In the Synchronization & Phase Coherency section of the XYNC campaign page, we mention that "if you’re looking to do direction finding or beamforming, you would need to take the extra step of calibrating the phases between all XTRXs". Here we would like to share the results of one of our clients, who carried out such calibration as a part of the AutoAir UK5G project.

Please note that this remains a work in progress, and we expect even better results after further tuning.

Test System Specification

• 16 Tx channels (8 pairs of coherent channels)
• All XTRX in the array share the same reference clock and the 1 pps signal
• 1 Rx calibration channel
• RF frequency: 3.5 GHz
• Sampling frequency: 7.68 MSPS

Below, you can see a high-level diagram of the test setup above. Both Tx channels of a single XTRX share the same PLL and are inherently phase-synchronized (typically to within 3-4 degrees). This allows us to calibrate only one channel out of each channel pair (Tx1) and apply the same correction to the other channel (Tx2).

XYNC Phase Calibration

Phase Stability Test Description

1. The calibration signal is transmitted on the Tx1 port of each of the eight XTRX’s
2. The signal is a CW (sine) signal with LTE FFT tone spacing, with each XTRX transmitting at a different frequency
3. The signals are then combined and routed to the calibration receiver
4. The calibration receiver performs FFT on the data and measures phase offset per tone over multiple test runs
5. The phase offset for Ch 2-8 is calculated relative to a reference channel (Ch1)
6. The XTRX array is kept running throughout the test

Test Results

• Measured phase offsets are stable over long periods of time (in the range of ~2 hours)
• The standard deviation of each phase offset is small (from 5 to 7 degrees)

Phase Offsets Between XTRXs as a Function of Time