GC5318 Datasheet, PDF (18/45 Pages) Texas Instruments – HIGH-DENSITY DIGITAL UPCONVERTER

GC5318
HIGH-DENSITY DIGITAL UPCONVERTER
SLWS187 – JULY 2006
Table 2-12. Programming
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VARIABLE
dither_ena
DESCRIPTION
When set, turns dither on. Clearing turns dither off.
The NCO spurious levels are shown in Figure 2-13 and Figure 2-14. Added phase dither randomizes the
ROM lookup slightly, thereby creating the ROM lookup error—spreading the spurious energy around
rather than concentrating it in a few frequencies. The phase dither is added below the LSB of the ROM
lookup. If the tuning frequency has no high bits more than 17 bits below the msb, the phase dither has no
effect. If the tuning frequency is a multiple of Fs/96, then an initial phase offset of four often reduces NCO
spurs. Figure 2-13 and Figure 2-14 show the spur level performance of the NCO without dither, with
dither, and with a phase offset value.
a) Worst Case Spectrum Without Dither
b) Spectrum With Dither (Tuned to Same Frequency)
Figure 2-13. Example NCO Spurs With and Without Dithering
a) Plot Without Dither or Phase Initialization
b) Plot With Dither and Phase Initialization
Figure 2-14. NCO Peak Spur Plot
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GC5318 Operation