HP 8530A, Руководство по эксплуатации и программированию, Страница: 239 / 439

Time Domain RCS Measurements
Figure 13-9. Gate Characteristics
T able 13-2.
Gate Shape Characteristics Using Dierent Window Settings
Gate
Shape
P assband
Ripple
Sidelobe
Levels
Cuto Time
T
2
= T
3
Minimum
Gate Span T
1
Minimum 60.40 dB 024 dB 0.6/f
span
1.2/f
span
Normal 60.04 dB 045 dB 1.4/f
span
2.8/f
span
Wide 60.02 dB 052 dB 4.0/f
span
8.0/f
span
Maximum 60.01 dB 080 dB 11.2/f
span
22.4/f
span
The P assband Ripple and Sidelobe Levels are descriptive of the gate (lter) shape . The Cuto
Time , T
2
= T
3
(see Figure 13-9 ), indicates how fast the gate lter rolls o. F or each gate shape ,
there is also a Minimum Gate Span (T
1min
= 2 x T
2
) which gives a lter passband of zero .
An easy way to calculate minimum Gate Span is shown in the far right column in T able 13-2 .
F or example , if you are using Normal Gate Shape , minimum Gate Span = 2.8 4 frequency span.
Entering a Gate span that is smaller than minimum will produce a distorted lter shape which:
Will have no passband.
Will have a wider shape .
May have higher sidelobe levels .
Will give an incorrect indication of gate Start and Stop times .
Therefore , you should always select a Gate Span that is higher than the minimum value . The
cuto time and the minimum Gate Span are inversely proportional to the frequency span of the
measurement as indicated in T able 13-2 .
F or best results using Gating, you should:
Always center the Gate around the response (or responses) that you want to retain in the
measurement.
Make the Gate Span wide enough to include all of those responses .
13-14 Introduction to Time Domain RCS and
Antenna Measurements