The applet above shows the harmonic images (red) and spurs (blue) for single frequency output from an AD9772. The model of the AD9772 is simplified and idealized - only SFDR is modeled and it is assumed frequency-independent. The response characteristics of the internal digital filter have been approximated. See datasheet for actual performance data.
For an ordinary DAC (see our interactive DAC Harmonic Images calculator), images are located at N*FDAC +/- FOUT. The AD9772 contains an integral interpolator which doubles the input data rate creating an image of the output frequency mirrored about FDATA/2. An interpolation filter suppresses the upper image in low-pass mode (MOD0=0), or suppresses the fundamental when in high-pass mode (MOD0=1). Both the filtered and unfiltered images then create further images and spurs at the DAC data rate, according to the N*FDAC +/- FOUT rule.
The AD9772 also has a "zero-stuffing" mode (MOD1=1) which allows the data stream to be doubled a second time by inserting zeroes between each sample. Zero-stuffing doubles again the number of images per FDAC harmonic, but FDAC is twice what it would be without zero-stuffing and the mathematics work so that the location of the images is the same as without zero-stuffing - only the amplitudes change. These new images are not filtered internally so that the upper image can be used for direct IF synthesis. MOD0 and MOD1 are often used together for this purpose.
Spurious 2nd or 3rd harmonics of each image are assumed to result from D/A nonlinearities and so are folded within the first Nyquist Zone (NZ) of FDAC. These spurs then have their own harmonic images that roll off as sin(x)/x (where x = pi*FSPUR / FDAC). The magnitude response of the AD9772 combining its internal interpolation with the sin(x)/x envelope is shown in light red.
To show external selection/suppression of desired/undesired images and spurs, the applet can apply a simulated post-DAC analog filter.
How to use this applet:
Enter FDATA and the Output Frequency in the provided text fields.
Hit "Enter" or click "Update" to recompute the display.
Select the Start and Stop Frequencies of the Analog Filter. A zero turns off that portion of the filter, i.e. entering a zero for the start frequency makes the filter low-pass only.
Select filter rolloff and type. The analog filter magnitude is shown in green.
Image frequencies and amplitudes are shown in a table at top right. The first column gives the DAC multiple, N, and then a sequence number for the image around that multiple. For non-zero-stuffing images, the order of these images for N>0 is N*FDAC + 1) -FOUT 2) +FOUT 3) -(FDATA-FOUT) and 4) +(FDATA-FOUT). The table data is selectable and can be copied and pasted into a spreadsheet. Use "Select all" to conveniently select the entire contents before copying.
SFDR is used to set the relative level of spurs, which are assumed here to be the result of D/A nonlinearity. SFDR Harmonic selects whether these distortion spurs are most prominent at
3X (default) an interpolation / zero-stuffing image.
In reality, SFDR depends on both the sample rate and output frequency, among other variables. However, a single compromise number is used here.
The images and spurs can be animated by clicking the "Anim." checkbox. Note: this feature requires significant additional memory and may not function correctly on all browsers. A symptom of insufficient memory is that the display will flicker during animation.