The radio bands of the electromagnetic spectrum are perhaps the most heavily restricted, regulated, and supervised resources on Earth. Precision, compliance, and safety are all essential and mandatory when producing and testing anything at all that operates in those bands.
Radio frequency (RF) analysers are a set of test and measurement instruments that are invaluable for such tasks.
In this article, discover the versatility of RF analysers by understanding their capabilities and industry applications.
What is an RF analyser?
An RF analyser (or RF spectrum analyser) is a special type of spectrum analyser for characterizing the response of a device or circuit to RF signals using network analysis. Network analysis treats the device under test as an electrical network to measure how radio frequency signals are modified by their interaction with the network as a whole.
RF analysers provide both spectrum and network analysis on RF devices and the characterization of parameters like:
- RF signal amplitude at different frequencies
- scattering parameters (S-parameters)
- impedance of the overall network
- gain in RF signal power as it’s transmitted through the network
- return and insertion losses in the power of a signal
- phase shifts in a signal
These aspects are critical for designing, testing, and troubleshooting RF circuits to ensure they satisfy functional requirements and comply with regulations.
Note that “RF analysers” is the general name for these instruments regardless of their bandwidth. Some models are limited to the RF band — 3 kilohertz (kHz) to 3 gigahertz (GHz) — while others can measure into the microwave band (up to 30 GHz) and even the millimeter wave, or mmWave, band (up to 110 GHz).
How are RF analysers different from spectrum analysers?
RF analysers are a special type of spectrum analyzers with the following differences:
- Functionality: RF analysers are specialized for analysing RF signals, focusing on parameters critical to radio frequency analysis like demodulation, modulation quality, and spectrum monitoring. In contrast, general-purpose spectrum analysers can measure a broader spectrum of signals, not limited to RF, making them suitable for a wide range of applications beyond RF analysis, including testing electronic components and circuits.
- Sensitivity and accuracy: RF analysers offer higher sensitivity and accuracy for RF signals due to their specialized design, whereas general-purpose spectrum analysers may have lower sensitivity and accuracy for RF signals, given their wider application range.
- Analysis features: General-purpose spectrum analysers typically include versatile features for spectrum analysis, such as fast Fourier transform (FFT), swept-tuned, and real-time modes. RF analysers additionally include network analysis features.
- Signal measurement: General-purpose spectrum analysers mainly focus on the scalar amplitude of signals, using simple detectors internally. RF analysers, particularly vector network analysers, are designed to measure both the amplitude and phase of RF signals, capturing the full vector form of the signal. They achieve this by using phase-sensitive detectors.
What key parameters do RF analysers measure?
Let’s understand some of the key network and spectrum parameters that RF analysers measure.
Amplitudes and phase shifts of a signal at different frequencies
Like regular spectrum analysers, RF analysers measure the amplitude or power at different frequencies. But they’re also vector network analysers that report not just the amplitudes but phase shifts as well.
S-parameters
Fig 1. S-parameters test setup
S-parameters indicate how much of the RF signal strength is reflected and how much is transmitted at the ports or connectors where signals enter or exit an RF device.
For a device with two ports, useful S-parameters include:
- S11: It tells us how much of the signal is reflected out of port 1 when a signal is sent into port 1.
- S21: It indicates how much of the signal power entering port 1 is intact when exiting port 2.
RF analysers measure the S-parameters while sweeping a band of frequencies and report them in decibels (dB) or decibel-milliwatts (dBm) at each frequency, as shown below.
Fig 2. S-parameters curve
Impedance measurements and matching
Ideally, we want to maximize the signal power transferred from a source (like a transmitter) to a load, like a software-defined radio (SDR) or antenna, at any frequency with minimal reflections. For this, impedance matching is necessary.
RF analysers allow sweeping a range of frequencies to measure the capacitive and inductive impedances of an RF assembly. They can produce Smith charts to visualize these complex impedances at different frequencies and help optimize the performance of RF devices. A Smith chart’s upper semi-circle represents inductive impedances while the lower presents capacitive impedances.
An example Smith chart is shown below.
Fig 3. Smith chart
Noise levels
RF analysers can report the displayed average noise level (DANL) in the selected span of frequencies. They also allow adjusting the resolution bandwidth (RBW) so that noise is reduced. Narrowing the resolution bandwidth reduces the noise and allows the detection of low-amplitude signals close to the noise floor.
Real-time spectrum analysis (RTSA)
Generally, spectrum analysis is done by sampling the frequency range for some time, processing it using FFT, displaying the data, then sampling again, and so on. These gaps in sampling can often miss the kind of fast signals and pulses that are common in Bluetooth communication or electronic warfare.
In contrast, RTSA continually samples the frequency range without any gaps while simultaneously processing the data in the background. This is extremely useful for digital wireless signals as well as security and military applications.
Modulation analysis
All radio communication uses some kind of modulation to carry data over radio waves.
Radio broadcasting uses amplitude and frequency modulation to transmit analog audio as well as associated digital metadata (like the song name). Other radio technologies like satellite positioning systems also use various modulation schemes.
Modern wireless systems use a variety of modulation schemes to scale up their communication capacity. They maximize the number of mobile subscribers who can make calls or receive data using schemes like orthogonal frequency division multiplexing (OFDM). Hundreds of Wi-Fi hotspots can coexist in dense urban areas using higher-order quadrature amplitude modulation (QAM).
With the right spectrum analyser software, RF analysers are capable of acting like signal analysers to identify the modulation scheme and often even demodulate some signals.
Selecting the right RF analyser
In this section, we analyse some key features and specifications that can help you choose the right RF analyser for your use case.
