Using 10 MHz and PPS with Multiple USRPs

[Pavel]

Short answer: Use a common 10 MHz reference to align sample-clock frequency and a common PPS edge to align device time. Either signal alone solves only part of synchronization; configure clock source, time source and the next-PPS time explicitly, then verify with a shared RF signal.

Before you start​

• Use UHD discovery and probe utilities before GNU Radio, OpenAirInterface or srsRAN. Save the exact UHD version, FPGA image and device arguments.
• Match the host interface to the requested stream: USB-controller limits matter for B-series radios, while N- and X-series radios require a planned Ethernet path.

Step-by-step method​

1. Step 1: Choose a reference source that meets every USRP input level and waveform requirement. Distribute 10 MHz and PPS through matched, properly terminated paths rather than passive tees that distort levels.
2. Step 2: Configure each USRP for the external clock source and verify lock sensors. Configure the PPS time source and observe that every device detects advancing PPS events.
3. Step 3: Set device time on the same future PPS edge, then issue timed stream commands. Record device times immediately after the event to catch a missed or double-counted edge.
4. Step 4: Split one coherent RF test tone to all receive channels and measure relative sample offset and phase over time. Repeat after restart to separate time alignment from deterministic phase behavior.

Concrete example​

Two USRPs sharing only 10 MHz can sample at the same long-term rate but start at different sample indices. Sharing only PPS can align a moment while oscillator frequencies drift. Use both when the experiment needs frequency and time coherence.

How to judge the result​

Synchronization is confirmed when all devices report reference lock, agree on device time after the chosen PPS and meet the project's repeatable sample/phase error using the shared RF test.

What to record​

• Monitor overflow or underflow indicators during a sustained test, not only during application startup.
• Clock and PPS solve different problems: 10 MHz supplies a frequency reference, while PPS supplies a time-alignment event.
• For coherent work, verify phase behavior and timestamp alignment with a known shared signal rather than only checking that two channels display data.

Common mistakes​

• Changing several hardware, software or RF variables at once, which removes the controlled comparison needed to identify the cause.
• Treating one autoscaled screenshot or one unusually good result as proof without recording the settings and repeating the test.
• Equal-length clock cables do not compensate for software failing to select the external sources or initialize device time on the same PPS.

Final check​

Reboot the host and power-cycle the radio, then repeat discovery, probe and the sustained stream. Keep the exact UHD, firmware and FPGA versions with the working result.