GC5316 datasheet(10/75 Pages) TI

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GC5316

SLWS154A − JANUARY 2004 − REVISED MARCH 2004

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2.3.2 Receive Number Controlled Oscillator (NCO)

Fre quency Word

Phase Offset

Dith er

Gen

Dither Sync

Frequency Syn c

Zero Phase Sync

Phase Offset Sync

Reg

Sin/ Cos

Table

Cos

Sin

Clr

Reg

Figure 8. Receive Number Controlled Oscillator

The NCO is a digital complex oscillator that is used to translate (or downconvert) an input signal of interest to

baseband. The block produces programmable complex digital sinusoids by accumulating a frequency word which

is programmed by the user. The output of the accumulator is a phase argument that indexes into a Sin/Cos ROM

table which produces the complex sinusoid. A phase offset can be added prior to indexing if desired for channel

calibration purposes. This changes the Sin/Cos phase with respect to other channels’ NCOs.

A 5-bit dither generator is provided and generates a small level of digital pseudo-noise that is added to the phase

argument below the bottom bit and is useful for reducing NCO spurious outputs.

Table 8. Programming

VARIABLE

DESCRIPTION

dither_ena(2)

When set turns dither on. Clearing turns dither off.

test_bits_1(1:0)

Test bits. MUST be cleared for normal operation.

The NCO spurious levels are better than –115 dBC. Added phase dither randomizes the periodic nature of the phase

accumulation process and reduces low-level spurious energy. For some frequencies (K

×Fs/24), dither is ineffective

– in these cases an initial phase of four reduces NCO spurs. Figure 9 and Figure 10 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 9. Example NCO Spurs With and Without Dither

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