Unexpected Harmonics When Enabling Full Mixer with NCO on NI 5783 Adapter Card

Issue Details

When configuring the NI 5783 adapter card to use the Full Mixer with Numerically Controlled Oscillator (NCO), the output spectrum may exhibit multiple harmonics spread across the frequency range. This behavior can occur even when the mixer is enabled without the NCO, and enabling the NCO alone does not change the output.

This issue is typically caused by missing the NCO reset sequence after configuring the NCO frequency registers.

Solution

To ensure correct operation of the Full Mixer with NCO, follow the recommended configuration and reset sequence:

 Configuration Steps

1. Set NCO Frequency Registers

   • config21 (0x15): 0x2012
   • config20 (0x14): 0xB797
     (These values correspond to a 100 MHz center frequency)

2. Enable Mixer and NCO

   • config2: 0x80D2
     (Mixer ON, NCO ON)

3. Reset NCOs via Serial Interface Sync

   • config31 (0x1F): 0x8882 (Set sif_sync bit)
   • config31 (0x1F): 0x8880 (Clear sif_sync bit)

This reset sequence ensures that the NCOs are properly synchronized and initialized, resulting in clean ±20 MHz tones around the 100 MHz center frequency.

Additional Information

1. Refer to (External) Table 10 of the DAC3484 datasheet for detailed register descriptions and initialization steps.
2. The coarse mixer using the NI FlexRIO documentation available at:
   C:\Users\Public\Documents\National Instruments\FlexRIO\Adapter Module Documentation\FRIOtransAMR.chm.