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The Noise block introduces noise onto specified carrier frequencies in a SimRF circuit-envelope simulation environment. For more information about simulating RF systems, see the example, Reduce Computations by Using RF Simulation Techniques.
To model noise in a SimRF model with a Noise block, you must select the Simulate noise check box in the SimRF Parameters block dialog box. Otherwise, the model simulates without noise.
The Noise block is useful for modeling localized noise as a current or voltage source for blocks other than Amplifier and Mixer blocks. To simulate global thermal noise in amplifiers and mixers, specify a noise temperature in the Temperature parameter in the SimRF Parameters block dialog box.
Specify the noise type as Ideal Voltage or Ideal Current. The default value of this parameter is Ideal Voltage.
Specify the height of the noise power spectral distribution (PSD). The default value of this parameter is 0.
Enter a vector of carrier frequencies whose elements are a subset of the frequencies specified in the SimRF Parameters block. Specify the units from the corresponding drop-down list. The default value of this parameter is 0 Hz.
In the SimRF User's Guide, the section Create a Low-IF Receiver model includes a model with a Noise block that models a thermal noise floor.
Model a Noisy Attenuator
The model
ex_simrf_attenuator
calculates the signal level, noise level, and signal-to-noise ratio (SNR) of an RF signal passing through an amplifier and noisy attenuator.
For comparison purposes, the model contains two systems: one where the signal is amplified before passing through the attenuator, and one where the signal is amplified after passing through the attenuator.
The noisy resistors in this model, labeled R1, R2, and Z0, are subsystems that contain Resistor and Noise blocks. The Noise blocks model thermal noise across the resistors as a power specral density (PSD), according to the equation
where kB is the Boltzmann constant, T is the noise temperature in Kelvin, and R is the resistance.
The model uses MATLAB code in the PreLoadFcn callback to define variables in the system:
R1 and R2 are resistance values that define a 10-dB attenuator. Additional possible values for different attenuation levels are listed in the table in the diagram. In the example, R1 is 25.97 ohms, and R2 is 35.14 ohms. These values are also used in the noise PSD calculation in the R1 and R2 subsystems.
Z0 specifies a source and load impedance of 50 ohms. This value is also used in the noise PSD calculation in the Z0 subsystem.
G and NF define the gain and noise figure of the amplifiers, respecively. In the example, G is 10 dB, and NF is 3 dB.
k and T define the Boltzmann constant and the noise temperature, respectively, which are used to simulate noise.
carriers_RF2 and sample_time define the RF carrier and the sample time.
For information on using callbacks in SimRF models, see the Model an RF Mixer example in the SimRF Getting Started Guide.
Running the model produces output on the Display blocks:
As the simulation runs, the 10-dB attenuator and amplifier system normalizes to a signal level of -110 dBm/Hz, a noise level of -161 dBm/Hz, and an SNR of 51 dB.
As the simulation runs, the amplifier and 10-dB attenuator system normalizes to a signal level of -110 dBm/Hz, a noise level of -169.38 dBm/Hz, and an SNR of 59.38 dB.
Although the signal level is the same in both setups, placing the amplifier first in the cascade reduces the overall noise level and increases the SNR. This result is in accordance with theoretical predictions made by the Friis equation.
Motchenbacher, C.D. and J.A. Connely. Low Noise Electronic System Design. New York, NY: John Wiley & Sons, 1993.
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