Sometimes finding a hidden signal can be very challenging, especially in cases where the signal is deliberately hidden in a way that makes it difficult to detect. High-performance spectrum analyzers help users detect and locate hidden signals. Their low phase noise and wide dynamic range are especially important when searching for traditional hidden signals, also known as bugs.
One common way to hide a traditional bug is to place it near a strong carrier, such as a broadcast AM signal. There is a narrow range of frequencies just above and below the carrier that has no modulation. See Figure 1. This particular station had a serious hum problem and the 60 and 120 Hz hum sidebands are plainly visible. Look between the carrier at the left edge of the screen and the 60 Hz sideband. This is a good place to hide with noise about 80 dB below the carrier. To be able to see this area, a spectrum analyzer needs at least 90 dB dynamic range, a 1 Hz resolution bandwidth selection and phase noise at least -90 dBc/Hz at 30 Hz offset.
Figure 1. AM signal showing hum sidebands
There are multiple trace display modes and several detectors that can be combined in useful ways to help detect signals. In Anritsu spectrum handheld spectrum analyzers, you can set a trace to normal, maximum hold, minimum hold or average. Detectors include a peak detector, a minimum detector and an RMS detector. There are several measurements made for each display point. The peak detector displays the largest measurement for each point, the minimum detector displays the smallest while the RMS detector does an RMS average of the measurements made for each display point.
Ways to use the peak detector
You can combine the peak detector with the Minimum Hold trace setting. This combination causes signals that are present continuously to remain visible while signals that come and go drop away. For example if you are looking for a signal in the 2.4 GHz ISM band, signals from nearby WiFi access points can make this very difficult. However, by using a minimum hold trace, the frequency hopping WiFi signals disappear from the trace, leaving only signals that are steadily at one frequency as shown in Figures 2a and 2b.
Figure 2a: ISM band with maximum hold
Figure 2b: ISM band with minimum hold
Next you can combine the peak detector with the maximum hold trace setting. By doing this, any signal that is observed, even once, will be captured. This is a good way to catch bursty or intermittent signals.
The negative detector combined with the minimum hold trace setting and using a reasonably wide video bandwidth can be very useful. Signals that are present continuously will remain visible while signals that come and go drop away. The noise floor tends to drop by about 15 dB for a reasonably wide VBW allowing you to see signals that would otherwise be buried in the noise.
Combine the negative detector with the max hold trace setting so that any signal that is observed, even once, will remain visible. This is a good way to catch bursty signals and is very useful for spectrum usage surveys since the noise floor tends to be about 3 dB lower than with peak detection
In recent times, the ubiquitous Bart’s head shape of many digital modulation formats such as CDMA provides a new way to hide signals — hide them in plain sight. By creating a signal that looks like a CDMA signal, it may very well be overlooked as a known quantity. However the actual modulation inside the Bart’s head may be something completely different. The signal could also be made to look like GSM, LTE, W-CDMA just as well. An instrument that has the capability of demodulating 3G and 4G signals can confirm that a signal is actually what it appears to be — or maybe is something else entirely.
The desire to hide signals will never cease, and it is impossible to prove that a signal isn’t hidden beyond finding at the current state of the art. Having a high-performance handheld spectrum analyzer puts all of this capability in your grasp and gives you the best chance of discovering and shutting down hidden signals.
For more information on Anritsu’s portfolio of high-performance handheld spectrum analyzers, visit www.anritsu.com.
Want more on interference hunting? Watch Anritsu’s on-demand webinar: RF Interference Hunting Techniques
