How to Generate Multi-Channel Phase-Stable and Phase-Coherent Signals
2019-04-10 | 5 min read
Testing multi-antenna systems, such as spatial diversity, Multi-input Multi-output (MIMO), and beamforming, requires a test system capable of providing multiple signals and a constant phase relationship between the signals.
In an earlier post, “What Timing Synchronization Means for Your Multi-Channel RF Signal Generation”, I discussed the importance of accurate triggering among the instruments to ensure that all measurements begin at the right time. However, potential phase errors may result from timing synchronization test systems. Let’s take a look at these errors and learn how to minimize them.
Phase-Stable Multi-Channel Signal Generation
A commercial signal generator has an independent synthesizer to upconvert an IF signal to an RF signal. Using commercial signal generators to simulate multi-channel RF systems may result in errors even with a common reference clock, such as phase draft and uncorrelated phase noise. Let’s take a two-channel RF system as an example.
The signal generators have separate oscillators, each with their own phase-locked loops (PLL). Figure 1 shows two signal generators with baseband generators synchronized using a triggering signal and a common 10 MHz time base. This results in phase drift between the signal generators, as shown to the right of Figure 1. Most of the time, PLL lock the phase drift within the constraints of the loop bandwidth (PLL’s loop filter). However, PLL cannot completely track out higher order responses.
Figure 1. Phase drift between two time-synchronized signal generators
In MIMO test systems, slow phase drift between channels is less of an issue, so test channels that share a common frequency reference may deliver acceptable performance.
Uncorrelated Phase Noise
Uncorrelated phase noise contributes to phase error between reference-locked signal generators. Inside the loop bandwidth of PLL, the frequency reference has the most impact on phase noise performance. Outside the loop bandwidth, the PLL’s oscillator determines the phase noise.
Using high-quality stable references and instruments with low phase noise can improve phase drift and phase error. Applications such as MIMO and spatial diversity can use these “phase-stable” multi-channel signals for testing. However, for precise component characteristics testing, using a common local oscillator (LO) remains appropriate in order to achieve the best performance.
Phase-Coherent Multi-Channel Signal Generation
Share a Common LO to Achieve Phase Coherence
To minimize the sources of coherency errors, use a common LO for multiple signal generators. Figure 2 represents two MXG N5182B vector signal generators configured for a phase-coherent test system. The system takes the LO of the top signal generator, then splits it and uses it as the LO input (red lines) for both signal generators. With this configuration, the RF paths of the two signal generators are fully coherent.
Figure 2. Setup for two phase-coherent RF channels with a common LO
Time and Phase Skew
Even if you are using a shared LO, you will still encounter some static time and phase skew between instrument channels. Cable lengths and connectors cause static time and phase variations. For multi-antenna systems, such as MIMO, a receiver’s adaptive equalizer can remove these linear errors by dynamically creating and applying a finite impulse response (FIR) compensating filter. The skews have a lower impact on receiver sensitivity tests.
However, for multi-channel component characterization and beamforming test systems, you need to perform highly phase-aligned and phase-controllable multi-channel signal generation. You must correct these skews and ensure the measured differences come from the device under test, and not from the test system. Learn about how to correct these skews and adjust phase differences between multiple channels in my next post.
See related posts to learn about timing synchronization and phase coherence: