Keysight EXG and MXG X-Series Signal Generators User’s Guide 305Using Custom Digital Modulation for N5172B/82B with Option 431 and 653/655/656/657Differential EncodingDifferential EncodingDifferential encoding is a digital–encoding technique whereby a binary value is denoted by a signalchange rather than a particular signal state. Using differential encoding, binary data in any user–defined I/Q or FSK modulation can be encoded during the modulation process via symbol tableoffsets defined in the Differential State Map.For example, consider the signal generator’s default 4QAM I/Q modulation. With a user–definedmodulation based on the default 4QAM template, the I/Q Values table editor contains data thatrepresent four symbols (00, 01, 10, and 11) mapped into the I/Q plane using two distinct values,1.000000 and –1.000000. The following illustration shows the 4QAM modulation in the I/QValues table editor.The following illustration shows a 4QAM modulation I/Q State Map.Differential encoding employs relative offsets between the states in the symbol table to encodeuser–defined modulation schemes. The Differential State Map table editor is used tointroduce symbol table offset values which in turn cause transitions through the I/Q State Mapbased on their associated data value. Whenever a data value is modulated, the offset value storedin the Differential State Map is used to encode the data by transitioning through the I/Q State Mapin a direction and distance defined by the symbol table offset value.1st SymbolDistinct values: +1, +1Data = 000000002nd SymbolDistinct values: –1, +1Data = 000000014th SymbolDistinct values: +1, –1Data = 000000113rd SymbolDistinct values: –1, –1Data = 00000010123 4
