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

[Brett Trotter]

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: address@hidden
> [mailto: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?
> 
> Thanks,
> Mike
> 
> -----Original Message-----
> From: address@hidden
> [mailto: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??
> 
> 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)"
> <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
>> 
>> 
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>> 
> 
> 
> George Barrinuevo
> address@hidden
> 
> 
> 
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