Three key considerations for RF & Microwave test

21st October 2024

Caitlin Gittins

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What should engineers consider before embarking on testing a device? This article, in Q&A format, considers important questions spanning device characteristics and test outcomes required, the type of test equipment required and the necessary features and performance of the equipment to ensure a satisfactory outcome.

By Dean Gooroochurn, Field Application Engineer, Anritsu EMEA

1) What characteristics are of interest when testing a device? What needs to be measured and what results or limits are developers hoping to see?

When testing an RF (Radio Frequency) or MW (Microwave) device, several characteristics are measured to ensure the device performs as intended and meets specifications. These characteristics depend on the specific device and its intended application. The device in question can be a transmitter, an active or passive component, a receiver, and many others.

Engineers and technicians are interested in:

Power: Measure the output power of the RF/MW device to ensure it meets the specified power levels

Frequency: Verify that the device operates within the specified frequency range. This includes checking frequency coverage, tuning range, and any bandwidth requirements

Phase Noise: Assess the signal purity of devicessuch as oscillators or frequency synthesisers

Sensitivity: Determine the device's sensitivity, critical in receiver applications, and assess its ability to detect weak signals

Intermodulation Distortion (IMD): Evaluate the device's susceptibility to intermodulation distortion, which can occur when multiple signals interact within the device

Noise Figure: Determine the device's noise figure, which quantifies the amount of noise it adds to a signal. Lower noise figures are desirable, especially in sensitive receiver applications

Spurious Emissions: Check for unwanted harmonic or spurious emissions outside the intended frequency range and ensure that the device complies with regulatory emission limits

We can also examine S-Parameters(Scattering Parameters), a set of standardised measurements used in RF/MW engineering to characterise the behaviour of linear, time-invariant electrical networks, components, and devices. S-parametersare mostly usedin network analysis and describe how electrical signals interact with a device or network, making them a fundamental tool for designing and analysing RF and microwave circuits. These parameters are particularly useful for modelling and understanding the performance of complex networks like amplifiers, filters, and transmission lines.

Primary types of S-parameters

There are two primary types of S-parameters:

Forward Scattering Parameters (S21, S12):

S21 (Transmission coefficient): S21represents the ratio of the output signal to the input signal when the signal travels from port 1 to port 2 of the device or network. It quantifies how much of the input signal is transmitted to the output

S12 (Reverse transmission coefficient): S12 represents the reverse transmission from port 2 to port 1. It quantifies how much of the signal at port 2 is coupled back to port 1

Reverse Scattering Parameters (S11, S22):

S11 (Reflection coefficient at port 1): S11 measures how much of the input signal is reflected back towards port 1 when it reaches the input port. It quantifies the impedance mismatch at port 1

S22 (Reflection coefficient at port 2): S22 is similar to S11 but measures the reflection at port 2. It quantifies the impedance mismatch at port 2

Each S-parameter is typically a complex number, with both magnitude (amplitude) and phase information. Magnitude provides information about signal attenuation or amplification, while the phase describes the phase shift introduced by the device.

As well as basic S-parameters, higher-order S-parameters (e.g., S31, S41, etc.) or differential parameterscan be defined for multi-port devices, but the most commonly used S-parameters are for two-port devices.

S-parameter measurements are essential for various RF and microwave design tasks, with the most essential being characterising and modelling components. Engineers use S-parameters to understand how devices like amplifiers, filters, and antennas behave within a specific frequency range.

2) What test equipment is required?

Performing high frequency measurements requires specialised test equipment to accurately analyse and characterise signals in these frequency ranges. Test equipment can vary depending on requirements, but common test equipment includes:

Signal Generator: A signal generator (Figure 1) generates precise RF and MW signals at specific frequencies and power levels, providing stimulus signals for testing and calibration. Signal generators can be either vector signal generators for more complex modulation schemes or analog signal generators for simpler modulated signals

Spectrum Analyser: Spectrum analysers (Figure 2) are essential for analysing the frequency domain of RF and MW signals. They display signal amplitudes versus frequency, allowing measurement of signal characteristics such as frequency, power, harmonics and spurious emissions.Modern spectrum analysersmay also perform additional measurements, like RTSA (Real Time Spectrum Analysis), phase noise measurement or vector signal analysis (VSA), making the instrument more versatile

Vector Network Analyser (VNA): VNAs (Figure 3)are crucial for characterising the S-parameters of devices, measuring reflection and transmission coefficients, and determining impedance matching. Depending on DUT (Device Under Test), VNAs can measure characteristics such as gain, insertion loss, coupling and isolation. Traditional VNAs can also conduct advanced measurements such asmulti-domain measurements, noise figure measurements and differential measurements.

