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Time Domain Gate Filtering of Vector Network Analyzer (Part II)

The time domain measurement section is divided into two parts (Part I). In this part, we will continue to introduce the Gate Form .

Similar to window wave filter, the gate also has corresponding wave filter form and Measure can be optimized by selecting reasonable wave filter form:

(1) Minimum gate: the wave filter has highest sidelobe level, with sharpest edge falling and maximum band-pass ripples.

(2) Maximum gate: the wave filter has lowest sidelobe level, with slowest edge falling and minimum pass band ripples.

1. Passband Ripples of the Gate

The gate is a pass band (or band-stop) wave filter on time domain and each form of wave filter has different filter property. Figure below indicates band-pass ripples of four filters in different forms and the scale in the figure is 0.5dB/grid. It can be seen from the figure that the minimum gate has the largest band-pass ripples.

Fig.1 Band-Pass ripples of different gate wave filters

2. Sidelobe Level of the Gate

Four wave filters of different forms have different sidelobe levels, in actual Measure, there must be a balance between lower sidelobe level and faster cut-off ratio.

(1) The minimum gate is with highest sidelobe level and fasted cut-off speed, which is very applicable to filtering undesired interference near the response.

(2) The maximum gate is with lowest sidelobe level, slowest cut-off speed, widest gate band-pass and maximum attenuation outside the gate band.

Fig.2 Sidelobe level of different gates

3. Property Index of the Gate

Different forms of gate have different properties and the properties are defined by index below:

(1) Pass band ripples and sidelobe level: to descript the gate form.

(2) 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.

(3) Minimum gate span: it equals to end time minus start time and it is twice of the cut-off time.

Table 1 Property Index of Different Gates

 Gate Form Pass Band Ripple Sidelobe Level Cut-off Time Min. Gate Span Min. ±0.10dB -48dB 1.4/frequency span 2.8/frequency span Standard ±0.10dB -68dB 2.8/frequency span 5.6/frequency span Width ±0.10dB -57dB 4.4/frequency span 8.8/frequency span Max. ±0.10dB -70dB 12.7/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:

(1) The gate form is distorted, without pass band.

(2) Indication of start and end time is incorrect.

(3)  Sidelobe level would be increased.