GC5018 Datasheet, PDF (18/115 Pages) Texas Instruments – 8-CHANNEL WIDEBAND RECEIVER

GC5018
8-CHANNEL WIDEBAND RECEIVER
SLWS169 – MAY 2005
www.ti.com
3.2.3 DDC Number Controlled Oscillator (NCO)
Frequency Sync
Frequency Word ³²
Reg
32
Reg
32
23
23
Clear
Zero Phase Sync
Phase Offset Sync
Phase Offset ¹⁶
Reg ¹⁶
Aligned
to top
32 bits
Aligned
5 _{to bottom}
5 bits
Dither
Generator
Dither Sync
20 cos
sin/cos
table
20 sin
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 will change 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 bits and is useful for reducing NCO spurious outputs. This dither
generation is enabled by setting the dither_ena bit; the magnitude of the dither can be reduced by setting
one or both of the dither_mask bits
VARIABLE
dither_ena
dither_mask(1:0)
DITHER PROGRAMMING
DESCRIPTION
When set turns dither on. Clearing turns dither off.
Masks the MSB and MSB-1 dither bits, respectively, when set.
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 (KxFs/24)
dither is ineffective – in these cases an initial phase of 4 reduces NCO spurs. The figures below 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
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RECEIVE DIGITAL SIGNAL PROCESSING