About Filters (Custom IQ)

Menu Path: MeasSetup > Custom IQ Demod Properties > Filter tab

The following topics explain the custom IQ filters and configuration parameters:

Custom IQ Filter Overview

Data filtering is used in custom IQ 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 further details, see Alpha / BT.

The shape and width of a filter is defined by the alpha (for cosine filters) or the BT Bandwidth Time Product (for Gaussian filters). The Alpha/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.

The VSA's Custom IQ demodulator produces two signals: a measured and a reference signal. These signals are called IQ Measured (IQ Meas) and IQ Reference (IQ Ref).

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.

Selecting the Correct Filter

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 Alpha/BT.

As an example, if the communication system under test filters the signal partially at the transmitter and partially at the receiver, 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

Selecting the Alpha or BT for a Filter

The shape and width of 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 21 symbols in length by default.

For Root Raised Cosine filters, the Filter Length that should be used is dependent on the filter alpha:

Alpha Filter Length

>= 0.2

30 symbols

< 0.2

40 symbols

< 0.1

80 symbols

< 0.03

120 symbols

For more information, see Alpha / BT.

List of Available Digital Demod Filters

See Also

Filter tab (Custom IQ Properties)