S parameter data are very useful to estimate the insertion loss and return loss. Many applications require S-parameter measurements between ASIC pads and SMA connector.
Engineers may consider using differential probe without any ground pin to capture mixed- mode S parameters and differential-mode impedance.
S Parameter Measurement Tips
- Use the the calibration substrate and probe provided by the same vendor to avoid calibration error
- Always check the calibration using short transmission line on calibration substrate
- Probing structure designs should be included in the PCB or package final release check list. Too many designers ignore this requirement and can't extract S parameter in lab due to missing adjacent ground pin or missing deemed structures.
- A missing ground pad at the ASIC area can prevent S-parameter measurement.
- Add simple through structure as a validation coupon to verify the loss effect of the stack up material.
- The phase value of a pair of differential traces provides useful information to detect the resonance beyond 7 GHz due to bad FR4 material.
S Parameter Probing Demonstrations
S-parameter calibration with substrate and VNA
Parameter measurement on high-speed trace with VNA