Specifications


SLWS142J − JANUARY 2003 − REVISED AUGUST 2007
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17
Frequency Word
48
Sine/Cosine Out
48
16
Phase
Offset
23
7
Dither
Generator
18
Sine/Cosine
Lookup Table
20
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.
a) Worst Case Spectrum Without Dither b) Spectrum With Dither (Tuned to Same Frequency
)
−150
−100
−50
0
0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0
Frequency − f
S
FREQ = 5/24 f
S
NCO Output Power − dB
NCO OUTPUT POWER
vs
FREQUENCY
−105 dB
−150
−100
−50
0
0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0
Frequency − f
S
FREQ = 5/24 f
S
NCO Output Power − dB
NCO OUTPUT POWER
vs
FREQUENCY
−116 dB
Figure 7. Example NCO Spurs With and Without Dithering