GC5316 Datasheet, PDF (10/75 Pages) Texas Instruments – HIGH-DENSITY DIGITAL DOWNCONVERTER AND UPCONVERTER

GC5316
SLWS154A − JANUARY 2004 − REVISED MARCH 2004
2.3.2 Receive Number Controlled Oscillator (NCO)
www.ti.com
Frequency Syn c
32
32
Fre quency Word
32
R eg
R eg
Clr
Zero Phase Sync
20
23
18 _{Sin/ Cos}
Cos
Table
20
S in
5
Phase Offset Sync
R eg
16
Phase Offset
D ith er
G en
Dither Sync
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
dither_ena(2)
test_bits_1(1:0)
DESCRIPTION
When set turns dither on. Clearing turns dither off.
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
10