3-14
Making Time Domain Measurements
Time Domain Bandpass Mode
Transmission Measurements Using Bandpass Mode
The bandpass mode can also transform transmission measurements to the time domain.
For example, this mode can provide information about a surface acoustic wave (SAW) filter
that is not apparent in the frequency domain. Figure 3-11 illustrates a time domain
bandpass measurement of a 321 MHz SAW filter.
Figure 3-11 Transmission Measurement in Time Domain Bandpass Mode
Interpreting the Bandpass Transmission Response Horizontal Axis
In time domain transmission measurements, the horizontal axis is displayed in units of
time. The time axis indicates the propagation delay through the device. Note that in time
domain transmission measurements, the value displayed is the actual delay (not twice the
delay). The marker provides the propagation delay in both time and distance.
Marker 2 in Figure 3-11 (left) indicates the main path response through the test device,
which has a propagation delay of 655.6 ns, or about 196.5 meters in electrical length.
Marker 4 in Figure 3-11 (right) indicates the triple-travel path response at 1.91 µs, or
about 573.5 meters. The response at marker 1 (at 0 seconds) is an RF feedthrough leakage
path. In addition to the triple travel path response, there are several other multi-path
responses through the test device, which are inherent in the design of a SAW filter.
Interpreting the Bandpass Transmission Response Vertical Axis
In the log magnitude format, the vertical axis displays the transmission loss or gain in dB;
in the linear magnitude format it displays the transmission coefficient τ. Think of this as
an average of the transmission response over the measurement frequency range.
Table 3-1 Time Domain Reflection Formats
Format Parameter
LIN MAG
REAL
LOG MAG
SWR
Reflection Coefficient (unitless) (0 < ρ < 1)
Reflection Coefficient (unitless) (−1 < ρ < 1)
Return Loss (dB)
Standing Wave Ratio (unitless)
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