Testing Wideband Radio Equipment with State-of-the-Art Signal Analyzers

The mmWave operating frequencies specified for cutting-edge radio applications like 5G mobile and automotive radar are needed to ensure adequate bandwidth to meet system-performance demands: a wide bandwidth gives 5G systems the channel capacity needed for the specified maximum data-downlink speed of 20Gbps, while automotive-radar resolution is directly related to bandwidth.

Equipment developers need suitable test gear for wideband designs at mmWave frequencies. Some of today’s digital oscilloscopes offer a measurement bandwidth up to the mmWave frequency range. Oscilloscopes with lower bandwidths can be combined with additional circuitry such as mixers to acquire a sample signal at mmWave frequency. Rohde & Schwarz has a selection of suitable mixers, such as the R&S FS-Zxx series, as well as oscilloscopes capable of compensating the losses caused by additional components in the signal path. Users can take advantage of built-in basic measurement tools and use the R&S VSE Vector Signal Explorer software for a more in-depth analysis.

With modern signal and spectrum analyzers such as the R&S FSW, the entire spectrum from 2 Hz to 85 GHz can be measured and displayed in a single measurement. Recent generations of instruments have extended the analysis bandwidth from 500MHz-1GHz to as much as 8.3GHz in the latest R&S FSW series analyzers.

The simplified block diagram (figure 1) shows the input circuitry of a cutting-edge signal analyzer. Sophisticated measurements can be made directly on the instrument, which, typically, also provides the advantage of a high dynamic range that helps identify a small signal in the vicinity of a large interference signal. In addition, excellent frequency selectivity simplifies examining signals within a tightly defined region of interest. 

Figure 1. Signal analyser block diagram

Testing 5G Wideband Amplifiers 

The first mmWave 5G services are expected to be in the 26 GHz “pioneer band” nominated by Europe’s Radio Spectrum Policy Group (RSPG). For these applications, wideband power amplifiers are needed to operate from 24 GHz to 28 GHz. Achieving the desired combination of linearity in the target frequency bands, power output, and optimum efficiency is challenging. Operating the amplifier close to its compression point enhances efficiency, and Digital Predistortion (DPD) is applied to preserve linearity. 

To validate the design, a suitable test setup needs a convenient DPD tool and a means of measuring parameters such as the Error Vector Magnitude (EVM), AM/AM and AM/PM distortion. Rohde & Schwarz has developed a closed loop solution for wideband amplifier testing that combines the R&S FSW spectrum and signal analyser with the R&S SMW200A vector signal generator (figure 2).

Figure 2. Wideband tests using R&S FSW signal analyser (right) to control a vector signal generator

The R&S FSW allows adding options for specific measurement applications and standards. A wide range of these are available, including the R&S FSW-K18 Amplifier option for mobile amplifiers and integrated frontends. Leveraging the measurement application to control the signal generator, the user can gain insights into the causes of signal degradation without knowing detailed information about the test signal, which is usually needed for demodulation. The signal generator applies the DPD model directly to the signal in real time without any waveform recalculation. Characteristics such as EVM, adjacent channel leakage ratio (ACLR), AM/AM and AM/PM, gain and gain compression can be derived from only one RF measurement.

This measurement application also performs an automated parameter sweep that simplifies characterization over frequency and power range. Measurements are taken at each sweep point and the results are presented in 3D graphics to help quickly overview the acquired data. 

High-Resolution Automotive Radar 

Ultra-wideband radar is essential for advanced driver assistance systems (ADAS) such as collision avoidance and adaptive cruise control, and will be critical for fully self-driving vehicles of the future. Automotive radar tends to use the frequency-modulated continuous wave (FMCW) principle, which allows measuring the target’s range and speed simultaneously. A continuous carrier is transmitted, modulated by a periodic function such as a sinusoid or sawtooth wave. When a signal reflected from an object in the field of view arrives at the receiver, the range and speed are measured from calculations performed on the transmitted and received signals. 

The accuracy, range, azimuth and radial velocity resolutions are key performance parameters. Because very high resolution is needed, compared to the target size and motion, to measure details of the target and distinguish between different objects, equipment is planned to operate in the 79 GHz band with bandwidth up to 4GHz. 

The latest signal analyzers, with their extended internal measurement bandwidth, simplify the testing of ultra-wideband radar. The R&S FSW-K6 pulse measurement application captures pulse parameters such as the duration, period, rise and fall times, power drop, and intra-pulse phase modulation, and compiles a trend analysis over many pulses. It features a segmented I/Q capture function that increases the analysis period almost 1000-fold for pulse lengths less than 1 µs and a 1 kHz pulse repetition interval (PRI).  

Additional measurement options are available for measuring pulse compression parameters and evaluating effects such as modulators and exciters that can degrade radar performance. In addition, transient analysis and chirp measurement (figure 3) can be performed to help optimize the radar sensor by calculating the chirp rate and the deviation from the ideal FMCW chirp. 

ssFigure 3. FMCW radar signal analysis with R&S FSW-K60/-K60C measurement options

Real-Time Signal Analysis

To complete the toolbox of today’s RF engineers, there is an emerging need for instruments that can handle dynamic and transient RF signals, such as in frequency-hopping radios and wideband VCOs (Voltage-Controlled Oscillators) used in radar systems and satellite ground stations. 

These call for real-time signal-analysis capabilities, to capture all relevant information about the signal behavior. Conventional signal analyzers that perform swept measurements can miss signal variations that occur while the captured signal is being analysed and the instrument is not taking measurements. Innovations such as shorter FFT compute times and fast-acting digital resolution-bandwidth (RBW) filters have significantly reduced the duration of this “blind” period. R&S FSW analyzers can now perform more than 1000 sweeps per second. 

By ensuring sampling and FFT calculation happen concurrently, real-time signal analysis can be performed. There are two key requirements: the FFT algorithm must be executed at high speed to prevent unprocessed data accumulating, and high-bandwidth analog-to-digital converters (ADCs) are needed that can capture a wide frequency range in a single shot without having to move the local oscillator (LO).

Various real-time measurement packs are available for the R&S FSW (such as the R&S FSW-K161R, R&S FSW-B512R, R&S FSW‑B800R), which enable users not only to analyze frequency-hopping algorithms but also to create alternative algorithms if needed to prevent collisions between the signals from different systems such as Bluetooth and WLAN operating in the 2.4GHz or 5GHz frequency band. 

Conclusion

Developers of wideband equipment operating at high frequencies are increasingly dependent on high-performance signal analyzers to gain the insights they need develop new products to meet demanding wireless specifications. The latest instruments offer a high maximum input frequency and a wide measurement bandwidth suitable for wideband applications at millimetre-wave frequencies. Additional measurement options enable the capabilities to be further extended for purposes such as radar pulse measurements and real-time analysis.