What is the significance of frequency range and resolution in RF analysers?
Most modern RF systems operate over wide bandwidths. An RF analyser’s ability to cover these frequencies in one sweep enables efficient and accurate measurements.
Another key specification is the resolution bandwidth. The RBW determines how well the RF analyser can differentiate between two different closely spaced, narrow waveforms or pulses as shown below. If the RBW is too wide, important behaviors at specific frequencies may be missed.
Additionally, narrow RBW means less noise, improving the ability of the RF analyser to detect weak signals.
The drawing below shows the difference between wide and narrow resolution bandwidths.
Fig 4. The c
What features and capabilities should be considered when selecting an RF analyser?
Key capabilities to look for in an RF analyser include:
- wide frequency range with fast sweeps
- resolution bandwidth that can detect very narrow signals prevalent for the use case
- vector network analysis to characterize S-parameters and similar aspects
- real-time spectrum analysis
- channel scanning to check coverage and power in multiple channels
- interference analysis and spectrogram
- signal analysis capabilities like modulation analysis and demodulation
- tracking generator, a type of signal generator that’s synchronized with the sweep frequency for characterizing frequency response of antennas, RF devices, and cables
- built-in signal calibration to ensure accurate readings
In what industries or applications are RF analysers commonly used, and what challenges do they address?
In this section, we look at how RF analysers help solve challenges in various industries.
Wireless telecommunications
RF analysers are extensively used to test all the RF equipment and consumer devices involved in wireless voice and data communications, including:
- mobile radio technologies like 6G, 5G, and Long-Term Evolution (LTE)
- Wi-Fi networks
- inter-device data channels over Bluetooth
For example, RF analysers are used for the following challenges in mobile radio network testing:
- Path loss characterization: 5G/6G mmWave coverage is adversely affected by rain, foliage, buildings, mountains, and more. The subsequent loss in power of mobile radio waves is called path loss. RF analysers help measure the path loss and ensure the quality of mobile coverage in poor terrain and weather. Typically, two handheld RF analysers are used. The transmitter is fitted with a gain-horn antenna. The receiver is fitted with an omnidirectional antenna, set to RTSA mode, and driven (or flown) around the target area.
- Base station coverage testing: Since 5G and 6G use multiple-input multiple-output and beamforming, RF analysers are fitted with phased-array antennas to measure signal power along with geographical coordinates, azimuth, and elevation.
- Component carrier power measurements: Mobile providers must measure the component carrier power levels to ensure that base stations are transmitting with optimal power. Engineers can use the channel-scanning features of RF analysers for such tests.
- Over-the-air interference testing: Interference due to other base stations, satellite ground stations, and other radio technologies is a common problem in mobile radio. The RTSA capabilities of RF analysers enable the detection of such interference.
- Network quality and beam performance verification: RF analyser software can report key performance metrics like the reference signal received power (RSRP), reference signal received quality (RSRQ), received signal strength indicator (RSSI), and more.
- Electromagnetic field (EMF) exposure evaluation: Telecom operators must verify EMF exposure levels in the field for compliance with regulatory limits and to maintain a safe environment for the public and workers. RF analysers enable them to measure these levels.
Defense and electronic warfare
RF analysers are used for a variety of defense and electronic warfare use cases, like:
- testing and characterizing pulse-modulated radar systems
- detecting offensive measures like signal jamming or electromagnetic pulses
- drone signal detection and jamming
Satellite ground station testing
RF analysers can help with satellite testing by verifying the performance of amplifiers, frequency converters, over-the-air signal quality, transmit power, and more.
Interference and compliance testing
Electromagnetic interference (EMI), whether unintended or malicious, is a key challenge in any kind of RF system. Signal hunting can become complex in today’s crowded frequency spectrum environment.
In addition, electromagnetic compatibility (EMC) testing is essential to get regulatory approvals for any RF-based equipment or consumer device. The spectrum analysis and channel scanning features of RF analyzers help establish that a device is not transmitting at prohibited frequencies or power levels above regulatory limits.
In what ways do RF analysers help in the detection and analysis of interference?
RF analysers can be used in the following ways to help mitigate interference:
- RF analysers fitted with directional wand antennas and geolocation software are used to triangulate the source of an interfering signal.
- Real-time spectrum analysis capabilities enable the detection of fast-disappearing signals.
- A capable RF analyser with a large memory to accommodate hundreds of thousands of FFT data points allows a very narrow resolution bandwidth like 1 kHz. That helps in detecting narrow and weak interfering signals as well as reducing the noise floor.
Cable testing
Many critical RF applications require the transmission of signals through physical cables. They include:
- feed lines to antennas placed on towers or masts
- connections between RF transceivers and their antennas
- signal distribution to various endpoints in broadcasting stations
- connections between test equipment in lab and production environments for quality assurance purposes
- interference prevention
Cable testing is a crucial process to verify the integrity and performance of RF cables and connections by looking for faults and damages using techniques like distance-to-fault (DTF) measurement and time domain reflectometry.
Handheld RF spectrum analysers like the FieldFox can display the signal impedance through the length of a cable. Sharp angles and damages result in more reflection, higher impedance, and less transmission of a signal as shown in the below illustration.
Fig 5. Cable faults and cable testing results using RF analyser
Use the power of RF analysers in your testing
This article gave you a glimpse into these powerful and versatile measurement instruments that can comprehensively test and verify every kind of RF use case.
TestEquity are an approved UK partner for Keysight
Blog: Prevent Interference and Boost Network Coverage With RF Analyzers
Content Source: https://www.keysight.com/blogs/en/tech/educ/2023/rf-analyzer
