The applet above shows the harmonic images (red) and spurs (blue) for single frequency output from an AD9777. The model of the AD9777 is simplified and idealized - only SFDR is modeled and it is assumed frequency-independent. See datasheet for actual performance data.
For an ordinary DAC (see our interactive DAC Harmonic Images calculator), images are located at N*FDAC +/- FOUT and follow a sin(x)/x envelope (shown by default). The AD9777 contains an integral interpolator which doubles, quadruples or octuples the input data rate by stuffing zeroes between successive samples and then (optionally) filtering the result. 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 magnitude response of the AD9777 combining its internal interpolation with the sin(x)/x envelope is shown in light red.
The AD9777 can optionally modulate the filtered data stream by FDAC / 2, 4 or 8, shifting the output spectrum and creating mirror images around the modulating frequency. These images follow separate magnitude envelopes so two envelopes are shown in different colors in this case. This release only models real modulation; complex modulation is planned for a future release.
Spurious 2nd or 3rd harmonics of each image are assumed to result from D/A conversion 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).
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 interpolation mode: none, 2X, 4X or 8X.
Select the modulation mode: none, FDAC/2, FDAC/4 or FDAC/8.
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 analog filter rolloff and type. The analog filter magnitude is shown in green.
Image frequencies and amplitudes are also shown in a table that is hidden behind the graph by default. Clicking on "Table" at top brings it to the front (and clicking on "Graph" pushes it to the back). The first column gives the DAC multiple, N, and then a sequence number for the image around that multiple. The table data is selectable and can be copied and pasted into a spreadsheet.
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.