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DAC5687 Application design case - IF generation, BPSK Modulation
Hello TI E2E Community members,
I am currently working in an RF project using the DAC5687 and I decided to open a design case in order to get some feedback from the community members who are more experienced on such applications. I will keep this post updated with results and waveforms in order to work as reference for others who are interested in similar applications.
In the application I am working on, I am planning to use the DAC5687 to perform BPSK modulation to a 1 Mbps bitstream. Result will be a 70 MHz IF frequency which will be later used for up-conversion to RF and transmission. The custom designed board I am using, connects only the CLK2, CLK2C pins of the device to the clock source which makes possible only the external clock mode. The external clock source is a PLL device which gives me the possibility to select among a variety of frequencies for clocking the DAC.
Below I am summarizing some of my design decisions and some questions hoping for some feedback on them.
Regarding your questions to the FIR filters, you may refer to the High Speed Digital to Analog Converters Basics for more information. Basically, the FIR filters allows your interface rate to be slower while keeping the sample rate (DAC update rate) higher.
Since your BPSK modulation is a 1Mbps bitstream, you may need to upsample this to 32.768MSPS sample rate before going to the DAC5687. The DAC5687's 4x interpolation filter will then upsample this again to 131.072MHz.
I am concerned about your NCO frequency. If you refer to page 36 FINE MIXER section, the NCO output is limited to -fs/2 to fs/2. In another words, the output of the NCO is limited to with the 1st Nyquist zone. If your NCO frequency requirement needs to be at 71.68MHz, you may need increase your CLK2 frequency (i.e. perhaps use x8 interpolation with CLK2 frequency of 2x 131.072 = 262.144MHz. Keep in mind the input data rate of the DAC still remains at 32.768MSPS. (i.e. 1/8 of CLK2).
The 2's complement/offset binary configuration really applies to the input data format of the DAC5687 input. This set's the scale of the digital code whether to start from 0 to 65536 (offset binary) or start from -32768 to +32767. This has nothing to do with the phase inversion of the BPSK. The phase inversion will need to come from the data stream generated in the baseband.
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