Page 26

EETE MAY 2015

Test & Measurement Fig. 2: PXI 10 MHz clock for backplane time synchronization. Fig. 3: Multi-channel phase-coherent sources with a common Achieving a multi-channel phase-coherent test system Phase-coherent channels are one key element of a multi-channel test system. Two signals are said to be coherent if they have a constant relative phase at all instances in time: There are several levels of synchronization that need to take place to achieve a fully phase-coherent system: synchronization of clocks; channel-to-channel time and phase synchronization This may seem simple, but is actually quite difficult to achieve. Modular PXI platforms are ideal for multi-channel synchronized systems due to the scalability, size and precise synchronization that can be achieved . Signals can be time aligned using a locked reference signal. As shown in figure 2, the PXI 10 MHz backplane clock is used to align and start all actions at the same time. Clock synchronization ensures that the waveform playback or waveform capture starts synchronously. The common reference clock provides some level of time synchronization but not phase synchronization. For phase alignment, the signals must have a constant relative phase at all instances in time, then coherence is a statistical property between signals. To achieve true phase-coherent measurements, a common local oscillator (LO) can be shared across source or analyzer channels so that all channels share the same phase properties. In figure 3, a single master synthesizer is used to enable a stable phase relationship. In this example, the synthesizer used in the Keysight M9381A PXI vector signal generator exposes 4 LO connections that can be used with up to 4 different VSG modulators to achieve phase synchronization. For more than 4 channels, a LO distribution network can be used to split and amplify the common LO across the multiple chassis. Calibration of the phase-coherent system The offsets in magnitude and phase between channels can greatly impact multi-channel performance. Without calibration, synthesizer. there will be degraded results in the beamforming signals and the measurement results will be in question. Even when using a shared LO, some static time and phase skew will exist between instrument channels. In addition, there will be fixed magnitude and phase offsets due to cables, connectors, and signal conditioning. Compensating for these offsets will ensure that any measured differences are due to the device under test and not the test equipment. A common calibration technique for a traditional multiplechannel source uses a multi-channel oscilloscope to measure the channel-to-channel performance of the signal generating system as shown in figure 4. This is typically a manual process and requires multiple steps to calculate the channel-to-channel delay at multiple frequencies, which is time consuming and expensive. A new alternative method is to perform source channel-tochannel calibration using Keysight’s patent- pending automated routine that uses one signal analyzer to extract the signal data and calculate the timing and phase skew between each channel. In this case, a known reference signal is played on each source and fed through a 4-way passive power combiner. The combined signal is then fed to one signal analyzer for analysis. With the automated routine the corrections are calculated, stored, and, ready to be applied during the multichannel signal generation. With this method, only one analyzer is required and the automated routine makes it easier, faster, and saves cost. A similar automated routine is used for correction of the multi-channel signal analysis system as shown in figure 4. Fig. 4: Multi-channel phase-coherent calibration techniques. 26 Electronic Engineering Times Europe May 2015 www.electronics-eetimes.com


EETE MAY 2015
To see the actual publication please follow the link above