Transformer-Coupled Loopback Test for Differential Mixed-Signal Specifications

Loopback tests for a differential mixed-signal device under test (DUT) have rarely been attempted, since any imbalance introduced by a design for test (DfT) circuitry on differential signaling delivers an imperfect sinusoidal wave to the DUT input, thereby degrading the DUT performance. In addition, this methodology inherently suffers from fault masking. These problems result in low test accuracy and serious yield loss. This paper presents a novel methodology for efficient prediction of individual DUT dynamic performance parameters with a radio-frequency (RF) transformer in loopback mode to overcome the imbalance problem of DfT circuitry. Cascaded RF transformer in loopback mode produces differently weighted loopback responses, which are used to characterize the DUT dynamic performance. Hardware measurement results show that this approach can be effectively used to predict the specifications of a DUT.