Internal GPSDO Application Notes (USRP-X3x0 Models)

Table of Contents

• Specifications
• Installation Instructions
• Using the GPSDO in Your Application
  • Device Time to GPS time
  • GPS sensors

This application note describes the use of the board-mounted GPS Disciplined OCXO, as used with the USRP X300/X310. For information regarding the GPSDO that is compatible with the USRP N2xx or E1xx, please see Internal GPSDO (USRP-N2x0/E1X0 Models).

Specifications

• Receiver type: 50 channel with WAAS, EGNOS, MSAS
• 10 MHz ADEV: 5e-11 over >24h
• 1PPS RMS jitter: <50ns 1-sigma
• Holdover: <20us over 3h

Phase noise:

Offset	OCXO Phase noise power
1Hz	-75dBc/Hz
10Hz	-110dBc/Hz
100Hz	-132dBc/Hz
1kHz	-142dBc/Hz
10kHz	-145dBc/Hz
100kHz	-150dBc/Hz

Antenna Types:

The GPSDO is capable of supplying a 3V for active GPS antennas or supporting passive antennas.

Installation Instructions

To install the GPSDO, you must insert it into the slot on the board near the 10 MHz Reference SMA. Keep in mind that the two sides of the GPSDO have a different number of pins. When inserting the GPSDO, make sure to press down firmly and evenly. When turning on the USRP X3x0 device, a green LED should illuminate on the GPSDO. This signifies that the unit has successfully been placed.

NOTE: The pins on the GPSDO are very fragile. Be sure to press down evenly, or the pins may bend or break. Once the GPSDO is in place, we very highly discourage further removal, as this also risks damaging the pins.

Using the GPSDO in Your Application

If a GPSDO is detected at startup, the USRP will have gpsdo clock and time source options.

Select these by using UHD's

usrp->set_clock_source("gpsdo");
usrp->set_time_source("gpsdo");

That will lock the reference clock to the disciplined 10MHz oscillator, and will make the USRP listen for time signal pulses coming from the GPSDO.

However, you will still need to explicitly set the device time on such a PPS edge, using the set_time_next_pps or set_time_unknown_pps calls.

GPS data is obtained through the mboard_sensors interface. To retrieve the current GPS time, use the gps_time sensor:

usrp->get_mboard_sensor("gps_time");

The returned value will be the current epoch time, in seconds since January 1, 1970. This value is readily converted into human-readable format using the time.h library in C, std::chrono in C++, etc.

Device Time to GPS time

If you want to set the device time to GPS time, the following procedure makes sure you don't encounter timing problems while doing so:

1. Wait for GPS lock, and then set the time source to GPSDO,

   while(! (usrp->get_mboard_sensor("gps_locked",0).to_bool()) ) {
       std::this_thread::sleep(std::chrono::seconds(2));
   }
   usrp->set_time_source("gpsdo");

2. poll on usrp->get_time_last_pps() until a change is seen, and then sleep 200ms to allow new time data coming from the GPSDO to propagate through the system,

   uhd::time_spec_t last = usrp->get_time_last_pps();
   uhd::time_spec_t next = usrp->get_time_last_pps();
   while(next == last) {
       std::this_thread::sleep(std::chrono::milliseconds(50));
       last = next;
       next = usrp->get_time_last_pps();
   }
   std::this_thread::sleep(std::chrono::milliseconds(200));

3. use

   usrp->set_time_next_pps(uhd::time_spec_t(usrp->get_mboard_sensor("gps_time").to_int()+1));

   to set the time,
4. poll on usrp->get_time_last_pps() until a change is seen and sleep 200ms (allow NMEA string to propagate),
5. Verify that usrp->get_time_last_pps() and usrp->get_mboard_sensor("gps_time") return the same time.

GPS sensors

Other information can be fetched as well. You can query the lock status with the gps_locked sensor, as well as obtain raw NMEA sentences using the gps_gprmc, and gps_gpgga sensors. Location information can be parsed out of the gps_gpgga sensor by using gpsd or another NMEA parser.