About Adaptive Equalization (Digital Demod)

Adaptive equalization removes linear errors from modulated signals by dynamically creating and applying a FIR Finite Impulse Response (feed-forward) compensating filter. Linear errors can come from filters in a transmitter or receiver's IF or from the presence of multiple paths in the transmission path, such as reflections in a cable system. These types of problems appear as group-delay distortion, frequency-response errors (tilt, ripple), and reflections or multipath distortion.

Equalization allows measurement of some impaired channels and can be used to isolate linear from non-llinear error mechanisms. Equalization does not require symbol lock or prior knowledge of the signal (such as a training sequence) and is compatible with recorded data.

Filter Coefficients

By default, the equalization filter has a unit impulse response which yields a flat frequency response (only one tap in the filter has a non-zero value and data simply passes through the filter).  The position of the unit impulse is a function of the filter length and is positioned to provide the most optimum efficiency for most situations.  The position cannot be adjusted.

The equalization filter has a unit impulse response when:

Aside from the above conditions, the application uses the last computed coefficients when equalization is enabled.  For example, if equalization is used in a previous measurement, the application uses the coefficients from the previous measurement unless the equalization filter is reset or Points / Symbol is changed.  Therefore, it is good practice to reset the Equalization Filter to initialize the filter coefficients before a measurement is started.

Important Concepts

The following paragraphs highlight important concepts about Digital Demod Adaptive equalization:

See Also

Using the Equalization Filter