Out of curiosity, what generation USRP do you have? My Rev 3 board has quite a lot of clearance on the TX-B side (1/4"ish- not measured)
Weber, Michael J. (US SSA) wrote: > > 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 > knows?) > > 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. :-) > > Best, > Mike > > > -----Original Message----- > From: [EMAIL PROTECTED] > [mailto:[EMAIL PROTECTED] > On Behalf Of Weber, Michael J. (US SSA) > Sent: Friday, March 02, 2007 11:58 AM > To: George Barrinuevo; Discuss-gnuradio@gnu.org > 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? > > Thanks, > Mike > > -----Original Message----- > From: [EMAIL PROTECTED] > [mailto:[EMAIL PROTECTED] > On Behalf Of George Barrinuevo > Sent: Thursday, March 01, 2007 8:56 PM > To: Discuss-gnuradio@gnu.org > Subject: Re: [Discuss-gnuradio] USRP + case = electrical short?? > > Hello, > > 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? > > Thanks, > > --- "Weber, Michael J. (US SSA)" > <[EMAIL PROTECTED]> 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 >> Discuss-gnuradio@gnu.org >> > http://lists.gnu.org/mailman/listinfo/discuss-gnuradio >> > > > George Barrinuevo > [EMAIL PROTECTED] > > > > _______________________________________________ > Discuss-gnuradio mailing list > Discuss-gnuradio@gnu.org > http://lists.gnu.org/mailman/listinfo/discuss-gnuradio > > > > _______________________________________________ > Discuss-gnuradio mailing list > Discuss-gnuradio@gnu.org > http://lists.gnu.org/mailman/listinfo/discuss-gnuradio > > > > _______________________________________________ > Discuss-gnuradio mailing list > Discuss-gnuradio@gnu.org > http://lists.gnu.org/mailman/listinfo/discuss-gnuradio > > -- View this message in context: http://www.nabble.com/RE%3A-USRP-%2B-case-%3D-electrical-short---EXPLAINED-tf3358833.html#a9350646 Sent from the GnuRadio mailing list archive at Nabble.com. _______________________________________________ Discuss-gnuradio mailing list Discuss-gnuradio@gnu.org http://lists.gnu.org/mailman/listinfo/discuss-gnuradio
