Spectrum Analysis Basics: Part 5 - Time Gating
2020-12-16 | 6 min read
Welcome back to the Spectrum Analysis Basics blog series! In part 4, I went over video filtering and trace averaging. This time, I’ll explain time gating. Time-gated spectrum analysis enables you to obtain spectral information about signals that occupy the same part of the frequency spectrum but differ in the time domain. This describes that two or more signals are in the same frequency channel that are active at different times. Using an external trigger signal to coordinate the separation of these signals, you can measure any one of the signals separated in time. You can also measure the spectrum of a signal in one time slot of a time division multiple access (TDMA) system and exclude the spectrum of interfering signals.
Time gating helps you perform measurements that are very difficult to make using traditional frequency-domain spectrum analysis. These measurements include pulsed radio frequency (RF) signals, time multiplexed signals, TDMA signals, and more.
Time Gating Methods
When making measurements with time gating, you need these four basic items:
- An externally supplied gate trigger signal
- A gate control or trigger mode (edge or level)
- A gate delay setting (determines how long after the trigger signal the gate becomes active)
- A gate length setting (determines how long the gate is on)
With these parameters, you can look at the signal during a desired time length. Sometimes, the desired signals will coincide with your periods of interest. If this is the case, you can use level gating, shown in Figure 1. Unfortunately, in most cases, the signal will not perfectly line up with the time you want. For these times, you can use edge triggering with a specified gate delay and gate length to define the time period in which to measure the signal.
Figure 1: Time gating using level triggering, where the spectrum analyzer only measures the frequency spectrum when the signal is above a certain level.
The most common methods to perform time gating are gated fast Fourier transform (FFT), gated local oscillator (LO), and gated video.
Using the built-in FFT capabilities in signal analyzers like Keysight’s X-Series, the signal analyzer will acquire data for an FFT starting after a chosen delay following a trigger. The analyzer calculates an FFT based on the data and displays the results on the spectrum, meaning you will see the spectrum that existed at a particular time of known duration. This is the fastest gating technique when the signal span is narrower than the FFT width.
Tip: to get the maximum possible frequency resolution, choose the narrowest available resolution bandwidth (RBW) with a capture time that fits within the time period of interest.
With LO gating, or gated sweep, you control the voltage ramp produced by a scan generator to sweep the LO, shown in Figure 2. Like any signal analyzer, the LO ramps up in frequency when the gate is active. When the gate is blocked, the analyzer freezes the voltage out of the scan generator, causing the LO to stop rising in frequency.
Figure 2: Block diagram of gated LO mode
Many spectrum analyzers feature the gated video analysis technique, including the Keysight ESA series. In this technique, the analyzer switches video voltage off during the time periods in which the gate is blocked. With the detector set to peak detection, you then set the sweep time so that the gates occur at least once per display point, or bucket. This allows the peak detector to see real data during that time interval. Otherwise, there will be trace points with no data, leaving an incomplete spectrum.
The gated video technique, shown in Figure 3, can lead to long sweep times. For example, in Global System for Mobile communications (GSM), a full frame lasts 4.615 ms. For an ESA spectrum analyzer set to its default value of 401 display points, the minimum sweep for GSM-gated video measurements comes out to 401 times 4.615 ms, or 1.85 s. Some TDMA formats have cycle times as long as 90 ms, leading to even longer sweep times.
Figure 3: Block diagram of a spectrum analyzer with gated video time gating.
When dealing with overlapping signals, time gating provides a method to analyze signals that may share a single frequency channel. Faster and more recent time gating methods like gated FFT and gated LO are available on Keysight X-Series signal analyzers. In Part 5 of Spectrum Analysis Basics, we will go over Digital IFs.
Find out more in Application Note 150.