3-19
Making Time Domain Measurements
Time Domain Low Pass Mode
Figure 3-14 Low Pass Step Measurements of Common Cable Faults
(Real Format)
Transmission Measurements in Time Domain Low Pass
Measuring Small Signal Transient Response Using Low Pass Step
Use the low pass mode to analyze the test device’s small signal transient response. The
transmission response of a device to a step input is often measured at lower frequencies,
using a function generator (to provide the step to the test device) and a sampling
oscilloscope (to analyze the test device output response). The low pass step mode extends
the frequency range of this type of measurement to the maximum frequency of the
analyzer.
The step input shown in Figure 3-15 is the inverse Fourier transform of the frequency
domain response of a thru measured at calibration. The step rise time is proportional to
the highest frequency in the frequency domain sweep; the higher the frequency, the faster
the rise time. The frequency sweep in Figure 3-15 is from 10 MHz to 1 GHz.
Figure 3-15 also illustrates the time domain low pass response of an amplifier under test.
The average group delay over the measurement frequency range is the difference in time
between the step and the amplifier response. This time domain response simulates an
oscilloscope measurement of the amplifier’s small signal transient response. Note the
ringing in the amplifier response that indicates an under-damped design.
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