Agilent Spectrum Analysis Basics - Agilent Technologies
Agilent Spectrum Analysis Basics - Agilent Technologies
Agilent Spectrum Analysis Basics - Agilent Technologies
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Gated sweep<br />
Gated sweep, sometimes referred to as gated LO, is the final technique.<br />
In gated sweep mode, we control the voltage ramp produced by the scan<br />
generator to sweep the LO. This is shown in figure 2-41. When the gate is<br />
active, the LO ramps up in frequency like any spectrum analyzer. When the<br />
gate is blocked, the voltage out of the scan generator is frozen, and the LO<br />
stops rising in frequency. This technique can be much faster than gated video<br />
because multiple buckets can be measured during each burst. As an example,<br />
let’s use the same GSM signal described in the gated video discussion earlier<br />
in this chapter. Using a PSA Series spectrum analyzer, a standard, non-gated,<br />
spectrum sweep over a 1 MHz span takes 14.6 ms, as shown in Figure 2-42.<br />
With a gate length of 0.3 ms, the spectrum analyzer sweep must be built up<br />
in 49 gate intervals (14.6 divided by 0.3), or. If the full frame of the GSM<br />
signal is 4.615 ms, then the total measurement time is 49 intervals times<br />
4.615 ms = 226 ms. This represents a significant improvement in speed<br />
compared to the gated video technique which required 1.85 s for 401 data<br />
points. Gated sweep is available on the PSA Series spectrum analyzers.<br />
RF<br />
step<br />
attenuator<br />
Mixer<br />
IF resolution<br />
bandwidth<br />
filter<br />
IF log<br />
amplifier<br />
Envelope<br />
detector<br />
(IF to video)<br />
Video<br />
bandwidth<br />
filter<br />
Peak/sample<br />
detector<br />
Analog-digital<br />
converter<br />
RF<br />
input<br />
Display logic<br />
Local<br />
oscillator<br />
Gate<br />
control<br />
Scan generator<br />
Display<br />
Figure 2-41. In gated sweep mode, the LO sweeps only during gate interval<br />
Figure 2-42. <strong>Spectrum</strong> of the GSM signal<br />
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