China Electronics Technology Instruments AV3672 Series, User Manual

Page: 344 / 424

Appendix
Appendix 2 Time Domain Measurement
340
2.3 Window Filtering
Affected by the side lobe and pulse width, the time resolution of time domain transformation is limited to a certain
degree. The window filtering function can be applied to reduce the influence of such factor. The analyzer has the
window function for time domain transformation to distinguish various responses more effectively. This function
can be used to change the response pulse width, side lobe level and step response rising time.
●
Window filtering
···································································································
378
●
Correct setting of window value
··················································································
379
●
Window characteristics
····························································································
379
2.3.1 Window Filtering
2.3.1.1 Concept of Window Filtering
Sudden changes may occur to the start and stop frequency in the time domain measurement. This may cause over-
travel and ringing in the time domain step response. The window filtering function can be applied to reduce the
influence of sudden changes of the frequency domain. The network analyzer also has the following disadvantages:
1) Pulse width of impact response or rising time of step response: caused by the band limits of the system,
limiting the capability of the analyzer to distinguish two adjacent response signals. The pulse width is inversely
proportional to the frequency span of measurement. The frequency span should be increased to reduce the pulse
width.
2) Side lobe of impact response: caused by sudden changes of the stop frequency. The adjacent high-level
response side lobe may result in hiding of low-level response signal, limiting the dynamic range of time domain
measurement. The window filtering function can be used to inhibit the side lobe, as shown in Fig. 2.7.
Fig. 2.7 Side Lobe Suppression by Window Filtering
2.3.1.2 Advantages of Window Filtering
1) The narrow window can be applied to reduce the pulse width of impact response, improve the response
resolution and better distinguish two adjacent responses.
2) The wide window can be applied to reduce the side lobe level of the impact response, improve the dynamic
range and better measure the low-level response.
2.3.1.3 Influence of Window Filtering
Fig. 2.8 Influence of Window on Response Signal
As shown in Fig. 2.8, the window filtering function of the analyzer for frequency domain measurement data has the
following influence on the time domain measurement response of the DUT:
1) It can influence the side lobe level and pulse width and improve the measurement resolution in the impact
transformation mode.