Frequency offset

[llg9012]

 Hi,I want to use three USRP to communicate with each other.But,I encouter the question of frequency offset.In order to eliminate frequency , firstly,I want to use external clock

to synchornize the three USRP.I know I need to use REF in Port,but do I need to use PPS in?.Also,does anyone have good ideas to reduce frequency offset?I'm a beginner to USRP,you answer will be highly appreciated.

[Sarah_Y · Accepted solution]

Hi llg9012,

There are 2 ways to synchronize your USRP.  If you only have 2 USRPs, you can use a MIMO cable to share the reference clock and PPS clock between the 2.  Since you have more than 2 USRPs, you will need to provide a reference clock (10 MHz) and a PPS (pulse per second) clock to all 3 units.  After you have them connected this way, you will need to use a property node to set the reference clock source and timebase clock source (which is the PPS clock) to external.

Sharing these clocks will keep the relative phase between all 3 signals constant.  However, each time you stop and start the device the differences in phase will not be the same.  If you are only concerned that your frequency and phase do not drift over time, using external clock sources is the best solution.  If you need your 3 USRPs to have the same phase, you can use a training sequence to align the phases.  There are examples posted on the community here that may be useful to get your application working:

https://decibel.ni.com/content/groups/ni-usrp-example-labview-vis?view=documents

Check out the angle of arrival detection and 6x6 MIMO examples to see some examples of how others are synchronizing their USRPs.

[0711]

How to make the  external clock sources? Is there any instructions if I want to make one external clock source by myself? Thanks verymuch

[Anthony_F]

Hello,

If you have acess to two function generators you can create the 10 MHz reference clock and the 1 Hz PPS signals manually. Just ensure the voltage levels are correct by reviewing the specifications in the help file.