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Simulate High-Power Out-of-Band Interfering Signal

This example shows how to test the Zigbee-like RF receiver designed in the previous chapter in presence of a high-power out-of-band interfering signal and perform multi-carrier simulation. You will determine the specifications of DC offset cancellation algorithm to compensate for the non-linear effects excited by the interfering signal.

Add Wideband Interfering Signal and Perform Multi-Carrier Simulation

This section modifies the circuit envelope model to create this circuit envelope with interferer model. The circuit envelope with interferer model includes a wideband interfering signal and these impairments:

  • WCDMA-like out-of-band blocker of -30 dBm at 2500 MHz

  • LO-RF isolation of 105 dB in the quadrature demodulator

  • OIP2 equal to 55dBm in the quadrature demodulator

This simulation models a non-standard-compliant interfering signal that has power and spectral distribution characteristics realistic for a WCDMA signal. The simulation of the wideband interfering signal requires a larger simulation bandwidth of 16MHz. Therefore the 1MHz OQPSK signal is oversampled by 16, and the Circuit Envelope simulation bandwidth is also increased to 16MHz.

First run the simulation to evaluate the impact of the out-of-band interfering signal. The interfering signal is within the passband of the SAW filter, but it is removed by the channel selection filter of the demodulator. Although the interfering signal is filtered out, due to its high power it excites the demodulator non-linearity (IP2) causing a large DC offset of -42 dBm that degrades the ChER.

model = 'TopDownRFReceiverDesignCircuitEnvelopeWithInterferer';
open_system(model);
ws = warning('off','simrf:simrfV2errors:DerivedNoiseNotPassive');
set_param([gcs '/SW'],'sw','1'); % bypass DC offset
sim(model);
warning(ws)

Determine the Specifications of DC Offset Cancellation Algorithm

The design requires a DC offset compensation algorithm to achieve the desired ChER.

Run the simulation including a DC blocker, that essentially consists of a very narrowband notch filter. As the filter is very selective, it introduces a long latency. For this reason, the ChER computation delay has been increased. As the DC offset compensation algorithm is required to make the receiver more robust to interfering signals, the LO isolation specification of the demodulator can also be relaxed while still achieving acceptable ChER.

set_param([gcs '/SW'],'sw','0') % include DC offset compensation
loToRFIsolation = 90; %dB
open_system(model);
ws = warning('off','simrf:simrfV2errors:DerivedNoiseNotPassive');

sim(model);

The spectrum centered at 0 Hz shows that the interfering signal and the finite IP2 of the demodulator combined with the finite LO isolation, cause a DC spectral component equal to -36dBm. As you run the model, the DC blocker compensation reduces the DC offset until it is eventually completely removed.

Conclusion

Following a top-down design methodology, RF receiver components specifications were derived. Impairments, interferer, and RF receiver subcomponent models were iteratively refined to increase fidelity and validated at each stage to confirm overall system performance goals were achieved.