Anritsu MS2024B Manual page 35

VNA Measurements
To calculate cable loss, the far end of the cable is shorted, and the resulting S
measurement is divided by two (S
Smoothing can be applied to remove any ripples in the 1-port cable loss response. The ripples
that are seen in TR1 (Trace 1) are caused by the phase interactions of the large reflection at
the short at the end of the cable and the small reflection of the connector at the near end of
the cable. In normal cable use, the far end of the cable is terminated with some device, which
would eliminate the large reflection in the image with the short. To accurately measure the
loss in the cable, however, the signal must be fully reflected from the far end of the cable,
which is why a short or an open is used. The resulting undesired ripple can be removed by
using smoothing. A smoothing setting of 2% to 5% is usually sufficient to remove the ripples.
If you apply smoothing when the sweep has more than 2000 points, the smoothing
may slow the sweep time of the trace and the responsiveness of the instrument.
Note
This slowing can be significant, and it increases as the number of sweep points is
increased.
This particular cable shows (at MK2) unwanted dips (troughs) in the insertion loss frequency
sweep. The optional time domain or distance-to-fault measurement can reveal potential
causes for the poor frequency response of the cable.
In this example, a loose connector that is located 6 feet down the cable was causing a sizable
degradation in performance. Marker 1 shows the mismatch of the near end cable connector,
Marker 2 highlights the loose connection, and Marker 3 shows the full reflection of the short
at the end of the cable. (MK3 is not a full 0 dB reflection, but somewhat less. This can be
attributed to the loss of the cable and the previous smaller reflections at MK1 and MK2,
which both contribute to the reduced magnitude of the reflected signal at the far-end short.)
Vector Network Analyzer MG
/2) to compensate for the round-trip loss of the cable.
11
PN: 10580-00289 Rev. K
4-2 1-Port Cable Measurement
11
return loss
4-3