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Design Notes

Signal Clipping

Signal Clipping is a form of distortion that limits a signal once it exceeds a threshold, which is often intentionally applied to keep a signal within a desired range.  The most common clipping algorithms are

- Magnitude (Vector) Clipping: magnitude of I/Q constellation points is limited, sqrt(I*I+Q*Q) < value

- Scalar Clipping: absolute value of real and imaginary components of I/Q constellation points is limited, abs(I,Q)<value

In the case of Hard Clipping the signal is strictly limited at the threshold, producing a flat cutoff.  In the case of Soft Clipping the clipped signal continues to follow the original at a reduced gain.  Hard clipping results in many high frequency harmonics; soft clipping results in fewer higher order harmonics and intermodulation distortion components.  Soft clipping may be implemented as hard clipping followed by filtering.

Many signal generators provide clipped outputs to protect the devices under test (DUT).  This example shows how a soft magnitude clipper may be implemented so that the signal matches the output of commercially available signal generators, e.g., Rohde-Schwarz SMU200A Vector Signal Generator.

A WiMAX 802.16d signal source is used.  The clipper is implemented as a subcircuit with the following adjustable parameters:

            - Clipping level (%)

            - Low pass filter order

            - Low pass filter cut-off frequency

These parameters may be tuned to achieve the desired clipping level and behavior, which can be evaluated by the CCDF measurement.

System Diagram - CCDF Measurement

Graph - CCDF

Graph - IQ Constellation

Graph - IQ Waveforms

Graph - IQ Constellation Clipped