3- 13
Making Time Domain Measurements
Time Domain Bandpass Mode
Figure 3-10 A Reflection Measurement of Two Cables
The ripples in reflection coefficient versus frequency in the frequency domain measurement are caused by
the reflections at each connector "beating" against each other.
One at a time, loosen the connectors at each end of the cable and observe the response in both the
frequency domain and the time domain. The frequency domain ripples increase as each connector is
loosened, corresponding to a larger reflection adding in and out of phase with the other reflections. The time
domain responses increase as you loosen the connector that corresponds to each response.
Interpreting the Bandpass Reflection Response Horizontal Axis
In bandpass reflection measurements, the horizontal axis represents the time it takes for an impulse
launched at the test port to reach a discontinuity and return to the test port (the two-way travel time). In
Figure 3-10, each connector is a discontinuity.
Interpreting the Bandpass Reflection Response Vertical Axis
The quantity displayed on the vertical axis depends on the selected format. The common formats are listed
in
Tab le 3-1. The default format is LOG MAG (logarithmic magnitude), which displays the return loss in
decibels (dB). LIN MAG (linear magnitude) is a format that displays the response as reflection coefficient .
This can be thought of as an average reflection coefficient of the discontinuity over the frequency range of
the measurement. Use the REAL format only in low pass mode.
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