GC5316 datasheet(38/75 Pages) TI

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GC5316

SLWS154A − JANUARY 2004 − REVISED MARCH 2004

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The NCO spurious levels are better than –115 dBC. Added phase dither randomizes the ROM lookup slightly, hence

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 32 and Figure 33 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 32. Example NCO Spurs With and Without Dithering

a) Plot Without Dither or Phase Initialization

b) Plot With Dither and Phase Initialization

Figure 33. NCO Peak Spur Plot

The tuning frequency is specified as a 32-bit frequency word and is programmed as three sequential 16-bit words

over the control port. The NCO operates at the same speed as the txclk / (tadj_interp_decim + 1). The frequency

resolution is simply the Fclk/ 232. The NCO frequency resolution is simply the Fclk/ 232. As an example, at an input

clock rate of 61.44 MHz, the frequency step size would be approximately 14 MHz. The frequency word is determined

by the formula:

Frequency word (in decimal) = 232 x Tuning Frequency / Fclk

NOTE: Frequency tuning words can be positive or negative valued. Specifying a positive frequency value translates

baseband frequencies upward. Specifying a negative tuning frequency translates baseband frequencies downwards.

Table 13. Programming

VARIABLE

DESCRIPTION

phase_add_a(31:0)

32-bit tuning frequency word for the A signal when in CDMA mode. Also for UMTS mode.

phase_add_b(31:0)

32-bit tuning frequency word for the B signal when in CDMA mode. Not used in UMTS mode.

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