Modeling a Direct Conversion Receiver

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Create a Direct Conversion Receiver Model Modeling IMD in System-Level Components Examining DC Impairments

Direct-conversion receivers are sensitive to second-order intermodulation products because they transfer the RF signal directly to baseband.

Create a Direct Conversion Receiver Model

The model

ex_simrf_dc

models a direct-conversion receiver within the SimRF environment. The RF system consists of a low-noise amplification (LNA) stage, a direct-conversion stage, and a final amplification stage. The receiver specifications are similar to the specifications used in the Design and Simulation of a Direct Conversion Receiver demo, which elaborates on the impairments shown in this example.

To run the model:

1. Open the model by clicking the link or by typing the model name at the Command Window prompt.

2. Select Simulation > Start.

Models that contain SimRF Amplifier and Mixer blocks generate files at update time. Before you can successfully update and run models with these blocks, you have to set up a compiler by running mex -setup.

By default, SimRF software generates files in the current MATLAB folder. However, you can change the output location for these files by specifying a cache folder in the Simulink Preferences dialog box. To specify a cache folder:

1. Open the Simulink Preferences dialog box (File > Preferences).

2. Specify a location for the Simulink cache folder parameter.

For more information about the Simulink interface, see Simulink Preferences Window.

Set Up the SimRF Environment

The model runs according to the following environment settings:

• In the SimRF Parameters Block Parameters dialog box, the Carrier frequencies parameter specifies the carriers in the SimRF environment:

  • f_RF = f_LO, the carrier of the RF and the local oscillator.

  • f_BL, the blocker carrier

  The SimRF environment always simulates the 0 Hz carrier, regardless of whether the SimRF Parameters block specifies it.

• In the Solver Configuration Block Parameters dialog box, the Use local solver box is selected. This setting causes the SimRF environment to simulate with a local solver with the following settings:

  • Solver type is Trapezoidal rule.

  • Sample time is sample_time, defined as 1.25e-4 in the model initialization function.

  Since the model uses a local solver, the global solver settings do not affect the simulation within the SimRF environment. For more information on global and local solvers, see Choosing Simulink and Simscape Solvers.

View Simulation Output

The model uses subsystems with a MATLAB Coder implementation of a fast Fourier transform (FFT) to generate four plots:

• The RF Display plot shows the power level of the RF signal.

  

  The power level of the RF is about 100 dBm.

• The Blocker Display plot shows the power spectrum centered at the carrier f_BL..

  

  The power level of the blocker is about 90 dB higher than the signal power of the RF..

• The In-Phase Output plot shows the power spectrum of the in-phase signal at baseband.

  

  In the figure, DC power is a direct result of the blocker and the IP2 in the mixers.

• The Quadrature Output plot shows the power spectrum of the quadrature signal at baseband.

  

  The quadrature output only contains noise because the input signal and blocker have no quadrature components.

If you have DSP System Toolbox software installed, you can replace the MATLAB Coder subsystems with Vector Scope or Spectrum Scope blocks.

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Modeling IMD in System-Level Components

The IP2 and IP3 parameters specify the second- and third-order intercept points of Amplifier and Mixer blocks:

• The amplifiers have infinite IP2 and IP3, so the amplifiers are linear.

• IP2 of the mixer is -10 dB

Amplifier and Mixer components have specified gains and noise figures:

• The gain and noise figure in the LNA stage are 25 dB and 6 dB, respectively.

• The gain and noise figure in the mixing stage are 10 dB and 10 dB. The Input impedance (ohms) parameters of the two mixers are both 100, which sum in parallel to a resistance of 50 Ω to match the output impedance of the LNA.

• The gain and noise figure in the final amplification stage are 20 dB and 15 dB, respectively.

To calculate RF system noise figure, use the Friis equation:

where F_i and G_i are the noise factor and gain of the ith stage. For more information on RF system noise figure, see the demo Impact of RF Receiver on Communcations System Performance.

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Examining DC Impairments

In addition to intermodulation distortion from IP2, direct-conversion receivers are subject to additional DC impairments. For example, coupling between mixer input and local oscillator (LO) ports causes self-mixing of the LO. For more information, see the demo Executable Specification of a Direct Conversion Receiver.

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Intermodulation Distortion

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