X440: TX inter-channel phase alignment using RFNoC Replay

[Latifah2003]

Hello,

I am not sure whether this is expected behavior or a software issue, so I would appreciate some clarification.

Configuration

• USRP X440
• UHD 4.8.x
• FPGA image: X4_400
• RFNoC Replay
• Python
• TX0–TX3
• Center frequency: 1 GHz
• Sample rate: 368.64 MS/s
• 10 MHz complex tone
• Same waveform transmitted on all four TX channels

I removed timed tuning after reading the X440 documentation because timed tuning is not supported on X440.

Instead, I use a timed RFNoC Replay stream start (stream_now = False with a future time_spec) so that all Replay ports begin transmitting simultaneously.

Measurements

Using a Keysight oscilloscope with equal-length SMA cables, I consistently measure approximately:

• CH1–CH2 ≈ 4.4 ns
• CH1–CH3 ≈ 260–280 ps
• CH1–CH4 ≈ 975–980 ps

These delays appear to be repeatable across multiple application restarts.

Questions

• Is this expected behavior on a single X440?
• Does RFNoC Replay guarantee deterministic sample alignment only, while fixed RF path delays remain?
• Is there an official calibration procedure for TX phase alignment between channels?

If needed, I can provide my Python application, oscilloscope screenshots, and uhd_usrp_probe output.

Thank you.