The following topics explain the digital demodulation filters and configuration parameters:
Data filtering is used in digital demodulation to limit bandwidth and reduce intersymbol interference. This VSA includes several commonly used filter types and has the ability to apply user-defined filters. All filters are computed to 20 symbols in length by default. If the filter alpha is < 0.4, the VSA uses extended filter length (40 symbols) to compute filter length (for most PSK and QAM formats). For further details, see Alpha / BT.
The shape and width of a filter is defined by the alpha (for cosine filters) or the BT (for Gaussian filters). The 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.
The VSA's digital demodulator produces two signals: a measured and a reference signal. These signals are called IQ Measured () and IQ Reference () or, for FSK and CPM (FM) measurements, FSK Measured and FSK Reference.
Different filters can be selected for the measured and reference signals, as shown in the following, generic block diagram.
For FSK and CPM (FM) measurements, filtering occurs after the FM demodulator.
The measured signal is the signal that results after demodulating the waveform. The reference signal is the signal that would result after demodulating the signal if the signal were ideal (contained no errors).
Notice that there are separate filters for the measured and reference signals. The correct filter MUST be selected for both 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 VSA'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 .
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), 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 detected symbols. Therefore, raised-cosine filtering must be chosen 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: |
The measurement filter should be: |
The reference filter should be: |
|---|---|---|
|
root raised cosine |
root raised cosine |
raised cosine |
|
raised cosine |
none |
raised cosine |
|
Gaussian |
none |
Gaussian |
The shape and width of Gaussian and Nyquist (cosine) filters can be modified. The shape and width is defined by the alpha (for cosine filters) or the BT (for Gaussian filters). All filters are computed to 20 symbols in length by default. If the filter alpha is < 0.4, the VSA uses 40 symbols to compute filter length (for most PSK and QAM formats).
For further details, see Alpha / BT.
Matched filtering of a demodulated signal is not available with the MSK demodulators. The measured filter is normally off. A user defined filter can be selected. Its primary use is for additional band-limiting and channel equalization.
See Also
Filter tab (Digital Demod Properties)
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