Altera NCO MegaCore Function - Manual (Page 23)

Figure 3-4: Frequency Hopping Block Diagram
Numerically
Controlled
Oscillator
fcos_o
out_valid
Avalon-MM
Interface
clk
reset_n
reset_n
address
write_sig
increment
freq_sel_sig
16 to 1
MUX
clken
RAM
fsin_0
phi_inc_i
clken
clk
NCO MegaCore Function
The RAM stores all hopping frequencies. The RAM size is <width>×<depth>, where <width> is the
number of bits required to specify the phase accumulator value to the precision you select in the
parameter editor, and <depth> is the number of bands you select in the parameter editor.
Phase Dithering
All digital sinusoidal synthesizers suffer from the effects of finite precision, which manifests itself as spurs
in the spectral representation of the output sinusoid. Because of angular precision limitations, the derived
phase of the oscillator tends to be periodic in time and contributes to the presence of spurious frequencies.
You can reduce the noise at these frequencies by introducing a random signal of suitable variance into the
derived phase, thereby reducing the likelihood of identical values over time. Adding noise into the data
path raises the overall noise level within the oscillator, but tends to reduce the noise localization and can
provide significant improvement in SFDR.
The extent to which you can reduce spur levels is dependent on many factors. The likelihood of repetition
of derived phase values and resulting spurs, for a given angular precision, is closely linked to the ratio of
the clock frequency to the desired output frequency. An integral ratio clearly results in high-level spurious
frequencies, while an irrational relationship is less likely to result in highly correlated noise at harmonic
frequencies.
The Altera NCO IP core allows you to finely tune the variance of the dither sequence for your chosen
algorithm, specified precision, and clock frequency to output frequency ratio, and dynamically view the
effects on the output spectrum graphically.
Related Information
NCO Multichannel Design Example on page 4-1
3-6 Phase Dithering
UG-NCO
2014.12.15
Altera Corporation NCO IP Core Functional Description
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