These Portable Analyzers Find Interference Outdoors

Summer usually brings an increase in outdoor activities, and more time outside with portable wireless equipment such as cellular telephones and Wi-Fi devices. Of course, just as when used indoors, those devices do not always work outdoors as intended and often the reason is because of interference. Indoors, radio frequency interference (RFI) may come from a leaky microwave oven working within the same 2.4 to 2.5 GHz frequency range as a wireless Bluetooth device. But outdoors, without the walls of buildings to suppress signals from AM and FM radio rations and television towers, and even cellular telephone repeaters, interference can come from all directions and be difficult to track down. But with the right test equipment, including a portable spectrum analyzer with antenna attached, frequency scans can help detect and record RFI. Alleviating the effects of the interference is something sought not only by wireless users but especially by wireless service providers hoping to deliver the best quality possible to their customers.

The importance of unhampered wireless signals in all industries cannot be denied. Whether for commercial or military communications, for industrial applications, even for a growing number of medical devices and health-care monitors, interference of any form can be restrictive or even damaging, and the interference environment can differ a great deal when wireless users are outdoors compared to when they are indoors. Spectrum analyzers equipped with an omnidirectional or directional antenna can detect, identify, even locate interference signals and their sources to help minimize the damage.

A spectrum analyzer is a radio receiver with adjustable passband that can be tuned across a wide frequency range. Interference signals can be constantly present, such as continuous wave (CW) signals with modulation, or short pulses or intermittent signals that occur at random intervals. A spectrum analyzer used as an interference hunter must have enough frequency range to surround the interference signals within its measured bandwidth and it must provide the frequency-sweep speed to measure the bandwidth of interest during the time in which the interference signals occur. In addition, a spectrum analyzer for RFI measurements should have a wide dynamic range to detect low-level signals even when they are surrounded by higher-power signals at nearby frequencies.

Portability makes it possible to bring the measurement capability to the scene of the interference, or at least to one of the sites where the effect of the interference is being felt, such as dropped cellular telephone calls at a roadway intersection. Modern portable spectrum analyzers leverage solid-state technology to achieve measurement power that compares favorably with much larger, benchtop units, but in housings that can be carried or held in hand and powered by rechargeable battery or a vehicle’s power connection. Here at Axiom Test Equipment, we provide many options with performance and functionality well suited for interference hunting. A starting point in the search for test equipment to detect and record RFI is to ensure that the analyzer’s frequency range covers the full bandwidth of interest, such as reaching 4 GHz when tracking Bluetooth or Wi-Fi interference sources at 2.4 to 2.5 GHz.

Antennas for use with the portable spectrum analyzers are also available from Axiom Test Equipment, both directional and omnidirectional antennas. A directional antenna makes it possible to find the location of the highest level of interference power. With an omnidirectional antenna, the interference signal level displayed on the spectrum analyzer’s screen will increase as the antenna and analyzer move towards the interference source.

Investigating Interference

An example of a portable interference-hunting spectrum analyzer is the Keysight N9960A. It has a frequency range of 9 kHz to 32 GHz in a housing measuring 11.5 × 7.4 × 2.8 in. (292 × 188 × 72 mm) and weighing 7.2 lb. (3.2 kg) with a rechargeable 10.8-VDC lithium-ion (Li-ion) battery. It runs for about 3.5 h on the battery and can also be powered by an external AC-to-DC adapter or external DC supply from 15 to 19 VDC. It uses a pair of 2.4-mm male connectors for the RF ports and shows measurements on a backlit 6.5-in. LED/LCD display.

This portable spectrum analyzer is available with a variety of options, including a built-in Global Positioning System (GPS) receiver to help pinpoint the location of interference. With its internal reference source, the typical frequency measurement accuracy of the analyzer is a respectable ±0.4 ppm through 32 GHz. When locked to the GPS receiver, the typical frequency measurement accuracy improves to ±0.01 ppm.

