[Discuss-gnuradio] USRP grounding

[Jonathan Jacky]

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