Feature: RF design
communication links, and there must be a high degree of confidence that what’s designed in the lab will operate in the field. Measuring noise figure can be done in a variety of ways and with a range of different test equipment, including spectrum analys ers, noise figure meters, noise figure analysers and vector network analysers. Regardless of testing technique, any noise figure measurement requires an accurately- calibrated broadband noise source. Te maximum data transmission rate of a communications system
and the sensitivity of a radar system are largely dependent on their signal-to-noise ratio (SNR). Te SNR mainly depends on the noise factors of the components in the signal path, particularly with low input levels. Te noise factor is the ratio of input SNR to output SNR of a linear two-port device, such as an amplifier. Te noise factor is frequency-dependent and is typically given as a logarithmic value in decibels (dB), called the noise figure. Tus, knowledge of the exact noise figure value is essential for the development, optimisation and production of virtually all RF systems. In the past, noise factors were determined using a dedicated noise
tester, but now noise figures and gain are oſten measured with a R&S FPC1500 spectrum analyser, shown in Figure 4, set up for this measurement. In this configuration are included an R&S FSW-K19 noise generator, switch, calibration, isolator and amplifier under test. Te spectrum analyser measures the values of the noise figure of the DUT, displaying the results graphically and numerically. In fact, each graphical result display shows the noise figure from a different perspective. In the default configuration, the application shows the noise figure of the DUT, its gain and corresponding Y-factor. In addition, it shows the numerical results of the measurement, where the noise figure is the ratio of the SNR value at the DUT input (SNRin at the DUT output (SNRout
) to the SNR value ), by the following relation: Noise figure = SNRin / SNRout From a physical point of view, the noise figure and noise temperature
levels always have a positive range (including zero). But, due to the mathematical operations the application performs, the results can be negative. Sometimes this happens due to incorrect calibration or variance of measurement values. Tis can happen when using the Y-factor method, which yields accurate results even with small noise figures. For this measurement, a noise source with a known excess noise ratio (ENR) is used in addition to the spectrum analyser. Te ENR indicates the increase in spectral intensity of the noise, known as power
spectral density (PSD) when the noise source is switched on. Noise figure values can be measured with various test equipment
pieces from different manufactures, but the measurement process always relies on an accurately-calibrated broadband noise source for reliable, repeatable measurements. In the Figure 4 example, a spectrum analyser is being used to perform the noise figure measurement and furnish the DC voltage to power the noise source. Te noise source is first used without the amplifier under test switched in, to calibrate the test system. Ten, to perform the noise figure measurement, the amplifier is switched back on. Te isolator and low-noise amplifier in the setup are used to reduce measurement uncertainty by reducing reflected power between components in the system, thus reducing the noise figure of the test setup itself. Te overall accuracy of the noise figure measurement will primarily be determined by the accuracy of the noise-source calibration.
OTA testing and chamber calibration As chipsets for 5G become more integrated, with on-board power amplifiers and antennas, and as frequencies increase to mmWave range, conducted RF power measurements are now physically impractical or even impossible to perform. Tese new MIMO devices provide minimal access to test points for making measurements, and physical connection for conducted testing is not possible; instead, radiated or OTA testing is required. Advanced MIMO technology uses multiple antennas at the
transceiver to improve transmis sion performance. Either multiple trans ceivers transmit via separate antennas over uncorrelated propagation paths to achieve higher throughput for one or more users (spatial multiplexing), or the same output signal is transmitted via multiple antennas and combined in the receiver (Rx) to improve signal quality (Rx diversity). Tanks to the large number of antenna elements in massive MIMO systems, both concepts can be combined. Tus, an antenna radio system that supports beam - forming as well as spatial multiplexing is known as a massive MIMO system. Although massive MIMO is now used only in base stations, wireless devices are also using increasing numbers of anten nas to implement MIMO techniques. In an OTA testing system, a calibrated noise source outside the
chamber is connected to a transmit antenna inside the chamber; see Figure 5. Receive antennas inside the chamber are connected to an adequate instrument outside the chamber. Te noise source can have one or two known ENR values with calibration data for the
Figure 4: Noise figure measurement with a calibrated noise source
www.electronicsworld.co.uk November/December 2020 21
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