NI PXIe-5641R Digital Downconverter (DDC) Operation

The NI PXIe-5641R digital downconverters inside each ADC perform IF to I/Q conversion. This operation consists of the following tasks:

• Frequency translation
• Band-selective filtering
• Sampling rate reduction

The band of interest is centered at a specified carrier frequency. The DDC extracts the band of interest from the digital data stream produced by the ADC, and reduces the sampling rate because only a rate sufficient to cover the bandwidth of interest is needed. Sampling rate reduction vastly reduces computational burden; demodulation and other signal processing can be performed on a much lower data rate commensurate with modulation bandwidth and independent of carrier frequency. These signal processing operations are done in the digital domain.

The following figure illustrates DDC operation. Specific values are shown only for illustration purposes.

In the preceding figure, the 100 MS/s input digital data rate is reduced to a 10 MS/s I/Q rate to cover a theoretical maximum signal bandwidth of 10 MHz. Practically, the bandwidth of the signal of interest must be less than 10 MHz in this example to allow for the digital decimation filter transition band.

Note  The output sample rate is independent of the carrier frequency and only depends on the bandwidth of the signal of interest.

The filters provided with the NI-5640R driver software are designed to reduce the chances of aliasing. If you are not using the NI-5640R instrument driver or the LabVIEW FPGA example VIs, the following considerations are important to remember if you design your own DDC filters:

• Proper scaling to avoid numerical overflows in the digital data path
• Appropriate alias rejection and passband flatness

Note  An image frequency is generated in the DDC digital data path as a result of digital mixing of the NCO and digitized input data stream. The DDC digital filters must be designed to properly attenuate this image frequency to prevent artifacts such as distortion and overflow of the digital data path.

If the image frequency is not attenuated properly, numerical overflows can occur inside the DDC because the signal and the image frequency can combine to create larger peaks than those of the main signal frequency alone.

At input frequencies that are integer multiples of 0.5 × F_S, the signal and the image frequency overlap, making it impossible to attenuate the image. For example, using an ADC sampling rate of 100 MS/s and an input frequency of 50 MHz, the image is impossible to attenuate. Thus an input power spectrum shows wide variation in signal power level because of a phase-dependant cancellation or summation of signal frequency and image frequency.