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[DAC5688] digital up-conversion using FMIX

Chang Jin-Yeong
Prodigy 10 points
Other Parts Discussed in Thread: DAC5688

hello

 

I want to design digital up-conversion in DAC5688.

in dac5688, it support complex mixer.

so, To get  dac fout = 70MHz,  i used fdata = 92.16Msps,  fin= -22.16MHz, fdac = 368.64MHz, nco = 92.16MHz(fdac/4), 4x interpolation

but it shows two freqency fout = 70MHz and fout = 114.32MHz

possibly, if FMIX is operated in real mode, FMix output is Iout = Iin(t)*cosine(fnco) and Qout = Qin(t)*sine(fnco). so i get two frequency

how do i controll dac5688 for one fout = 70MHz??  how can use compelx mix in DAC5688?

 

I am sorry for my awkward English. ^^;;;;

 

dac5688 register value is below;

 Address Data

 01  0A

 02  01

 03  00

 04  00

 05  12

 06  00

 07  00

 08  00

 09  00

 0A  00

 0B  40

 0C  00

 0D  00

 0E  00

 0F  36

 10  00

 11  00

 12  00

 13  00

 14  00

 15  00

 16  00

 17  10

 18  80

 19  00

 1A  8E

 1B  88

 1C  00

 1D  00

 1E  00

 

 

 

  • 0
    TI__Guru* 78174 points

    Hi Jin-Yeong,

    There could be two possibilities for this to happen. The first one is that the input digital signal to the DAC5688 is not a complex signal (I + jQ). In order for the NCO and complex mixer to function correctly, the input signal must be complex. The second possibility is that the NCO and complex mixer was not synchronized correctly. Your default register configuration set the NCO to look for a low to high transition on the TXENABLE pin to synchronize the NCO. For an easier synchronization and debugging purpose, you may want to try using software sync. You can first set the NCO to look for the software sync signal (0x16 = 0xA0). To synchronize the NCO, simply toggle bit 6 of the register 0x05. 

    To eliminate the possibility that the error is in the digital data, try to set the DAC5688 data path with a constant input. The NCO will then output a tone at the exact NCO frequency that you have set. For instance, if the input format is offset binary, the constant data of 0 or 65535 will create a full-scale tone at 92.16MHz. 

    CONFIG4, bit7 to enable ser_dac_data_ena

    CONFIG 20 and 21: constant data. Value depends on 2’s complement or offset binary