3-16
Making Time Domain Measurements
Time Domain Low Pass Mode
Minimum Allowable Stop Freque ncies
The lowest analyzer measurement frequency is 50 MHz, therefore for each value of n there
is a minimum allowable stop frequency that can be used. That is, the minimum stop
frequency = n × 50 MHz. Table 3-2 lists the minimum frequency range that can be used for
each value of n for low pass time domain measurements.
Reflection Measurements in Time Domain Low Pass Figure 3-12 shows the time domain response of an unterminated cable in both the low-pass
step and low-pass impulse modes.
Figure 3-12 Time Domain Low Pass Measurements of an Unterminated Cable
Interpreting the Low Pass Response Horizontal Axis
The low pass measurement horizontal axis is the two-way travel time to the discontinuity
(as in the bandpass mode). The marker displays both the two-way time and the electrical
length along the trace. To determine the actual physical length, enter the appropriate
velocity factor as described in "Time Domain Bandpass Mode" on page 3-12.
Interpreting the Low Pass Response Vertical Axis
The vertical axis depends on the chosen format. In the low pass mode, the frequency
domain data is taken at harmonically related frequencies and extrapolated to dc. Because
this results in the inverse Fourier transform having only a real part ( the im agina r y part i s
zero), the most useful low pass step mode format in this application is the real format. It
displays the response in reflection coefficient units. This mode is similar to the traditional
TDR response, which displays the reflected signal in a real format (volts) versus time (or
distance) on the horizontal axis.
51 50 MHz to 2.55 GHz 801 50 MHz to 40.05 GHz
Table3-2 Minimum Frequency Ranges for Time Domain Low Pass
Number of Points Minimum Frequency Range Number of Points Minimum Frequency Range