Test and Measurement
T/R modules are only as good as their amplifi ers
By Gabi Duncan, Keysight RF product portfolio marketing manager for aerospace & defence and non-terrestrial network technologies T
/R modules only operate as well as their internal amplifiers perform. Operational success depends on power amplifier and low-noise amplifier
performance quality; however, amplifiers prove the most difficult system components to fully characterise. Learn how the flexible hardware and advanced software capabilities of modern network analysers simplify amplifier performance verification while improving measurement accuracy and repeatability.
Trends in component technologies like multiple T/R modules per chip and GaN power amplifiers (PA) continue to reduce the size and cost of phased arrays, shown in Figure 1, expanding market application potential. As implementation of phased arrays in satellite and 5G communications systems becomes more feasible, the focus for engineers shifts from achieving basic functionality to optimising performance. T/R modules set up system performance in a phased array through three key functions: boosting transmission signals to the maximum radiated power establishing system noise figure during receive operations providing beam steering control and angular accuracy
Characterising T/R module behaviour places high demands on a test system’s
performance and flexibility. The test system needs to support both transmit and receive test modes while maintaining accuracy and maximising throughput. Though many component parts contribute to successful T/R module operations—phase shifters, attenuators, limiters—the amplifiers most strictly limit the modules transmit and receive performance. So, to optimise the performance of a T/R module, shown in Figure 2, and thereby the phase array, engineers must start by characterising the receiver low noise amplifier (LNA) and transmitter PA.
Figure 1: Example of 4x4 phase array antenna. Four individual beamformer integrated circuits (BFIC), outlined in red, drive the antennas.
20 October 2024
The LNA and PA function as the fi rst stage of the receive path and last stage of the transmit path, respectively. As such, engineers optimise receive sensitivity using data from LNA noise fi gure and gain measurements. Likewise, for transmission, they also optimise linearity and effi ciency based on PA distortion behaviour. During receive operations the LNA determines the system link budget, noise fi gure, and consequently, the minimum detectable signal for the T/R module. According to the Friis formula for noise factor, shown in Figure 3, the noise fi gure of the fi rst amplifying stage, F1, sets the minimum noise fi gure for the overall receiver.
Regarding the transmitter path, distortion effects critically impact performance—
Components in Electronics
Figure 2: For every antenna connected to a BFIC, there is an associated T/R module.
Figure 3: Friis’s formula shows that the total receiver noise factor, Ftotal, accounts for the noise factor, Fn, and gain, Gn, of each stage. The fi rst stage noise factor, F1, impacts the overall receiver noise fi gure most.
particularly nonlinear distortion effects contributed from the power amplifi er. (The power amplifi er, being the fi nal component before the antenna, plays a critical role. Its distortions directly affect the quality of the signal we transmit.) In-band distortion contributions cause particular concern since fi ltering proves ineffective. The communication systems industry considers error vector magnitude (EVM), defi ned in Figure 4, the benchmark fi gure of merit for in-band distortion.
Modulation standards, such as 802.11ac and 5G New Radio (NR), set the minimum acceptable EVM level. As standard stringency increases so does the need to accurately capture and optimise PA modulation distortion and EVM.
The power amplifi er also contributes the most to energy consumption. Biasing determines whether an amplifi er behaves more linearly, but less effi ciently, or effi ciently but with signifi cant distortion. This linearity- effi ciency tradeoff means that engineers must optimise the PA effi ciency while maintaining linearity compliant to strict regulatory standards to get the best performance from the T/R module.
For all typical continuous wave (CW) and two-tone testing, including measurements like IP3 or 1 dB compression point, the instrument of choice is a vector network analyser (VNA).
Figure 4: The defi nition of EVM is the root mean square (RMS) of the error vector over time at the instants of the symbol clock transitions. The error vector is the vector difference at a given time between the ideal refer- ence signal and the measured signal.
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