How To Prevent Undersampling - Keysight Technologies C Series User Manual

Fieldfox analyzers

TDR (Time Domain Reflectometry) - Option 215

TDR Measurement Settings

An un-terminated cable (with NO perfect load at the end) will show faults that

appear to be beyond the end of the cable. These are NOT alias faults. These

faults appear as the signal reflects off the open at the end of the cable and

travels back down the cable toward the connection at the FieldFox.

Re-reflections are measured at the FieldFox as mirror images of the original

faults. The largest fault is the open end of the cable. To avoid confusion, set

the Stop distance shortly after that fault.

How to Prevent Undersampling

Undersampling is when the TDR measurement is inaccurate because the

frequency step size (resolution) is too large. The predominant symptom of this

is when the measured impedance is incorrect. For example, when measuring a

75Ω cable, a marker that is on a part of the trace without reflections shows

To prevent undersampling, set TDR Sweep to Cable. If undersampling still

occurs, set TDR Sweep to Auto. Then set Auto Max Frequency to a lower

value. Manually increase the Resolution so that the Start Frequency setting is

less than about 500 kHz.

Keysight N9915-90020 User's Guide