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.
TDR Step Generator Rise Time
Resolution Parameter
Transition speed of a voltage pulse determines the spatial resolution of impedance measurements in high speed transmission lines. The tdr step generator rise time defines the smallest distance between two discontinuities that the instrument can distinguish. Shorter rise times allow for the detection of smaller features like vias and connectors.
Discontinuity Detection
Pulse width and edge speed govern the frequency content of the signal sent into the device under test. A slow edge will filter out high frequency reflections, making a small pad or a thin trace look like a uniform impedance. Fast edges reveal the parasitic capacitance and inductance that cause signal reflections at every transition.
System Bandwidth
Practical resolution is limited by the combined performance of the generator and the probe assembly. As the pulse travels through the cable, the high frequency components are attenuated, which effectively slows down the signal before it reaches the board. High quality coaxial cables and low loss probes are necessary to preserve the fast edge provided by the generator.
The effective rise time at the probe tip is the square root of the sum of the squares of the individual component speeds. Measurements of ultra-fast signals require the source to be faster than the expected signal transition on the board.