UG-272
Evaluation Board User Guide
SELECT EXTERNAL MCLK FREQUENCY
FSK AND PSK FUNCTIONALITY
Having selected the digital interface specifics, next use the
EXTERNAL MCLK box to choose which frequency to use. The
boards are supplied with a 25 MHz general oscillator. If a different
clock source is required, the CLK1 SMB connector can be used
to supply a different MCLK value.
In software mode, the AD9833 can be set up for FSK or PSK
functionality by simply entering the bit rate in milliseconds and
selecting the push-button option (see Figure 8).
Two options for the general oscillator include the AEL3013
oscillators from AEL Crystals and the SG-310SCN oscillators
from Epson Electronics.
Figure 8. FSK and PSK Functionality
WAVEFORM OPTIONS
The output waveform can be selected as a sinusoidal waveform
or a ramp waveform. The internal comparator in the AD9833
can be disabled or enabled (see Figure 9). The MSB or the
MSB/2 of the phase accumulator can be selected as the output
on the SIGN BIT OUT pin.
Figure 6. EXTERNAL MCLK Input
LOADING FREQUENCY AND PHASE REGISTERS
The desired output frequency and output phase can be loaded
using the inputs shown in Figure 7. Either the FREQ 0 register or
the FREQ 1 register can be loaded with frequency data. The
frequency data is loaded in megahertz, and the equivalent hex code
is shown to the right once data is entered; click Enter to load
data. Once data is loaded, the output appears on the IOUT1 and
IOUT2 pins. Similarly, either the PHASE 0 register or PHASE 1
register can be selected, and the phase data is loaded in degrees.
Figure 9. Waveform Profile and SIGN BIT OUT Pin
Power-Down Options
The AD9833 has various power-down options selected through
the control register. The part can disable the MCLK or disable
the DAC if just the MSB output is used on the SIGN BIT OUT
pin, or it can power down both sections for a lower power sleep
mode (see Figure 10).
The analog output frequency from the AD9833 is defined by
f
MCLK/228 × FREQREG
where FREQREG is the value loaded into the selected frequency
register in decimals. This signal is phase shifted by
2π/4096 × PHASEREG
where PHASEREG is the value contained in the selected phase
register in decimals.
Figure 10. Power-Down Options
Figure 7. Frequency and Phase Load
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