User manual MATHWORKS SIMRF 3

[. . . ] Please see www. mathworks. com/patents for more information. Revision History September 2010 Online only New for Version 3. 0 (Release 2010b) Contents Simulating Sensitivity Measurements 1 Example -- Modeling System Noise Figure . 1-2 1-2 1-6 1-7 1-9 1-9 1-12 1-13 Simulating RF Interference 2 Example -- Carrier to Interference Performance of a Weaver Receiver . 2-2 2-2 2-7 2-7 2-8 2-8 2-11 Simulating Intermodulation Distortion 3 Example -- Modeling a Direct Conversion Receiver . . 3-2 3-6 3-7 iv Contents 1 Simulating Sensitivity Measurements · "Example -- Modeling System Noise Figure" on page 1-2 · "Designing a Receiver with an ADC" on page 1-9 1 Simulating Sensitivity Measurements Example -- Modeling System Noise Figure RF receivers amplify signals and translate them to lower frequencies. [. . . ] Due to their finite resolution, ADCs introduce quantization error into the system. The resolution of the ADC is determined by the number of bits and the full-scale (FS) range of the ADC. The preceding figure illustrates an RF signal that falls within the dynamic range (DR) of an ADC. The input signal and noise at the carrier fRF has high signal-to-noise ratio (SNR). The received signal at fIF has reduced SNR due to system noise figure. However, if the quantization error is near or above the receiver noise, system performance degrades. To ensure that the ADC contributes no more than 0. 1 dB of noise to the signal at fIF, the quantization noise floor must be 16 dB lower than the receiver noise. This condition can be met by: · Reducing the full-scale range or increasing the resolution of the ADC, which lowers the quantization noise floor. · The value 1. 76 is a correction factor for a pure sinusoidal input. Therefore, the quantization noise floor is -116 dBm/Hz, in agreement with the measured output levels. 1-12 Designing a Receiver with an ADC Improving Receiver-ADC Performance Increasing the gain in the mixer raises the receiver noise without increasing the noise figure. Calculate the mixer gain required to achieve a 16-dB margin between the quantization noise floor and the receiver noise: Gmixer = (QNFADC + 16) - (-174 + Gsys + NFsys ) = (-116. 1 + 16) - (-174 + 40 + 10. 3) = -100. 1 + 123. 7 = 23. 6 dB To simulate a receiver that clears the quantization noise floor: 1 Set the Available power gain parameter of the mixer to 23. 6. 2 Select Simulation > Start. The figure shows that the receiver noise is 16 dB above the quantization noise floor. Therefore, the SNR at the output is dominated by the receiver noise. 1-13 1 Simulating Sensitivity Measurements 1-14 2 Simulating RF Interference · "Example -- Carrier to Interference Performance of a Weaver Receiver" on page 2-2 · "Modeling LO Phase Noise" on page 2-8 2 Simulating RF Interference Example -- Carrier to Interference Performance of a Weaver Receiver A classic superhetorodyne architecture filters images prior to frequency conversion. In contrast, image-reject receivers remove the images at the output without filtering but are sensitive to phase offsets. The preceding figure illustrates two input signals at the carriers fRF and fIM that both differ from the LO frequency, fLO1, by an amount fIF1. Perfect image rejection in the final stage of the receiver removes the image signal from the output entirely. Creating a Model with RF Interference The model ex_simrf_ir simulates image rejection in a Weaver architecture. The receiver downconverts the signals at fRF and fIM to fIF1 and fIF2 in two sequential stages. 2-2 Example -- Carrier to Interference Performance of a Weaver Receiver 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, Mixer, or S-Parameter blocks generate files in the current MATLAB directory at runtime. However, you can configure the output location for these files by specifying a cache folder in the Simulink Preferences dialog box. To specify a cache folder, open the Simulink Preferences dialog box (File > Preferences) and specify a location on your file system for the Simulink cache folder parameter. For more information about the Simulink interface, see Simulink Preferences Window. Setting Up the SimRF Environment The model runs according to the following settings: 2-3 2 Simulating RF Interference · In the SimRF Parameters Block Parameters dialog box, the Carrier Frequencies parameter specifies the carriers of the SimRF environment: - fRF, the RF carrier. fIF1, the intermediate frequency of the signal after the first mixing stage, equal to fLO1 ­ fRF and fIM ­ fLO1. fIF2, the intermediate frequency of the signal after the second mixing stage, equal to fLO2 ­ fIM. · 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 below the Nyquist frequency of the modulation. Since the model uses a local solver, the global solver settings do not affect the simulation within the SimRF environment. [. . . ] If you have Signal Processing Blockset software installed, you can replace the Embedded MATLAB subsystems with Vector Scope or Spectrum Scope blocks. Modeling 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 3-6 Example -- Modeling a Direct Conversion Receiver 100, which sum in parallel to a resistance of 50 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. [. . . ]