Predistort Waveform (Channel Quality)

The Predistort Waveform expanding parameter group is used to compensate the provided waveform file for the currently measured channel response and saves it as another file. This is useful if you measure the channel with the Channel Quality Measurement and then want to predistort some other waveform before sending to the signal generator, perhaps to compensate a modulated signal for the IQ impairments for an IQ modulator, and then .demodulate to see the EVM Error vector magnitude (EVM): A quality metric in digital communication systems. See the EVM metric in the Error Summary Table topic in each demodulator for more information on how EVM is calculated for that modulation format. that could be achieved if the IQ modulator were compensated internally.

First, select the input and output waveforms (supported VSA recording types, single channel). Then choose the predistortion mode, which controls the way the filters are applied to the input waveform and affects the output waveform length and contents.

Only single channel input waveforms are supported.

If the predistortion is performed with a multi-channel Channel Quality measurement, the Input Waveform will be predistorted according to the response for Measurement Channel 1.

Input Waveform opens a dialog to select an input waveform file.

Output Waveform opens a dialog to select an output waveform file.

Mode specifies how to handle filter settling during predistortion.

Circular (for periodic waveform): This is for waveforms that are exactly periodic and will be transmitted in a repeating pattern. The predistortion is applied such that the filter effects wrap around from the end of the waveform to the beginning. The first sample in the output still corresponds to the first sample in the input.

Linear (for bursted waveform): This is for waveforms that contain bursted data (on for a period of time and then off). Filtering is applied with settling (no wraparound) and then the settling is truncated so that the output waveform is the same length as the input. The first sample in the output waveform corresponds to the first sample in the input waveform.

Linear, with settling present: This is the same as Linear (for bursted waveform) except that the settling points are left in the recording. This will avoid a potential discontinuity at the beginning and end of the input waveform, but will make the waveform longer and the first point will not correspond to the first point in the input waveform.

Predistort applies predistortion to the waveform. Clicking the Predistort button will read the input waveform, predistort it, and save to the output waveform path. Any existing output waveform file will be overwritten. The channel response will be resampled to match the input waveform. Predistorting a signal with a bandwidth wider than the stimulus bandwidth is not supported (although the sample rate can be larger). Also, short waveforms may require that the predistortion filters are truncated to match the input waveform when Mode = Circular.