GC5318 datasheet(18/45 Pages) TI

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a) Worst Case Spectrum Without Dither

b) Spectrum With Dither (Tuned to Same Frequency)

a) Plot Without Dither or Phase Initialization

b) Plot With Dither and Phase Initialization

GC5318

HIGH-DENSITY DIGITAL UPCONVERTER

SLWS187 – JULY 2006

Table 2-12. Programming

VARIABLE

DESCRIPTION

dither_ena

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.

Figure 2-13. Example NCO Spurs With and Without Dithering

Figure 2-14. NCO Peak Spur Plot

GC5318 Operation

GC5318 Datasheet(PDF) 18 Page - Texas Instruments