Specifications

43
Gated sweep
Gated sweep, sometimes referred to as gated LO, is the final technique.
In gated sweep mode, we control the voltage ramp produced by the scan
generator to sweep the LO. This is shown in figure 2-41. When the gate is
active, the LO ramps up in frequency like any spectrum analyzer. When the
gate is blocked, the voltage out of the scan generator is frozen, and the LO
stops rising in frequency. This technique can be much faster than gated video
because multiple buckets can be measured during each burst. As an example,
let’s use the same GSM signal described in the gated video discussion earlier
in this chapter. Using a PSA Series spectrum analyzer, a standard, non-gated,
spectrum sweep over a 1 MHz span takes 14.6 ms, as shown in Figure 2-42.
With a gate length of 0.3 ms, the spectrum analyzer sweep must be built up
in 49 gate intervals (14.6 divided by 0.3), or. If the full frame of the GSM
signal is 4.615 ms, then the total measurement time is 49 intervals times
4.615 ms = 226 ms. This represents a significant improvement in speed
compared to the gated video technique which required 1.85 s for 401 data
points. Gated sweep is available on the PSA Series spectrum analyzers.
RF
input
RF
step
attenuator
Local
oscillator
Mixer
Gate
control
IF resolution
bandwidth
filter
IF log
amplifier
Envelope
detector
(IF to video)
Scan generator
Display
Display logic
Analog-digital
converter
Peak/sample
detector
Video
bandwidth
filter
Figure 2-41. In gated sweep mode, the LO sweeps only during gate interval
Figure 2-42. Spectrum of the GSM signal