A classic superheterodyne 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 and that both differ from the LO frequency, , by an amount . Mixing translates both input signals down to . Perfect image rejection in the final stage of the receiver removes the image signal from the output entirely.
ex_simrf_ir performs image rejection with a Weaver architecture. The receiver downconverts the signals and to and in two sequential stages.
To maximize performance, the Fundamental tones and Harmonic order parameters are explicitly set in the
Configuration block. To create the minimal set of simulation frequencies, the following carrier frequencies are set or created within the model.
, the RF carrier, equals 100 MHz.
, the image of the RF carrier relative to , equals 200 MHz.
, the frequency of the LO in the first mixing stage, equals 150 MHz.
, the intermediate frequency of the signal after the first mixing stage equals: = 50 MHz.
, the frequency of the LO second mixing stage equals 75 MHz.
, the intermediate frequency of the signal after the second mixing stage equals: = 25 MHz.
In this system, every carrier is a multiple of . The largest carrier, , is the 8th harmonic of , so setting Fundamental tones to and the Harmonic order to 8 ensures that every carrier is in the set of simulation frequencies.
Solver conditions and noise settings are also specified for the
The Solver type is set to auto. For more information on choosing solvers, see the reference page for the Configuration block or see Choosing Simulink and Simscape Solvers.
The Step size parameter is set to 1/(mod_freq*64). This setting ensures a simulation bandwidth 64 times greater than the envelope signals in the system.
The Simulate noise box is checked, so the environment includes noise in the simulation.
The model uses
Spectrum Analyzer to generate four plots.
The RF spectrum scope shows the power spectrum of the carrier signal , specified as
carriers.RF in the Carrier frequencies parameter of the Input Sensor
The modulation of the RF carrier is a constant envelope generated by a
Continuous Wave block which generates a single peak centered at the carrier.
The Image Spectrum scope shows the power spectrum of the image. The signal is recovered from the carrier , specified as
carriers.IM in the Carrier frequencies parameter of the Input Sensor
Sinusoidal Source block generates a two-tone signal centered at .
The IF1 spectrum scope plot shows a power spectrum centered at the first intermediate frequency, measured between the first and second stages. The sensor outputs the modulation from the carrier , specified as
carriers.IF1 in the Carrier frequencies parameter.
The Output spectrum scope shows the complete effects of the RF system. The sensor outputs the modulation from the carrier , specified as
carriers.IF2 in the Carrier frequencies parameter.
To model more robust input signals, you can use a RF Blockset
Inport block to specify a circuit envelope signal generated using blocks from other Simulink™ libraries. For example, see the featured example
Impact of an RF Receiver on Communication System Performance. This example uses Communications System Toolbox™ to model a QPSK-modulated waveform of random bits with RF Blockset Inport that brings the signal into the RF Blockset environment.
The phase shifters have specified Phase shift parameters of 90. Deviation from this value results in a phase offset and causes imperfect image rejection. The featured example
Measuring Image Rejection Ratio in Receivers analyzes the IRR of the Weaver and Hartley architectures several times, calculating the image rejection ratio (IRR) for several different phase offsets.