National Instruments 5411 - Manual (Page 35)

Chapter 4 Arb Operation
DAQArb 5411 User Manual 4-10 © National Instruments Corporation
Frequency Resolution and Lookup Memory
For DDS-based waveform generation, you must first load one cycle of
the desired waveform into the lookup memory. The size of the DDS
lookup memory is 16,384 samples. Each sample is 16 bits wide.
Note: One cycle of the waveform buffer loaded into the memory should be exactly
equal to the size of the DDS lookup memory.
F
c
= update clock for the accumulator
Set the DAQArb 5411 at F
c
= 40 MHz.
F
a
= desired frequency of the output signal
N = accumulator size in bits
Set the DAQArb 5411 at N = 32.
FCW = frequency control word to be loaded into the accumulator
to generate F
a
.
This is calculated using the formula:
FCW = (2
N
* F
a
) / F
c
The frequency resolution is then given by:
frequency resolution = F
c
/ 2
N
= (40 x 10
6
) / 2
32
= 9.31322 mHz
For example, if you need to generate a frequency of 10 MHz, then the
FCW is (2
32
* 10E6)/40E6, which equals 1,073,741,824. If you need to
generate a frequency of 1 Hz, then the FCW is (2
32
* 1)/40E6, which
equals 107.
Note: On the DAQArb 5411, the maximum frequency of a sine wave you can
generate reliably is limited to 16 MHz. Other waveforms like square or
triangular waves are limited to 1 MHz.
You can also synthesize arbitrary waveforms using DDS. Generating
arbitrary waveforms this way will be very limited; you are restricted to
a single buffer, and this buffer should be exactly equal to the size of the
lookup memory.
To update every next sample of an arbitrary waveform in lookup
memory at the maximum clock rate of 40 MHz, write an FCW value of
2
(N-L)
, where N is the size of the accumulator and L is the number of
address bits of lookup memory (L = 14 bits for the AT-5411 and the
PCI-5411). Thus, the FCW value for the DAQArb 5411 equals 262,144.