Agilent Vector Signal Analysis Basics - Agilent Technologies

Selecting the correct demodulator baseband filtering may not be as
straightforward and intuitive as you might think, especially for a distributed
system. For example, consider a North American digital cellular (NADC)
standard which uses distributed filtering; a root raised cosine filter is
implemented in both the transmitter and receiver. See Figure 2-9. For
the VSA demodulator, you would use a root raised cosine filter for the I-Q
measured waveform (matching the receiver filtering in a system that includes
a similar filter at the transmitter). For the I-Q reference waveform, you
would use a raised cosine filter (matching the total system channel filtering).
This is because the product of a sqrt (raised cosine) times a sqrt (raised
cosine) is equal to a raised cosine filter.
Table 2-1 shows some commonly used filter types and examples of analyzer
measurement and reference filter selections based on the transmitter
filter type.
Table 2-1. Commonly used transmitter filter types and analyzer filters used
If the transmitter The measure filter The reference filter
filter is: should be: should be:
Root raised cosine Root raised cosine Raised cosine
Raised cosine None Raised cosine
Gaussian None Gaussian
Any type User defined User defined
Filter alpha and BT bandwidth time product
Another filter parameter that must accurately represent the system under
test is the filter bandwidth coefficient, specified as the filter alpha or BT.
Each filter type will have a filter bandwidth coefficient associated with it;
Nyquist filters use alpha and Gaussian filters use BT. The demodulator
uses the same alpha or BT value for both the measurement filter and the
reference filter.
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