292 Keysight EXG and MXG X-Series Signal Generators User’s GuideUsing Custom Digital Modulation for N5172B/82B with Option 431 and 653/655/656/657Using Finite Impulse Response (FIR) Filters with Custom ModulationUsing Finite Impulse Response (FIR) Filters with Custom ModulationFinite Impulse Response filters can be used to refine the transitions between symbol decision pointsof the generated waveforms.Finite Impulse Response filters can be created and used with both dual arbitrary waveformgenerated waveforms and real-time baseband generated waveforms.Figure 9-31 Filter MenuUnderstanding FIR FiltersFIR filters are used to limit the bandwidth of the input to the I and Q modulators. Several differenttypes of FIR filters exist. The NADC, PDC, PHS, and TETRA standards specify a root Nyquist filter inboth the transmitter and the receiver. The combined response is equivalent to a Nyquist filter. TheNyquist filter has an impulse response that rings at the data clock rate so nulls appear at all symboldecision points except the desired one at the center of the impulse response. Since each symbolcauses zero response at all undesired decision points, there can be no inter-symbol interference(ISI). The alpha term (a) defined for Nyquist-type filters identifies the frequency cutoff point werethe filter response is zero. The closer the alpha term is to zero, the steeper the filter roll-offbecomes. Alpha gives a direct measure of the occupied bandwidth of the system and is calculatedasMode > ARB Custom Modulation > Single Carrier Setup > FilterThis replaces the currentFIR filter with the factoryinstalled default filtersettings for the currentmodulation type.This softkey changes, dependingon the selected filter–type.Available only when the filter selected =Root Nyquist or NyquistAvailable only when the filterselected = Gaussianpage 299Only applies to Dual ARBModulation. Refer topage 151.For details on each key, use key help as described on page 56.Opens the IS–95filter selectionmenu.page 294
