SLAS837B April 2013 – January 2017 DAC3174
PRODUCTION DATA.
NOTE
Information in the following applications sections is not part of the TI component specification, and TI does not warrant its accuracy or completeness. TI’s customers are responsible for determining suitability of components for their purposes. Customers should validate and test their design implementation to confirm system functionality.
The DAC3174 is a single-channel, 14-bit, 500-MSPS DAC with a flexible input interface (full SDR, 14-bit interface; or DDR, 7-bit interface). DAC3174 supports independent input data clock and output DAC clock, and the FIFO can be used to absorb the timing difference of two clock domains. The DAC3174 can be widely used in many applications, such as real-IF transmitter for wireless infrastructure, arbitrary waveform generator, radar, cable head-end equipment, and so on.
Figure 56 below shows an example block diagram of the DAC3174 used as a real IF transmitter to generate a modulated communication signal.
A single-carrier, WCDMA-modulated waveform of 5-MHz bandwidth must be created. The WCDMA signal is modulated up to a 900-MHz carrier using a real mixer. A real mixer creates two images of the signal about the carrier frequency and some bleed-through of the local oscillator (LO); therefore, a band-pass filter is used to filter out the undesired signal image and the local oscillator.
The data pattern file that represents the desired 5-MHz, single-carrier, WCDMA signal is created with a pattern generation. Figure 56 shows the DAC3174 being clocked by an FPGA. The data pattern file is generated with the 5-MHz, WCDMA signal centered at an intermediate frequency of 100 MHz, and a local oscillator of 800 MHz is used to upconvert the modulated signal to 900 MHz. The real mixer creates an image of the desired signal centered about 700 MHz, and there is also a LO feedthrough spur present at 800 MHz. Figure 57 illustrates a band-pass filter following the mixer that is required to remove the lower image of the signal and the LO feedthrough spur.
The choice of the intermediate frequency has an impact on the design of the 900-MHz bandpass filter. The band-pass filter passes the WCDMA signal image that is centered at 900 MHz, but provides significant attenuation of the local oscillator feedthrough and the signal image. The distance between the signal and the image is equal to twice the intermediate frequency. If the intermediate frequency is too low, the image gets too close to the signal; therefore, a higher-order band-pass filter with steep rolloff is required. If the intermediate frequency is too high, the image is further away from the signal, but the signal is too far out towards the end of the Nyquist zone, and the sinx/x distortion becomes an issue. Centering the DAC output signal at an intermediate frequency of 100 MHz is a good, balanced choice in this example, and makes the design of the band-pass filter reasonably easy.
The DAC3174 does not have an interpolation option, so the data rate for the sample data are the same rate as the sample rate to the DAC3174. In this case, choose a sample rate of 500 MSPS (a commonly used telecommunications sample rate), so that the sample data rate into the DAC3174 is also 500 MSPS.
Figure 58 shows the DAC output ACPR of a single-carrier, WCDMA-modulated signal centered at an intermediate frequency of 100 MHz.