ZHCSFZ0D December 2016 – December 2023 DAC38RF80 , DAC38RF83 , DAC38RF84 , DAC38RF85 , DAC38RF90 , DAC38RF93
PRODUCTION DATA
Each DUC in the DAC38RFxx has digital quadrature modulator (DQM) blocks with independent Numerically Controlled Oscillators (NCO) that converts the complex input signal to a real signal with flexible frequency placement between 0 and fDAC/2. The NCOs are enabled by fields NCOAB_ENA and NCOCD_ENA in register MULTIDUC_CFG2 (7.5.14). The NCOs have 48-bit frequency registers (FREQ_NCOAB (7.5.25) and FREQ_NCOCD (7.5.26)) and 16-bit phase registers (PHASE_NCOAB (7.5.23) and PHASE_NCOCD (7.5.24)) that generate the sine and cosine terms for the complex mixing. The NCO block diagram is shown in Figure 7-22.
Synchronization of the NCOs occurs by resetting the NCO accumulators to zero. The synchronization source is selected by fields SYNCSEL_NCOAB and SYNCSEL_NCOCD in register SYNCSEL1 (7.5.29). The frequency word in the FREQ_NCOAB and FREQ_NCOCD registers are added to the accumulators every clock cycle, fDAC.
The frequency and phase offset of the NCOs are:
Treating the complex channels as complex vectors of the form I + j Q, the output of the DQM is:
Where t is the time since the last resetting of the NCO accumulator and the fields MIXERAB_GAIN and MIXERCD_GAIN in register MULTIDUC_CFG2 (7.5.13) are either 0 or 1.
The maximum output amplitude of the DQM occurs if IIN(t) and QIN(t) are simultaneously full scale amplitude and the sine and cosine arguments are equal to an integer multiple of π/4.
With MIXERAB_GAIN or MIXERCD_GAIN = 0, the gain through the DQM is sqrt(2)/2 or -3 dB. This loss in signal power is in most cases undesirable, and it is recommended that the gain function be used to increase the signal by 3 dB to compensate. With MIXERAB_GAIN or MIXERCD_GAIN = 1, the gain through the DQM is sqrt(2) or +3 dB, which can cause clipping of the signal if IIN(t) and QIN(t) are simultaneously near full scale amplitude and should therefore be used with caution.