Differential Phase Skew Verification Using High Bandwidth Time Domain Reflectometry
High bandwidth TDR isolates sub picosecond differential phase skew on test coupons to prevent glass weave induced mode conversion in high speed channels.
High Bandwidth Probing
Signal Interface
Physical measurements of sub-nanosecond transition times require high-fidelity measurement nodes that minimize electrical distortion. Employing high bandwidth probing preserves signal integrity during oscilloscope measurements of multi-gigabit serial links. This measurement method ensures that the fast rising edges of signals are not rounded off by the test interface.
Loading Effect
Input capacitance must remain extremely low to prevent the measurement system from detuning the circuit under test. When using high bandwidth probing, the probe tip usually presents less than one picofarad of loading, which protects the circuit from impedance changes. A high-impedance tip ensures that current drawn by the tester is minimal, keeping the trace voltage near its unprobed state.
Excessive tip capacitance can otherwise mask signal integrity issues by artificially damping reflection waveforms and trace ringing.
Calibration Routine
De-embedding routines remove the residual transfer function of the measurement cables and probe tips from the raw oscilloscope data. Accurate high bandwidth probing relies on measuring a calibration substrate to establish the probe response before executing board-level tests. This procedure measures known standards such as open, short, load, and transmission standards to calculate the correction filters.
Applying these digital filters to the measured waveforms exposes the true electrical activity occurring at the probe contact point.