Power Meter: Power meters (Figure 4) measure the power level of high frequency signals accurately. They can be used with various power sensors and detectors to measure CW, average, True-RMS, peak or very high power levels. Nowadays, it is more and more common to talk about USB power sensors instead of power meters. These use a PC as a meter, making the solution lighter, cheaper and more transportable than standard power meters. USB power sensors are also perfect for field measurementswhere the external PC supplies the sensor

3) What are the required features and performance of this test equipment?

Signal Generator: Frequency range: Defines frequencies thatcan be generated by the instrument. Industry wants wider bandwidth to comply with more measurement applications

Number of ports: Most signal generators come with a unique RF/MW port but some offer eight or more). This is particularly useful for multi-channel applications: Intermodulation, Phase-Coherence, Frequency-Converting Measurements, etc

Output power:A high output power can avoid the need for a power amplifier, boosting transmitted power and simplifying measurement setup
Step attenuator:This attenuatesthe generated signal power, at any frequency. This gives users control of the emitted signal power, allowing Signal conditioning, Signal-to-Noise Ratio control, and Swept & Stepped measurements
Signal purity & stability: Signal purity refers to phase noise, which should be as low as possible. Stability refers to instrument internal reference aging, where slowest values are expected, particularly in defence and metrologyapplications

Modulation formats:These can be analogueor digital. For example, radar applications will use pulsed modulation,whereas cellular applications will exploit digital modulation standards (GSM / 3G / LTE / 5G etc…) and will need digital demodulation to retrieve data

Instrument format:Signal generators are usually fixed, suitable for high performance applications.Some Test & Measurement manufacturers offer handheld versions, perfect for field applications.Usually, handheld instrumentsshow standard performances but have strong advantages like form factor and weight.

Spectrum Analyser:

Frequency range: Defines frequencies thatcan be measured and exploited by the instrument. Industry is seeking wider bandwidth to cover more measurement applications

Tracking generator:A spectrum analyser tracking generator is an additional feature or module found in some advanced spectrum analysers. It provides a controlled and known output signal used with the spectrum analyser to perform measurements and tests on high frequency devices and circuits.This saves costs, avoiding an external generator/synthetiser and placing two instruments in one box - a traditional spectrum analyser is a pure receiver that cannot generate a signal. The term "tracking" in tracking generator means that the output frequency of the tracking generator follows or tracks the frequency span being analysed on the spectrum analyser. This ensures that the tracking generator's output is always at the same frequency as the analyser's centre frequency, simplifying measurements and enabling accurate comparisons

A tracking generator makes a spectrum analysera versatile tool for RF testing, characterisation, and measurement of various components and circuits. It's particularly valuable for applications requiring precise control of the test signal and knowledge of its characteristics. This configuration can act as an SNA (Scalar Network Analyser), where phase cannot be measured due to the spectrum analyser’sinternal circuitry.

DANL (Displayed Average Noise Level):This critical specification characterises the analyser's ability to measure weak signals or detect low-level signals in the presence of noise.The DANL value (Expressed in dBm)represents the minimum detectable signal level that the spectrum analyser can display on its screen while maintaining a certain level of accuracy. In other words, it indicates how sensitive the analyser is to weak signals. A lower DANL value implies better sensitivity,as the analyser can detect and display weaker signals effectively.

Advanced capabilities: Modern spectrum analysers feature advanced hardware and software architectures, bringingadvanced and complexmeasurement capabilitiesthat make the instrument compatible with many more measurement applications: Millimetre Wave/Digital Demodulation/Noise Factor

Instrument type:The first spectrum analysers were benchtop units. Today, handheld spectrum analysers are emerging with performances as good as benchtop devices, and they can be used in the lab or in the field. The main difference between them is instrument form-factor, with a handheld device being more compact and lightweight

This article originally appeared in the September'24 magazine issue of Electronic Specifier Design – see ES's Magazine Archives for more featured publications.