In the lab, the VNA (Vector Network Analyzer) is used to measure alignment of multiple RF-DACs. Multiple RF-DAC channels are synchronized and output CW signals at the same frequency, VNA is configured in receiver mode, to capture the input CW signal and calculate the phase difference between ports in frequency domain. The results are showed as phase difference between one reference channel versus all other channels. For ADC, the same theory applies, CW signals from a multi-ports power splitter were fed into multiple RF-ADC channels, all these RF-ADC channels have been synchronized, data are captured at the same time, then FFT analysis will give the phase difference on each channel versus a reference channel.
Based on previous discussion in this application notes, the PCB should be well designed to match the trace of each converter channels and related clock signals, the ZCU1275 V2.0 verification board is used for measurements, other accessories such as cables, power splitter are also carefully picked up to avoid misalignment. In the testing the baluns are not used to avoid introducing potential phase imbalance. For both ADCs and DACs, only P legs were used for signal inputs and outputs, the N legs were 50Ω terminated. The following figure illustrates the set up.
A capture of VNA is showed as following for example, the phase difference of three RF-DAC channels versus one reference channel are measured.
