How to Perform Multi-Channel Timing and Phase Alignment
2019-04-18 | 7 min read
In an earlier post, “How to Generate Multi-Channel Phase-Stable and Phase-Coherent Signals”, I discussed a test system capable of providing multiple signals and a constant phase relationship between the signals for testing multi-antenna systems. However, for specific applications such as component characterization and beamforming test systems, you need to perform highly phase-aligned and phase-controllable multi-channel signal generation. Let’s take a look at the phase and time skews and how to adjust them for your test applications.
Why Phase-Aligned and Phase-Controllable Signals Are Important
Coherently-driven antennas with the appropriate phase delay between antenna elements can form signal beams. Phased array antennas use phase shifters in the beamforming network (BFN) to produce a uniform wave front traveling in a specific direction. The uniform wave front allows a group of low directivity antenna elements to behave like a highly directional antenna for either transmit or receive applications.
Figure 1 illustrates the impact of using multiple antenna elements at a specific spacing. As you increase the number of antenna elements (a half wavelength separation), the antenna beamwidth gets narrower (Figure 1a to 1d). By applying a 90-degree phase shift to the signal at each antenna, you can change the direction of the beam as shown in Figure 1e. By changing phase shifts between elements in different amounts, you can steer the beam in a range of directions.
To simulate such phased array test signals, you need to precisely control the phase difference between the channels for both transmitter and receiver tests.
Figure 6. Antenna pattern vs. the number of antenna elements and phase shifts between elements
Timing and Phase Skew Correction
When configuring multiple signal generators with a common local oscillator (LO), the external cable lengths, connectors, and signal conditioning (amplifiers or attenuators) cause static timing and phase skew between the channels. The delays or phase shift skews the phase relationship between the channels. You need to correct these offsets and ensure the measured differences are from the device under test, and not from the test system.
Figure 2. Remove the time and phase difference between the channels
Direct Channel to Channel Alignment
To measure the alignment of multiple vector signal generators (VSGs), you can use a wide bandwidth oscilloscope. This technique produces the best results because the channel to channel alignment of oscilloscopes is very good.
For time delay, you can adjust baseband “I/Q Delay” to align the channel timing; for phase shift, you can adjust baseband “I/Q Phase”. “I/Q Phase” rotates the relative I/Q plane so that you can align the phase of RF output signals.
Cross-Channel Measurements with Vector Signal Analysis Tools
Another technique is to use an oscilloscope and Keysight 89600 vector signal analysis software and measure the time delay and phase shift. You can play the same wideband modulated waveform on all VSG channels, and use AM/AM cross-channel measurement to calculate the time delay. Measure the phase shift by playing a waveform that has continuous wave (CW) frequency components, such as a multi-tone waveform, and using the cross-channel frequency response’s phase component to measure the relative phase at the tone.
Simplify Correction Setup with a Single Channel Vector Signal Analyzer
Keysight provides a correction utility that measures the timing and phase difference between multiple VSG channels using a combiner with output fed into a single channel VSA as shown in Figure 3. Keysight has a patent-pending solution that extracts the signal data and calculates timing and skew between each channel. You need to specify desired frequencies, waveform sample rates, and amplitudes in order to perform the corrections. When you change any of these three parameters, you need to redo the correction process.
Figure 3. Multiple VSGs’ timing alignments with single VSA
Figure 4 shows a four-channel LTE-Advanced beamforming antenna pattern with/without corrections. The main lobe appears deformed to the left of Figure 4 and the side lobes are larger prior to applying the corrections. To the right of Figure 4, the main beam is much more prominent after applying the correction.
Figure 4. The phase shift impacts on antenna beam
A best practice is that all cables from the splitter output to the instrument inputs have equal lengths. All test equipment requires a warm-up period to ensure accurate performance.
Tips for True Phase-Coherent Signal Generation
To simulate the multichannel test signals, ensure that the phase between test signals is coherent and controllable. To summarize, here are some tips for achieving true phase-coherent multi-channel signal generation:
- Use the same model signal generator and share a common LO
- Use phase stable and low phase noise LO
- Use reference clock synchronization and triggering to ensure all measurements begin at the right time
- Calibrate any time and phase skew between VSG channels
- Calibrate any losses and mismatches due to cables and connectors
See earlier posts to learn about timing synchronization and phase coherence: