Filtering; General Information; Filter Choices For The Measured And Reference Signals - Agilent Technologies 89410A Operator's Manual

Digital Demodulation Concepts (Opt. AYA)

Filtering

General information

Data filtering limits bandwidth and reduces intersymbol interference. This analyzer
includes several commonly used filter types and has the ability to apply
user-defined filters. All filters are computed to 20 symbols in length.
The shape and width of the filter is defined by the alpha (for cosine filters) or the
BT (for gaussian filters). The alpha or BT indicates the filter roll-off (or excess
bandwidth) of the selected filter which occurs due to the practical inability of filter
technology to build a perfectly square ("brick-wall") filter which would have an
alpha of 0 (no excess bandwidth). For example, a typical filter with an alpha of 0.3
has a bandwidth 30% greater than the theoretical minimum.

Filter choices for the measured and reference signals

Filtering for various communication systems may occur either at the transmitter or
the receiver; or the filtering may be distributed between the transmitter and the
receiver. This is an important concept which affects your filter selection for the
measured and reference signals. The analyzer's measured filter represents filtering
in the system's receiver while the reference filter represents filtering in the entire
system. Both filters share the same alpha/BT.
As an example, if the communication system under test filters the signal partially at
the transmitter and partially at the receiver (as with NADC signals), you choose a
root-raised-cosine filter for the measured signal to simulate receiver filtering in a
system which includes a similar filter at the transmitter. However, the reference
signal is a calculated ideal signal which is derived from the measured signal and
reflects only the filtering applicable to the measured signal. Therefore, you must
choose raised-cosine filtering for the reference in order to represent all the filtering
included throughout the simulated system. The following table shows some
examples of filter selection:
If the transmitter filter is:
root raised cosine
raised cosine
gaussian
any type
22 - 16
The measure filter should be: The reference filter should be:
root raised cosine
none
none
user defined
raised cosine
raised cosine
gaussian
any type