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Design Notes

Cartesian Feedback System

This example demonstrates the use of a feedback circuit that may be implemented in a cell phone to reduce spectral regrowth in the transmitted signal.

This is accomplished by coupling off part of the demodulated signal to pre-distort the input baseband I and Q signals.

The feedback circuit makes use of the complex envelope of the modulated signal.

The time step of the envelope is greater than that of the modulated signal. Thus, using the complex envelope minimizes the impact of the delay induced by the feedback loop. The delay element in the circuit is representative of the “real world delay” that naturally occurs in feedback systems.

For comparative purposes a spectrum of a transmitted signal without feedback correction is displayed with one that accounts for correction:

• You can see from the "Power Spectrum" plot that the feedback circuit significantly reduces the spectral regrowth of the transmitted signal

As the simulation is running you can tune:

• On the power level of the QPSK signal

• The 1dB compression point of the main amplifier

• The delay element in the feedback loop (at a certain point the delay value will have a major impact on the effectiveness of the feedback circuit.

To update the filter response graphs click the Analyze icon on the tool bar.

System Diagram - Without Feedback

System Diagram - Cartesian Feedback

Graph - Power Spectrum

Graph - Filter Response_Bandpass

Graph - Filter Response_Lowpass