Filter Length (Digital Demod)

Default: 3 to 99 symbols, odd values only.

Range: With first use, 21 symbols; otherwise, previous value.

The Filter Length parameter sets the length of the VSA's equalization filter.

This parameter is available only when the Equalization Filter check box is selected.

Selecting a Filter Length

In general, the best filter length is the smallest that meets the measurement requirements. For measurements at the transmitter, the filter length may only need to be a few symbols in length. Longer filter lengths may be needed to measure multi-path environments.

The number of symbols determines the maximum filter length that can be entered, as follows:

Filter Length £ [result length] - 2

For example, if the Result Length is 50 symbols, the maximum filter length is 47.

Filter Design

The filter length and Points / Symbol) determine the number of taps in the equalization filter, as follows:

Taps = [(filter length - 1) ´ (points per symbol)] + 1

The Filter Length also determines the placement of the impulse response in the equalization filter. For longer Filter Length, the VSA puts the initial unit impulse response closer to the beginning of the time record to accommodate multi-path measurements, as follows:

Filter Length (in symbols)	Unit Impulse Response Position (symbol)
3 to 31	(length - 1) / 2
31 to 75	15
75 to 99	(length) / 5

For example, if the Filter Length is 11, the unit impulse response is positioned at symbol 5. If the Filter Length is 35, the unit impulse response is positioned at symbol 15.

The VSA places the unit impulse response at the position listed in the table above when the Equalization Filter is reset. The About Adaptive Equalization topic lists the conditions that reset Equalization Filter.