Architectural Design of a Low-IF Receiver System
This example shows how to use the RF Blockset™ Circuit Envelope library to simulate the performance of a Low IF architecture with the following RF impairments:
- Component noise
- Interference from blocker signals
- LO phase noise
- ADC dynamic range
- Component mismatch
Design variables in the RF portion of the model include explicit specification of gain, noise figure, IP3, input/output impedance, LO phase offset, and LO phase noise. Carrier frequencies for waveforms entering RF Blockset subsystems are specified in the Inport blocks. Design variables for the transmitter side of the RF interface include carrier frequency, modulation scheme, signal power, and blocker power level. Baseband design variables are number of bits and full scale range of the ADC.
This model illustrates the design and simulation of an ISM Band Receiver. The model is comprised of blocks from RF Blockset, Communications System Toolbox™, DSP System Toolbox™, and Simulink® libraries. Primary subsystems include a digital transmitter, an RF receiver, an ADC, a phase noise block for noisy LO modeling, and a digital receiver. Remaining blocks are used for analysis. Blocks and plotted power spectral density signals are color coded:
- RF Blockset: Light Blue
- Communications System Toolbox: Green
- DSP System Toolbox: Grey
- Simulink: White
The digital transmitter consists of three FSK modulated waveforms and a high power tone. The target waveform at 2450 MHz has a 1 Ohm referenced passband power level of approximately -70 dBm. Similarly defined image and IMD blocker waveforms have passband powers of approximately -40 dBm and -33 dBm, respectively. The IMD tone that couples with the IMD blocker to generate in-band IM3 products has a passband power of -33 dBm. Since the Communications System Toolbox defines the complex envelope waveforms, computing passband power requires the insertion 1/sqrt(2) gain as shown in the design. An IF of 2 MHz can be inferred by inspecting the input signal spectrum.
The RF Blockset Low IF receiver is comprised of a receive band SAW filter, a frequency conversion stage, an image rejection stage, and two gain stages. Resistors are used to model input and output impedances. Each nonlinear block has a noise figure specification. Power nonlinearities in the LNA, IF amplifier and mixers are specified by IP3. Image rejection is accomplished with a Hartley design, and single LO and phase shift blocks provide cosine and sine terms to mix with the I and Q branches, respectively. The summation block recombines the signals on the I branch and the phase-shifted Q branch. Image rejection quality can be controlled directly by setting a non-ideal phase offset in the Phase Shift block. To capture the RF, Image, IMD Signal and IMD Tone waveforms/spectra, choose the Fundamental tones to be 2450 MHz, 1 MHz and the Harmonic Order as 1 for the first tone and 8 for the second tone within the Configuration block. To model a thermal noise floor in the RF Blockset environment, the Temperature within the System Parameters section in the Configuration block is set to a noise temperature of 290.0 K.
The ADC is modeled using an ideal sampler and a N-bit quantizer. The quantizer takes into account the full-scale and dynamic ranges of the ADC, properly modeling its quantization noise floor.
A digital receiver demodulates the waveform for bit error rate calculation.
Running the Example
Running the example simulates a design that meets an uncoded 0.1% BER spec. Modifications to the signals and component specifications in the receiver and ADC has a direct impact on the receiver performance. Manual switches enable the user to:
- Select a power level for the IMD blocker tone of -33 dBm or -45 dBm
- Select an ideal or noisy LO.
Other possible changes to the design include:
- Image rejection ratio (IRR) of the Hartley design. The IRR of the present design (dPhi=0.01 degrees) is -40 dB. For more information on calculating IRR, see the example Measuring Image Rejection Ratio in Receivers.
- Signal power levels
- Signal carrier frequencies
- Noise figures
- Non-linear gain parameters
- Modulation schemes
- Interstage matching
- ADC bit length and full scale range