It provides resolution bandwidth (RBW) filters from 10 Hz to 5 MHz for finding signals and can perform zero-span sweeps at speeds from 1 µs to 6000 s. Reference levels can be set from -210 to +90 dBm when looking for signals at distinct levels, although the analyzer’s typical displayed average noise level (DANL) is typically -130 dBm to 32 GHz. The analyzer includes input attenuators from 0 to 30 dB and a preamplifier with 15 dB or more gain to boost low-level signals. It features over-the-air (OTA) measurement capability from 1 MHz to 32 GHz for automatically scanning and identification of cell sites, their center frequencies, and channel power. To separate interference from known signals, the analyzer can store radio standards such as W-CDMA, LTE, and GSM. For those not requiring the 32-GHz range, the Keysight N9935A FieldFox Handheld Microwave Analyzer offers much of the same measurement capability in a slightly smaller package 6.6 lb. or 3.0 kg with battery) with a top frequency of 9 GHz.

Another small, but powerful, portable spectrum analyzer is the Anritsu MS2724C Spectrum Master™. It packs a frequency range from 9 kHz to 20 GHz in a housing measuring just 12.4 x 8.3 x 3.0 in. (315 x 211 x 77 mm) and weighing just 7.8 lb. (3.5 kg) with a rechargeable Li-ion battery that lasts for typically 2.5 h. The analyzer shows measurements on an 8.4-in. display screen upon which as many as 12 signals can be marked with identification (ID) at one time. As with the Keysight/Agilent analyzers, it has USB ports for connection of external power sensors for accurate power measurements. This analyzer has almost negligible noise, with a typical DANL of -160 dBm when measured in a 1-Hz RBW. RBWs can be adjusted from 1 Hz to 10 MHz and sweep speeds set from 10 µs to 600 s to scour frequency spans for noise and interference.

The MS2724C can be equipped with many automatic functions useful for interference tracking, including an optional interference analyzer capable of automatically recording field strength, occupied bandwidth (OBW), channel power, and adjacent channel power ratio to how the levels of nearby signals relate to signals of interest. The portable unit can also integrate spectrogram functionality for collecting data for as long as 72 h for measurements according to assigned bandwidths, reference levels, and sweep speeds.

When 20 GHz bandwidth is not required, the Anritsu S362E Site Master™ is a lightweight portable spectrum analyzer spanning 100 kHz to 6 GHz that includes a cable and antenna analyzer with frequency range of 2 MHz to 6 GHz. Along with tracking interference, it can perform such functions as measuring the distance to a fault in a cable and find lossy or leaking connectors in an antenna site. As with the MS2724C spectrum analyzer, the model S362E analyzer can be supplied with spectrogram capability for measuring signal frequency and amplitude as a function of time, to track intermittent sources of interference. It can also be equipped with an optional GPS receiver and connected to USB power sensors for precision power measurements over its frequency range. This unit shows results on an 8.4-in. color touch-screen display and runs for 3 h on a rechargeable battery.

With comparable frequency range, the compact Rohde & Schwarz FSH8.28 portable spectrum analyzer provides a wide dynamic range across a frequency range of 100 kHz to 8 GHz which can be scanned with RBWs of 1 Hz to 3 MHz. As with the other portable spectrum analyzers, it is available with a range of options, including a built-in GPS receiver and USB power sensors. It also provides some flexibility in remote, on-site testing with a second version of the rechargeable battery. The standard battery runs for typically 3.0 h while a high-capacity version of the battery can provide as much as 4.5 h running time in the field.

For those seeking a unique look at the interference within their frequency range of study, the Tektronix H500 portable spectrum analyzer uses the company’s novel DPX™ Live RF spectrum display technology to mark interference from 10 kHz to 6.2 GHz using assorted colors on the screen. Reference levels can be set from -160 to +20 dBm and, with its integrated GPS receiver, the H500 features a variety of interference tracking modes. To support its clear 10.4-in. VGA display screen (640 x 480 pixels resolution) requires a housing slightly larger than many of the portable analyzers at 13.0 × 10.0 × 4.8 in. (33.0 × 25.5 × 12.5 cm) and weighing 12.27 lbs. (5.56 kg), but the added size and functionality of this unit may help speed the search for interference.

These are just a few examples of the spectrum analyzers available from Axiom Test Equipment (www.axiomtest.com). Axiom’s sales and engineering professionals can help you make the best choice in the test gear needed to better know your portable spectrum analyzers. To meet the challenges of a particular application, users are invited to contact Axiom Test Equipment for help with sourcing equipment.