Altera NCO MegaCore Function - Manual (Page 19)

where M is the accumulator precision and f
clk
is the clock frequency
The minimum possible output frequency waveform is generated for the case where ϕ
inc
= 1. This case is
also the smallest observable frequency at the output of the NCO, also known as the frequency resolution
of the NCO, f
res
given in Hz by the equation:
f
RES
= f
clk
/2M Hz
For example, if a 100 MHz clock drives an NCO with an accumulator precision of 32 bits, the frequency
resolution of the oscillator is 0.0233 Hz. For an output frequency of 6.25 MHz from this oscillator, you
should apply an input phase increment of:
(6.25 x 10
6 /100 x 10 6 ) x 2 32 = 268435456
The NCO MegaCore function automatically calculates this value, using the specified parameters. IP
Toolbench also sets the value of the phase increment in all testbenches and vector source files it generates.
Similarly, the generated output frequency, f
FM
for a given frequency modulation increment, ϕ
FM
is
determined by the equation:
f
FM
= ϕ
FM
f
clk
/2 F Hz
where F is the modulator resolution
The angular precision of an NCO is the phase angle precision before the polar-to-cartesian transforma‐
tion. The magnitude precision is the precision to which the sine and/or cosine of that phase angle can be
represented. The effects of reduction or augmentation of the angular, magnitude, accumulator precision
on the synthesized waveform vary across NCO architectures and for different f
o
/f
clk
ratios.
You can view these effects in the NCO time and frequency domain graphs as you change the NCO IP core
parameters.
NCO IP Core Architectures
The NCO MegaCore function supports large ROM, small ROM, CORDIC, and multiplier-based
architectures.
Large ROM Architecture
Use the large ROM architecture if your design requires very high speed sinusoidal waveforms and your
design can use large quantities of internal memory.
In this architecture, the ROM stores the full 360 degrees of both the sine and cosine waveforms. The
output of the phase accumulator addresses the ROM.
The internal memory holds all possible output values for a given angular and magnitude precision. The
generated waveform has the highest spectral purity for that parameter set (assuming no dithering). The
large ROM architecture also uses the fewest logic elements (LEs) for a given set of precision parameters.
Small ROM Architecture
To reduce LE usage and increase output frequency, use the small ROM architecture.
In a small ROM architecture, the device memory only stores 45 degrees of the sine and cosine waveforms.
All other output values are derived from these values based on the position of the rotating phasor on the
unit circle.
3-2 NCO IP Core Architectures
UG-NCO
2014.12.15
Altera Corporation NCO IP Core Functional Description
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