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Keysight EXG and MXG X-Series Signal Generators User’s Guide
Custom Digital Modulation (Option 431)
Differential Encoding
Differential Encoding
Differential encoding is a digital–encoding technique whereby a binary value is denoted by a signal
change
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 table
offsets defined in the Differential State Map.
For example, consider the signal generator’s default 4QAM I/Q modulation. With a user–defined
modulation based on the default 4QAM template, the
I/Q Values
table editor contains data that
represent 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/Q
Values
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 encode
user–defined modulation schemes. The
Differential State Map
table editor is used to
introduce symbol table offset values which in turn cause transitions through the I/Q State Map
based on their associated data value. Whenever a data value is modulated, the offset value stored
in the Differential State Map is used to encode the data by transitioning through the I/Q State Map
in a direction and distance defined by the symbol table offset value.
1st Symbol
Distinct values: +1, +1
Data = 00000000
2nd Symbol
Distinct values: –1, +1
Data = 00000001
4th Symbol
Distinct values: +1, –1
Data = 00000011
3rd Symbol
Distinct values: –1, –1
Data = 00000010
1
2
3
4
