We are about to connect the USRP with LFRX and LFTX daughter boards to a laboratory data acquisition system where noise and ground loops are an issue. Can anyone confirm my understanding of the grounding on the USRP, daughter boards, and enclosure?
As I understand it, the USRP is floating with respect to the rest of the laboratory. The only ways the USRP grounds could be connected to grounds elsewhere in the lab are through the USB ground (depending on how the host computer is grounded), or through the shields of the cables connected to the SMA connectors on the daughterboards (see figure below). Here are the details as I understand them, from the circuit diagrams at http://gnuradio.org/trac/browser/usrp-hw/ On the USRP motherboard: - Power ground: The USRP is powered through a transformer. The power ground of the USRP motherboard (the ground pin of the power regulator IC) is connected to one of the terminals on the output side of this transformer. Therefore the USRP power ground is not connected to the rest of the lab (except for the coupling through the transformer). - Digital ground: The ground pins of the digital ICs (and the digital ground pins of the AD986X) on the USRP motherboard are connected to the power ground (same as the ground pin on the power regulator IC). - USB ground: The USB connector ground is connected to the digital ground. - Analog grounds: The analog ground pins of the two AD986X are connected to two separate analog grounds, AGND_1 and AGND_2. Each analog ground is connected at one point to the power/digital ground, through an inductor. - The mounting holes are not connected to any ground. LFRX and LFTX Daughterboards - Power and grounds on both daugherboards are connected to the USRP power and grounds through the daughterboard connector. - Analog grounds: Each daughterboard has its own analog ground AGND that is connected to the separate AGND_1 and AGND_2 on the USRP motherboard. On each daughterboard, there is no connection between its analog ground and its digital ground. - Analog inputs: The signal on the SMA connectors is connected to the analog input of the instrumentation amplifier IC through a resistor, and to the analog ground through resistors and capacitors. - The shield on the SMA connectors on the LFRX and LFTX boards is connected to the analog ground. Enclosure (we have a homemade enclosure, a painted metal box): - The USRP motherboard mounting holes are not connected to any ground, so it is possible to insulate the enclosure from the electronics - If the SMA connectors on the enclosure are not insulated from the enclosure, then the enclosure is connected to both analog grounds (through the SMA connector shields on the daughterboards), and there could be loops (see figure) SMA | USB | SMA | shield| ground | shield| Enclosure ......... | ................ | ................ | ......... | | | LFRX | AGND | AGND | LFTX Daughterboards +--+--- --+-- | --+-- ---+--+ | | | | | daughterboard | | | | | connector | | | | | | | | | | USRP Motherboard ---+---))))--+-----+--+--+--))))---+--- AGND_1 coil digital | coil AGND_2 and power| ground | | | + ))))))) Transformer ))))))) | | Lab power _______________________________________________ Discuss-gnuradio mailing list [EMAIL PROTECTED] http://lists.gnu.org/mailman/listinfo/discuss-gnuradio
