RF Noise and Nonlinearity Simulations

Excess noise and nonlinearities in amplifiers and mixers can degrade RF system performance. You can design a robust RF system by simulating noise and nonlinearities in amplifiers and mixers to determine the optimal noise and nonlinearities in your RF system. This topic discusses power amplifier (PA) characterization and spot noise measurements using Circuit Envelope library blocks as well as how you can use Idealized Baseband library blocks to simulate noise and nonlinearities in your RF systems.

PA Characterizations and Spot Noise Measurements

PA in RF transmitters is not immune to noise and other nonlinearities. To understand the effects of these impairments you must characterize your power amplifiers. Characterization of power amplifiers involves simulating and measuring AM/AM and plotting gain against the input power data. Characterization reveals the linearity of your PA for the input signal given to the system. In addition to nonlinear gain, you can also simulate the memory effect of the PA using the memory polynomial model. The memory polynomial model yields complex coefficients of PA. With these coefficients, you can perform fit and calculate root mean squared (RMS) errors. With this fitted data, you can visualize both fitted and measured output signals. You can also use the memoryless nonlinearity model to study the effects of amplitude and phase distortion. In a transmitter, to offset the effects of nonlinearities in the power amplifier you can perform a digital predistortion.

Spot noise parameters such as the noise factor, optimum reflection coefficient, and resistor noise help you to describe the noise introduced by a 2-port device. These parameters along with the source impedance Z_s uniquely determine the measured noise figure of the device. You can use noise circles plotted on a Smith chart generated by a Noise Figure Testbench block to show an interaction between Z_s and the noise figure.

Idealized Baseband Simulations

The RF Blockset™ Idealized Baseband library extends your Simulink^® environment with a library of blocks that model single-carrier complex-baseband systems with discrete-time signals. You can directly couple blocks from this library with DSP System Toolbox™ blocks or other Simulink blocks to estimate the impact of RF phenomena on overall system performance.

Idealized Baseband Amplifier and Mixer blocks can be used to simulate nonlinearities and noise in your RF system design. The Amplifier block provides four nonlinearity models: cubic polynomial, AM/AM-AM/PM, Saleh, and modified Rapp. Both the Amplifier and Mixer blocks provide three options to represent noise: noise temperature, noise factor, and noise figure. You can also visualize the power characteristics and noise characteristics of your systems using these blocks.

Simulation Workflows

You can simulate RF nonlinearities and noise in your system using these cross-product workflows:

Power Amplifier Characterization — This workflow shows how to characterize a power amplifier (PA) using measured input and output signals of an NXP Airfast PA. Optionally, you can use a hardware test setup including an NI PXI chassis with a vector signal transceiver (VST) to measure the signals at runtime.
Digital Predistortion to Compensate for Power Amplifier Nonlinearities — This workflow shows how to use digital predistortion (DPD) in a transmitter to offset the effects of nonlinearities in a power amplifier. by sending two tones and 5G-like OFDM waveform with a bandwidth of 100 MHz to a RF transmitter.
Spot Noise Data in Amplifiers and Effects on Measured Noise Figure — This workflow shows a testbench model to describe the noise introduced by a 2-port device. The spot noise data parameters, minimum noise figure, reflection coefficient, and resistor noise fully describe the noise introduced by a 2-port device. These parameters along with the source impedance Z_s uniquely determine the measured noise figure of the device. You can use noise circles plotted on a Smith chart to show an interaction between Z_s and the noise figure.
Idealized Baseband Amplifier with Nonlinearity and Noise — This workflow shows how to use the Amplifier block from the Idealized Baseband library to amplify a signal with nonlinearity and noise. The Amplifier block uses the cubic polynomial model with a linear power gain of 10 dB, an input IP3 nonlinearity of 30 dBm, and a noise figure of 3 dB.
Modulate Quadrature Baseband Signals Using IQ Modulators — This workflow shows how to modulate quadrature baseband signals using two different RF Blockset™ blocks. You can use a Mixer block from the Idealized Baseband library or a circuit envelope IQ Modulator block in your model to modulate quadrature baseband signals to the RF level. Observe the impairments in the modulated output signal due to gain imbalance, third-order intercepts (OIP3), and system noise in the complex output power density and output power spectrum analyzers.
RF Budget Harmonic Balance Analysis of Low-IF Receiver, IP2 and NF — This workflow shows how to use the harmonic balance solver of the rfbudget object to analyze a low intermediate frequency (low-IF) receiver RF budget for the second-order intercept point (IP2), and to compute a more accurate noise figure (NF) that correctly accounts for system nonlinearity and noise folding.