GC5016 Datasheet, PDF (17/88 Pages) Texas Instruments – WIDEBAND QUAD DIGITAL DOWN CONVERTER/ UP CONVERTER

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GC5016
SLWS142G − JANUARY 2003 − REVISED NOVEMBER 2005
Phase
Offset
Dither
Generator
48
16
7
48
Frequency Word
23
18
Sine/Cosine
20
Lookup Table
Sine/Cosine Out
Figure 6. Numerically Control Oscillator (NCO) Circuit
The NCOs can be synchronized with NCOs on other chips. This allows multiple down converter outputs to be
coherently combined, each with a unique phase and amplitude. The NCO’s frequency, phase and accumulator can
be initialized and synchronized with other channels using the freq_sync, phase_sync, and nco_sync controls. The
freq_sync and phase_sync controls determine when new frequency and phase settings become active. Normally,
these are set to Always so that they take effect immediately, but can be used to synchronize frequency hopping or
beam forming systems. The nco_sync control is usually set to Never, but can be used to synchronize the NCOs of
multiple channels.
The NCO’s spur level is reduced to below −113 dB through the use of phase dithering. The spectrums in Figure 7
show the NCO spurs for a worst case tuning frequency with and without dithering. Dithering decreases the spur level
from −105 dB to −116 dB. Dithering is turned on or off using the dith_sync controls. Holding dith_sync always on
freezes the dither value, effectively turning off dither.
NCO OUTPUT POWER
vs
FREQUENCY
0
FREQ = 5/24 fS
NCO OUTPUT POWER
vs
FREQUENCY
0
FREQ = 5/24 fS
−50
−105 dB
−50
−116 dB
−100
−100
−150
0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0
Frequency − fS
a) Worst Case Spectrum Without Dither
−150
0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0
Frequency − fS
b) Spectrum With Dither (Tuned to Same Frequency)
Figure 7. Example NCO Spurs With and Without Dithering
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