution is to insulate the side B mounting post from the
3.3V pad with a very thin insulating washer, scotch tape, or perhaps to
not mount that post. This prevents the enclosure from being connected to
the +3.3V buss when we really do want it grounded. If the washer is too
thick it will cause the daughterboard to bend or not fully seat in the
nearby connector.

IMHO, the long term solution for the next batch of USRPs is to increase
the clearance between the mounting hole and the 3.3V regulator heatsink
pad that the hexagonal mounting stud does not cover or touch the pad.
That will prevent problems for folks with overenthusiastic fingers. :-)


-----Original Message-----
From: address@hidden
On Behalf Of Weber, Michael J. (US SSA)
Sent: Friday, March 02, 2007 11:58 AM
To: George Barrinuevo; address@hidden
Subject: RE: [Discuss-gnuradio] USRP + case = electrical short??

George, thanks for the information!

I would be very surprised if the USRP design did not intend to allow for
RF grounding to a case, but I supposed it's possible. Good grounding is
essential to good RF performance, especially with the proximity of the
analog and digital circuits.

Matt, is this in fact the design, or is this behavior a fluke?


-----Original Message-----
From: address@hidden
On Behalf Of George Barrinuevo
Sent: Thursday, March 01, 2007 8:56 PM
To: address@hidden
Subject: Re: [Discuss-gnuradio] USRP + case = electrical short??


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 problem?


--- "Weber, Michael J. (US SSA)"
<address@hidden> wrote:

> Hello,
> 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 
> traces.
> Help?
> Thanks,
> Mike
> -----------------------------
> Mike Weber * 703-668-4516
> Network Engineer, BAE Systems
> _______________________________________________
> Discuss-gnuradio mailing list
> address@hidden

George Barrinuevo

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