Circuit provides low-cost QAM mapping and translation

This Design Idea presents an efficient way to do QAM (quadrature-amplitude-modulation) mapping and translation into two’s-complement values with only two inverters and no look-up tables.

Suppose you want to create a 256-level QAM signal using a microcontroller and two 10-bit DACs with a parallel input in two’s-complement notation. Because you can split a 256-level QAM signal into a 16-level ASK (amplitude-shift-keying) signal for the in-phase component and a 16-level ASK for the quadrature component, a symmetrical approach is feasible. The fully symmetrically circuit performs the 16-level ASK mappings and translations (Figure 1). Two inverters are the only glue logic you need for the conversion. Each part of the circuit converts four output bits of the microcontroller into a 10-bit two’s-complement vector, which feeds directly to the DACs (Table 1). The possible DAC-input values are equally distributed. The third column of Table 1 gives the normalized DAC output after an optional current-to-voltage conversion.

For 256-level QAM signals, you need 8 input bits, which exactly fit the width of a general-purpose-I/O bank on most microcontrollers. Simultaneously setting all 8 bits ensures synchronization between in-phase and quadrature signals. You can easily adapt this circuit for any QAM constellation or DAC resolution. Because this circuit is fully digital, you can also embed it in FPGAs or CPLDs, using the inverters available in the output buffers.

Caption

Figure 1 This circuit converts 234-bit outputs from the microcontroller into 2312-bit, 16-level ASK values.

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Figure 1, Table 1