Specifications

42
There are three common methods used to perform time gating:
• Gated FFT
• Gated video
• Gated sweep
Gated FFT
Some spectrum analyzers, such as the Agilent PSA Series, have built-in FFT
capabilities. In this mode, the data is acquired for an FFT starting at a chosen
delay following a trigger. The IF signal is digitized and captured for a time
period of 1.83 divided by resolution bandwidth. An FFT is computed based
on this data acquisition and the results are displayed as the spectrum. Thus,
the spectrum is that which existed at a particular time of known duration.
This is the fastest gating technique whenever the span is not wider than the
FFT maximum width, which for PSA is 10 MHz.
To get the maximum possible frequency resolution, choose the narrowest
available RBW whose capture time fits within the time period of interest.
That may not always be needed, however, and you could choose a wider RBW
with a corresponding narrower gate length. The minimum usable RBW in
gated FFT applications is always lower than the minimum usable RBW in
other gating techniques, because the IF must fully settle during the burst in
other techniques, which takes longer than 1.83 divided by RBW.
Gated video
Gated video is the analysis technique used in a number of spectrum
analyzers, including the Agilent 8560, 8590 and ESA Series. In this case,
the video voltage is switched off, or to “negative infinity decibels” during the
time the gate is supposed to be in its “blocked” mode. The detector is set to
peak detection. The sweep time must be set so that the gates occur at least
once per display point, or bucket, so that the peak detector is able to see
real data during that time interval. Otherwise, there will be trace points
with no data, resulting in an incomplete spectrum. Therefore, the minimum
sweep time is N display buckets times burst cycle time. For example, in GSM
measurements, the full frame lasts 4.615 ms. For an ESA spectrum analyzer
set to its default value of 401 display points, the minimum sweep time for
GSM gated video measurements would be 401 times 4.615 ms or 1.85 s. Some
TDMA formats have cycle times as large as 90 ms, resulting in long sweep
times using the gated video technique.
RF
input
RF
step
attenuator
Local
oscillator
Mixer
IF resolution
bandwidth
filter
IF log
amplifier
Envelope
detector
(IF to video)
Gate control
-$ dB
Scan generator
Display
Reset
Display logic
Analog-digital
converter
Peak/sample
detector
Video
bandwidth
filter
Figure 2-40. Block diagram of a spectrum analyzer with gated video