Spectrum Analyzers Help Find Harmful Interference

Frequency spectrum is filling steadily with growing numbers of wireless devices. Many of the wireless devices operate within short distances at narrow frequency bands, such as Bluetooth and wireless local are network (WLAN) equipment. However, as more users make wireless cellular radios their principal telephones, longer-distance wireless applications, including satellite communications (satcom) systems, are occupying more of the electromagnetic (EM) spectrum. Cellular communications networks are evolving from Fourth Generation (4G) technology to Fifth Generation (5G) technology and higher frequencies, well into the millimeter-wave frequency range above 24 GHz.

This growing number of wireless applications and signals creates increased opportunities for interference when signals are not properly monitored. When interference does take place, it degrades system performance and inevitably must be detected and eliminated. This requires spectrum monitoring and interference hunting with the right tools and the best equipment for that job is the spectrum analyzer. With a spectrum or signal analyzer teamed with a suitable antenna, it is usually possible to detect and measure a source of RF interference. In addition, with a portable unit running on battery power, it can be carried close enough to the source of the interference for accurate and repeatable measurements. The trick is finding a spectrum analyzer with the performance and functionality to speed and simplify interference hunting in a desired frequency range.

Spectrum analyzers employ several methods to find signals. They may operate as superheterodyne receivers that mix inputs with a tunable local oscillator (LO) and then process the resulting lower-frequency intermediate-frequency (IF) signals for their content. This type of spectrum analyzer is well suited for detecting continuous-wave (CW) signals but can miss on and off intermittent signals if they are not present when the analyzer tunes past that section of spectrum. The other main type of spectrum analyzer, the real-time analyzer (RTA), relies on analog-to-digital conversion (ADC) to digitize sections of spectrum and detect CW as well as short-term spurious and pulsed signals.

By measuring as many as the signals within a frequency range of interest, such as around 2.4 GHz for Bluetooth and wireless local area network (WLAN) signals and any additional, unwanted signals, potential interference can be detected and displayed with a spectrum analyzer. At present, higher frequencies are being employed for applications such as automotive radars and Fifth Generation (5G) cellular wireless communications networks. But the majority of the global 5G infrastructure is still in what has been designated as “frequency range 1” (FR1) of three frequency ranges for 5G, within spectrum at frequencies less than 6 GHz, and a spectrum analyzer covering a frequency range of approximately HF to 6 GHz will provide a good starting point for a great deal of practical signal monitoring and interference hunting, if armed with suitable functionality and performance.

Surveying the Spectrum

As an example, the Keysight N9342CN spectrum analyzer is a lightweight, handheld swept-tuned unit designed for in-field use such as connected to an antenna in search of radio-frequency interference (RFI). It can show frequency spectrum from 100 kHz to 7 GHz on a bright full-color screen with marker resolution of 1 Hz. It has a wide dynamic range, handling input levels as high as +20 dBm and detecting low-level signals that can be extracted from the unit’s displayed average noise level (DANL). The DANL is typically -117 dBm or better at all frequencies and can be improved with an optional full-range preamplifier. The spectrum analyzer runs on a rechargeable battery and delivers excellent frequency accuracy of ±1 ppm thanks to an internal 10-MHz reference oscillator. The compact unit measures 12.5 × 8.15 × 2.7 in. (318 × 207 × 69 mm) and weighs 7 lbs. (3.2 kg). When direction finding (DF) is part of tracking down interference, the model N9342CN is available with an optional built-in GPS receiver and GPS antenna.

In the same frequency range, the Tektronix RSA5106B RTA brings extensive digital signal processing (DSP) for battery-powered portable measurements from DC to 6.2 GHz. It allows users to select the real-time acquisition bandwidth (25, 40, 85, 125, or 165 MHz) to find short pulses and frequency events that may be causing interference, events as short as 6.2 ns when using the 165-MHz acquisition bandwidth tuned across the unit’s wide frequency range. The RTA can set triggers according to different signal parameters, such as signal power levels, and store as many as 64,000 spectrum records in a single acquisition when looking for short-term pulsed signals.

The versatile unit can function as a traditional spectrum analyzer or an oscilloscope-like time-domain RTA, performing measurements in the frequency and time domains. As a spectrum analyzer, for example, it is capable of a 1-GHz sweep across frequency with a resolution bandwidth (RBW) of 10 kHz in less than 1 s, to find and resolve multiple signals closely placed in frequency. A wide variety of options include programmed measurements according to popular wireless standards, such as WLAN 802.11 and Bluetooth Basic LE, with choice of measurement types, such as peak power, average power, adjacent-channel power, and in-band emissions helping to find signals that may be acting as interference for these wireless systems. When so much frequency range is not necessary, the model Tektronix RSA5103B covers the range of 1 Hz to 3 GHz.

For higher-frequency spectrum monitoring and interference hunting, the Rohde & Schwarz FSW13 spans 2 Hz to 13.6 GHz, but in a much larger, rack-mount benchtop unit housing. It is a powerful tool for finding low-level signals, with a dynamic range that handles signal levels as high as +30 dBm (1 W) and a low end defined by its DANL. The DANL is typically -150 dBm at the lowest frequencies and -154 dBm at 13.6 GHz.

For even higher-frequency interference hunting, with the benefit of portability, the Tektronix RSA518A RTA from covers a frequency range of 9 kHz to 18 GHz with aa USB connection to a laptop computer to take advantage of the additional processing power. It draws power from its own rechargeable battery while the USB connection provides access to Windows-based test software for control and signal analysis.

At even higher frequencies, the Keysight N9960A microwave spectrum analyzer is one of the company’s versatile FieldFox family of portable units. With a 103-dB dynamic range across a span of 5 kHz to 32 GHz, the compact, lightweight spectrum analyzer runs about 3.5 h in the field on a rechargeable lithium-ion battery. The self-contained unit stores data in 4-GB internal solid-state memory and shows test results on a full-color 6.5-in. LCD/LED screen.

These are a few of the portable and versatile spectrum analyzers well suited for spectrum monitoring and interference hunting in the 5G FR1 frequency range. For those working at higher frequencies and concerned with problems from higher-frequency fundamental and harmonic signals, higher-frequency analyzers can be found on the Axiom Test Equipment website at www.axiomtest.com.