Fixed Equalization (User Correction, Playback User Correction)

Input > Playback User Correction

The Fixed Equalization properties apply a fixed FIR Finite Impulse Response equalization filter to the time data for the specified Measurent Channel. The filter is defined by its frequency response rather than by its impulse response. The frequency response must be stored in a data register. See About Fixed Equalization for more information.

Measurent Channel: When the measurement hardware or recorded signal has more than one input channel, choose the appropriate Measurent Channel. The equalization settings on the tab apply only for the Measurent Channel selected. (For more information, see Measurent Channel in Analog Input Properties).

Fixed Equalization: Select the Fixed Equalization check box to enable fixed equalization. Clear the check box to turn off fixed equalization.

Register: Select a register in which the frequency domain data has been saved. This data defines the frequency response of the equalizer. A different register can be used for each channel.
Invert: The usual effect of equalization is to divide the unequalized spectrum by the frequency response in the data register. Select the Invert check box to effectively multiply rather than to divide. This means that if a saved two channel frequency response (Channel 2 / Channel 1) is used to define the equalizer, then either apply it un-inverted to channel 2, or inverted to channel 1. If the adaptive equalizer channel response is used to define the fixed equalizer, it is usually applied to either channel un-inverted.
Frequency Mapping: Indicates either absolute or relative frequency mapping.
Compute Time Coefficients: Enables or disables extraction of the time-domain filter coefficients of the filter used to correct the input data. The filter coefficients can be loaded into a DUT Device under Test: An acronym used to describe some type of electrical apparatus connected to test instrumentation. The apparatus can range from a single component to a complex subsystem such as a mobile phone, base station or MSC. to correct the IQ data, resulting in a lower 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.. When enabled (checked), Sample Rate and Truncation parameters become editable to adjust the variables used in the computation.

To use the Compute Time Coefficients feature, you first need a correction filter. It needs to be a frequency-domain channel response that is at least as wide as the VSA Measurement Span. This channel response can be measured in a variety of ways, including with the Channel Quality measurement, Channel Sounding measurement, and with other demodulation analysis measurements (equalizer frequency response). Once you've measured the channel frequency response, you can use File > Copy Trace to place the response into a Data Register for use with User Corrections > Fixed Equalization.
- Sample Rate: Sets the sample rate of the user coefficients (Channel X > Fixed Equalization Time trace). Sample Rate can be different from the VSA sample rate. Set to the desired sample rate for your DUT. When Auto is enabled, the Sample Rate will follow the VSA's sample rate (1.28 * Measurement Span).
- Truncation: (with Left and Right truncation lengths) When enabled, specifies the offset from the center of the filter at which to truncate the filter impulse response. This can be adjusted during a demodulation measurement to trade off between lower EVM and fewer filter coefficients.
This feature is not available for baseband signals (real-valued samples, starting from 0 Hz).

The frequency-domain content of the correction filter outside the correction bandwidth (VSA span) will be attenuated to zero using a cosine taper (Tukey).