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RF Noise Modeling

This example shows how to use the SimRF™ Circuit Envelope library to simulate noise and calculate noise power. Results are compared against theoretical calculations and a Communications System Toolbox™ reference model.

System Architecture

The RF system, shown in white, consists of:

  • A Configuration block, which sets global simulation parameters for the SimRF system. With the Simulate Noise option checked, noise is included in the simulation.

  • A Noise source with a power spectral density of $4kT_sR$ applied at the input. In this equation, $k$ is the Boltzmann constant, $T_s$ is the temperature of the source, and $R$ is the noise reference impedance. The calculated noise level of -174 dBm/Hz is used in this example. The Noise source is an explicit signal.

  • An Amplifier block with a specified power gain and noise figure.

  • An Outport block, with the Source type parameter set to Voltage.

  • Source and load resistors.

The Communications System Toolbox reference system, shown in green, consists of:

  • Gain blocks that model amplifier gain and loading effects.

  • Two Receiver Thermal Noise blocks that model both the external noise and the amplifier noise.

Calculate Power block computes RMS noise power. Note, that Communications System Toolbox signal is referenced to 1 Ohm, while SimRF power is computed for the actual load $R_load$ .

The example model defines variables for block parameters using a callback function. To access model callbacks, select File > Model Properties > Model Properties and click the Callbacks tab in the Model Properties window.

Running the Example

  1. Type open_system('simrfV2_noise') at the Command Window prompt.

  2. Select Simulation > Run.

The Display block verifies that the SimRF and Communications System Toolbox noise models are equivalent.

Computing RF System Noise

To enable noise in the SimRF circuit envelope environment:

  • In the Configuration block dialog, select Simulate noise.

  • Specify a Temperature. SimRF software uses this value to calculate the equivalent noise temperature inside the amplifier.

  • Specify the Noise figure (dB) parameter of any amplifiers or mixers in the system.

In the example, for a specified LNA gain of 4 dB and noise figure of 3 dB, the output noise is calculated using the following equations:

$$G_1 = 2.5119 \mbox{  (4 dB)}$$

$$F_1 = 1.9953 \mbox{  (3 dB)}$$

The next equation converts the noise factor to an equivalent noise temperature. $T$ is the Temperature parameter of the SimRF Parameters block.

$$T_e = (F_1 - 1)*T = 288.6261$$

The final equation calculates the output noise power. $T_s$ is the temperature of the external noise (noise source in this example)

$$N_{out,sys} = 10\log_{10}{\left(K(T_s + T_e)G_1\right)} + 30 = -166.97
\mbox{ dBm/Hz}$$

The available noise power is the power that can be supplied by a resistive source when it is feeding a noiseless resistive load equal to the source resistance. The Reference External Noise block generates an available power referenced to 50 Ohms.

The Gain Front end block models the voltage divider due to the source resistance and the input impedance of the amplifier.

The Amplifier noise and the Gain block model the noise added by the amplifier and the amplifier gain respectively.

The output of the Amplifier Gain block is equal to the voltage across the load resistor in the SimRF system.

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