Digital Upconverter

A digital upconverter (DUC) accepts a complex I/Q waveform as its input. The DUC then converts this complex waveform to a real waveform with the same frequency content but centered at a higher IF center frequency. In other words, the baseband signal is "upconverted" to IF, and this upconversion is done digitally in DUC firmware.

The DUC is a set of algorithms implemented in an FPGA. Two main operations are performed by these algorithms: resampling and upconversion. The FPGA resamples the baseband I/Q waveform from the given I/Q rate to the digital-to-analog converter (DAC) sample rate (100 MS/s). The DUC then multiplies the resampled I/Q waveform by an internally-generated complex IF carrier. This multiplication produces a waveform with a center frequency that matches the IF carrier frequency and frequency content that matches the baseband I/Q waveform, and this upconverted waveform is then fed into the DAC.


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Note The PXIe-5672 uses the DUC in the PXI-5442 AWG module when the I/Q rate is set to 8.33 MS/s or less.

The DUC offers the following benefits:

  • Lower I/Q rates configurable in smaller intervals:
    • Reduces the onboard memory required for a given waveform. For example, for a 1 MHz wide signal, you can set the I/Q rate to 1.25 MS/s, meaning that each second of the waveform duration uses 1.25 million samples of onboard memory. If you set the I/Q rate to 100 MS/s, each second of the waveform duration uses 100 million samples of onboard memory.
    • Increases the maximum duration of a waveform; the smaller the I/Q rate, the longer a waveform can be generated without repetition.
  • Guarantees that a phase-continuous input waveform remains phase continuous upon generation, regardless of the setting of the Phase Continuity Enabled property or the NIRFSG_ATTR_PHASE_CONTINUITY_ENABLED attribute.
  • Frequency accuracy is not constrained by the Frequency Tolerance property or the NIRFSG_ATTR_FREQUENCY_TOLERANCE attribute. NI-RFSG ignores the Frequency Tolerance property or the NIRFSG_ATTR_FREQUENCY_TOLERANCE attribute and sets the frequency as accurately as possible for the hardware.
  • Changing the RF frequency does not cause the arbitrary waveform to be rewritten to the AWG module. This capability can dramatically speed up applications that generate the same signal at multiple RF frequencies.