How good can USRPs with internal GPSDOs being synchronized?

classic Classic list List threaded Threaded
1 message Options
Reply | Threaded
Open this post in threaded view
Report Content as Inappropriate

How good can USRPs with internal GPSDOs being synchronized?

Hi all,

I am trying to synchronise two USRP N210 with the goal of receiving time-aligned samples. 
I've modified the rx_samples_to_file.cpp code according to this page:

It seems that the signals at the USRPs have between +0.2 and -0.2 us time difference. 
This time difference seems to be random, as it changes every time I run a new experiment.

Does anyone know why the samples receive at these two USRPs are still not aligned?
I thought the time difference should be less than 100ns. 
What's the right way to minimize the time difference??

Best regards,
Cheng Chi

Here is the setup and the code snippet that I've add to the example rx_samples_to_file.cpp.

- USRP N210 with internal GPSDO module, daughtherboard is DBSRX2
- Signal generator outputs a common signal and connects to the two USRPs through equal length cables

Transmitter part: Common signal is FM signal with 1MHz bandwidth, carrier frequency is 1.6GHz. 
Receiver part: Sampling rate is 5Msps

1. For Common Reference Signals

2. For synchronising the Device Time
Because each N210 has an internal GPDSO module, the device times are automatically synchronised

3. For Synchronising Channel Phase

To align CORDIC:
    uhd::stream_cmd_t stream_cmd((num_requested_samples == 0)?
    stream_cmd.num_samps = num_requested_samples;
    stream_cmd.stream_now = false;
    stream_cmd.time_spec = uhd::time_spec_t(time_to_receive);

To align LOs in the frontend:
    uhd::time_spec_t cmd_time = usrp->get_time_now() + uhd::time_spec_t(0.1);

For DBSRX2, does this method for aligning LO work?

Discuss-gnuradio mailing list
[hidden email]