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Fig.8-12 Sidelobe Level of Different Gates
8.4.3.3.
Property index of the gate
Different forms of gate have different properties and the properties are defined by index below:
Pass band ripples and sidelobe level: to descript the gate form.
Cut-off time: it is the time between end time of the gate (at -6dB of the filter edge) and the first sidelobe peak point.
Minimum gate span: it equals to end time minus start time and it is twice of the cut-off time.
Table 8-1 Property Index of Different Gates
Gate Form
Pass Band Ripple
Sidelobe Level
Cut-off Time
Min. Gate Span
Min.
Standard
Width
Max.
±0.10dB
±0.01dB
±0.01dB
±0.01dB
-48dB
-68 dB
-57 dB
-70 dB
1.4/frequency span
2.8/frequency span
4.4/frequency span
12.7/frequency span
2.8/frequency span
5.6/frequency span
8.8/frequency span
25.4/frequency span
Each form of gate has a recommended minimum gate span to operate correctly, which is determined by limited cut-off ratio of the
gate. If the gate span set is smaller than this minimum value, the Analyzer will produce effects below:
The gate form is distorted, without pass band.
Indication of start and end time is incorrect.
Sidelobe level would be increased.
8.5.
Time Domain Measure Data
This section will provide examples of various Measure types to facilitate understanding of time domain response of the measured
piece.
8.5.1.
Screening
Screening will affect the response observed during time domain Measure; therefore, when explaining time domain Measure data,
what is screening and its effect toward Measure result must be known clearly.
Screening occurs when discontinuous points or loss nearest to reference plane affects response of each discontinuous point.
Energy reflected or absorbed by the first discontinuous point won’t approach the subsequent discontinuous points.
Response of each subsequent discontinuous point is smaller than that without screening.
8.5.1.1.
Screening caused by energy reflection
With serious mismatch, screening by energy reflection happens, examples below illustrate such phenomenon:
When a section of 50Ω transmission line is connected with a section of 25Ω air line and connected with another 50Ω
