Internal GPSDO Application Notes (USRP-X3x0 Models)

Table of Contents

• Specifications
• Installation Instructions
• Using the GPSDO in Your Application

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

By default, if a GPSDO is detected at startup, the USRP will be configured to use it as a frequency and time reference. The internal VITA timestamp will be initialized to the GPS time, and the internal oscillator will be phase-locked to the 10MHz GPSDO reference. If the GPSDO is not locked to satellites, the VITA time will not be initialized.

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, boost::posix_time in C++, etc.

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.