User Manual DDS200 Digital Demodulation System 070-9952-00

Manual OperationSPECIAL FUNCTION MenuIn the SPECIAL FUNCTION menu that is selected in the MAIN FUNCTIONblock, special settings can be made for values that need not be changed in normaloperation (see Figure 2–29). In this menu, the instrument can be set to a statewhere normal operation is not always guaranteed. If a special function has beenselected, the yellow LED next to the key lights and turns off only after allfunctions have been reset to standard values. In the menu, default values aremarked in bold (refer to Preset Values and Menu Overview on page 2–58).Default values are selected in the following example.Figure 2–29: SPECIAL FUNCTION menuEQUALIZER. The operating mode of the channel equalizer is set with theEQUALIZER... key.1. EQUALIZER MODE:In the EQUALIZER MODE submenu, the equalizer can be frozen(FREEZE) in the adaptive mode. The equalizer can also be completelyswitched off (OFF).2. CENTRAL TAP POSITION:The equalizer is a complex 24-stage FIR filter giving 48 coefficients as aresult of the real and the imaginary parts. The processing of high-order QAMsignals places considerable demands on the equalizer capability. Consideringthe mentioned length, the type of equalizer used here is more complex thanmost other types. For simulating a shorter equalizer, the position of thecentral coefficient (CENTRAL TAP) my be set between 0 and 23.2–34 DDS200 Digital Demodulation System User Manual