I am using the Ettus B200 SDR to design and implement a Stepped Frequency Continuous Wave (SFCW) radar system for ground-penetrating radar (GPR) applications. While the system has shown promise, I am encountering persistent issues with the IQ signals that I am unable to resolve despite extensive efforts.
Problem Description:
1. IQ Signal Issues:
The received IQ signals often exhibit inconsistent behavior. Sometimes the I and Q signals appear inverted, and other times there is a phase shift between them.
This phase shift is not constant over time, making it extremely challenging to compensate for it during signal processing.
2. Synchronization Attempts:
I have attempted to synchronize the transmit (Tx) and receive (Rx) channels using various methods, including a time trigger. Despite these efforts, the phase shift between the IQ signals remains variable and unpredictable.
3. External Clock Usage:
To address potential clock drift or inconsistencies, I connected an external frequency reference to the RF clock input of the B200. Unfortunately, this did not resolve the issue, and the variable phase shift persists.
Troubleshooting Steps Taken:
Verified proper configuration of the USRP device using LabVIEW and the NI-USRP driver.
Checked the IQ imbalance and scaling in the received signal processing pipeline.
Used a direct Tx-to-Rx connection via a cable to isolate hardware-related issues, yet the phase inconsistency remains.
Attempted phase compensation techniques in software but found it impractical due to the non-constant nature of the phase shift.
Application Context:
This system is intended for an SFCW radar application, where accurate phase difference measurement between the transmitted and received signals is critical for distance and reflectivity calculations. The variable phase shift between the IQ signals significantly affects the accuracy of the radar system.
Request for Assistance:
I kindly request guidance on the following points:
1. Could this issue stem from internal clocking or LO mismatches within the B200?
2. Are there specific settings or configurations I should adjust to stabilize the IQ phase relationship?
3. What additional diagnostics or steps can I take to isolate and resolve the issue.
