Technical Insights > RF + Microwave

Why RTSA Is the Best When it Comes to Interference Hunting in the Field

2019-11-26  |  6 min read 

Interference is an inherent side effect of new technology deployments using wireless communications. In a society that is moving (quickly) toward all-wireless technology, it is pertinent to detect, troubleshoot, and mitigate interference for success. Traditional spectrum analysis techniques are great until you have to hunt down an elusive, intermittent signal. This blog explains where traditional spectrum analysis falls short and where real-time spectrum analysis (RTSA) exceeds in interference hunting in the field.

First, we’re going to compare two traditional spectrum analyzers: swept-tuned and snapshot FFT.

Swept-tuned analyzers – Swept-tuned analyzers, also called superheterodyne analyzers, have a ramp generator that sweeps the local oscillator (LO) over the frequency range of interest using a fixed resolution bandwidth (RBW) filter. They have fast sweep times over wide frequency spans with great dynamic range and sensitivity, plus precise sweep time control. However, when the RBW to span ratio is too small, the analyzer sweeps very slowly.

Because the swept-tuned analyzer sweeps over the frequency range with a set RBW and displays only one certain viewpoint of the signal at a particular time, it misses signals outside of the viewpoint it sweeps over. Because it can miss information, the swept-tuned analyzer is not the best choice for interference hunting. Figure 1 depicts this loss of information.

A swept-tuned analyzer misses data events that occur outside the fixed RBW filter as it sweeps over a frequency range
Figure 1. A swept-tuned analyzer misses data events that occur outside the fixed RBW filter as it sweeps over a frequency range

Snapshot FFT analyzers – Snapshot FFT (fast Fourier transform) analyzers capture complete blocks of time-domain samples and compute the frequency domain response from those samples. Unlike a swept-tuned analyzer that sweeps over a frequency range, a snapshot FFT analyzer steps through the frequency span one block at a time. It captures all of the signal content within the information bandwidth of the block, it allows for digital demodulation after capture, and it has a fast update speed at narrow RBWs.

If a snapshot FFT analyzer does not have high-speed processing, its calculation and processing time limit its ability to capture intermittent signals. The analyzer captures the data then converts it. The time that passes while the instrument is calculating and not acquiring data is called dead time. If the intermittent signal occurs during its dead time, then the snapshot FFT analyzer will not capture it. Figure 2 shows this information loss. Because of this dead time, snapshot FFT analyzers are not fit for interference hunting.

An FFT-based analyzer misses events that occur between time data sample blocks
Figure 2. An FFT-based analyzer misses events that occur between time data sample blocks

So, how do real-time spectrum analyzers compare? I’m glad you asked!

Real-time spectrum analyzers – Real-time spectrum analyzers, also called gap-free FFT analyzers, are architecturally similar to snapshot FFT analyzers but they use a circular memory buffer for full-signal capture without dead time. As long as the FFT engine keeps up with the buffer, it does not miss the signal. In RTSA, the analyzer samples, processes, and parallelly computes the data at the same time. This makes RTSA-capable analyzers the best choice for interference hunting.

Figure 3 shows an example of Keysight’s FieldFox capturing a signal using traditional spectrum analyzer mode. In the figure, you can see the dead time between each update. Alternatively, Figure 4 shows an example of the same signal captured by FieldFox in RTSA mode. You can see that all of the data information is represented, and no gaps are present.

A signal captured using traditional spectrum analysis mode with dead time between updates
Figure 3. A signal captured using traditional spectrum analysis mode with dead time between updates
A signal captured using RTSA mode with gap-free results
Figure 4. A signal captured using RTSA mode with gap-free results

To find out more about sources of signal interference and the advantages and disadvantages of the analyzers discussed above, check out this whitepaper.

Keysight’s FieldFox is the industry’s most integrated handheld analyzer, configurable to be a cable and antenna tester, a network analyzer, a spectrum analyzer, a signal source, and more. With a maximum real-time spectrum analysis bandwidth of 100 MHz, FieldFox can identify  5G control channels, capture signals as narrow as 5.52 us with 100 percent probability of intercept, and detect signals as narrow as 47 ns.

Visit the FieldFox webpage to find out more.