GC5316_14 Datasheet, PDF (38/76 Pages) Texas Instruments – HIGH-DENSITY DIGITAL DOWNCONVERTER AND UPCONVERTER

GC5316
SLWS154A − JANUARY 2004 − REVISED MARCH 2004
www.ti.com
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 F_clk/ 2³². The NCO frequency resolution is simply the F_clk/ 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 / F_clk
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
phase_add_a(31:0)
phase_add_b(31:0)
DESCRIPTION
32-bit tuning frequency word for the A signal when in CDMA mode. Also for UMTS mode.
32-bit tuning frequency word for the B signal when in CDMA mode. Not used in UMTS mode.
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