AD9858BSV Analog Devices Inc, AD9858BSV Datasheet - Page 17

• Manufacturer Part Number: AD9858BSV
• Manufacturer: Analog Devices Inc

Specifications of AD9858BSV

• Lead Free Status / RoHS Status: Not Compliant

Single-Tone Mode
When in single-tone mode, the AD9858 generates a signal, or
tone, of a single desired frequency. This frequency is set by the
value loaded by the user into the chip’s FTW register. This
frequency can be between 0 Hz and somewhat below one-half
of the DAC sampling frequency (SYSCLK). One-half of the
sampling frequency is commonly called the Nyquist frequency.
The practical upper limit to the fundamental frequency range of
a DDS is determined by the characteristics of the external low-
pass filter, known as the reconstruction filter, which must follow
the DAC output of the DDS. This filter reconstructs the desired
analog sine wave output signal from the stream of sampled
amplitude values output by the DAC at the sample rate (SYSCLK).
A DDS is a sampled data system. As the fundamental frequency
of the DDS approaches the Nyquist frequency, the lower first
image approaches the Nyquist frequency from above. As the
fundamental frequency approaches the Nyquist frequency, it
becomes difficult, and finally impossible, to design and construct a
low-pass filter that provides adequate attenuation for the first
image frequency component.
The maximum usable frequency in the fundamental range of
the DDS is typically between 40% and 45% relative to the SYSCLK
frequency, depending on the reconstruction filter. With a 1 GHz
SYSCLK, the AD9858 is capable of producing maximum output
frequencies of between 400 MHz and 450 MHz, depending on the
reconstruction filter and the application system requirements.
For a desired output frequency (f
the FTW of the AD9858 is calculated by
where:
N is the phase accumulator resolution in bits (32 in the AD9858).
SYSCLK is in Hertz.
FTW is a decimal number.
FTW = ( f
OUT
× 2
N
)/ SYSCLK
DELTA FREQUENCY RAMP RATE WORD (≥8ns)
DELTA FREQUENCY TUNING WORD
8ns
OUT
) and sampling rate (SYSCLK),
16ns
TIME
Figure 32. Frequency vs. Time Plots for a Given Sweep Profile
24ns
32ns
Rev. C | Page 17 of 32
When the decimal number is calculated, it must be rounded to
an integer and converted to a 32-bit binary value. The frequency
resolution of the AD9858 is 0.233 Hz when the SYSCLK is 1 GHz.
Frequency Sweeping Mode
The AD9858 provides an automated frequency sweeping capability.
This allows the AD9858 to generate frequency swept signals for
chirped radar or other applications. The AD9858 includes features
that automate much of the task of executing frequency sweeps.
The frequency sweep feature is implemented through the use
of a frequency accumulator (not to be confused with the phase
accumulator). The frequency accumulator repeatedly adds an
incremental quantity to the current FTW, thereby creating new
instantaneous frequency tuning words, causing the frequency
generated by the DDS to change with time. The frequency
increment, or step size, is loaded into the delta frequency
tuning word (DFTW) register. The rate at which the frequency
is incremented is set by the delta frequency ramp rate word
(DFRRW) register. Together these registers enable the AD9858 to
sweep from a beginning frequency set by the FTW, upwards or
downwards, at a desired rate and frequency step size. The result is
a linear frequency sweep or chirp.
The DFRRW functions as a countdown timer, in which the
value of the DFRRW is decremented at the rate of SYSCLK/8.
This means that the most rapid frequency word update occurs
when a value of 1 is loaded into the DFRRW and results in a
frequency increment at 1/8 of the SYSCLK rate. With a SYSCLK
of 1 GHz, the frequency can be incremented at a maximum rate
of 125 MHz (DFRRW = 1). The DFTW must specify whether
the frequency sweep should proceed up or down from the starting
frequency (FTW). Therefore, the DFTW is expressed as a twos
complement binary value, in which positive indicates up and
negative indicates down.
40ns
80ns
TIME
120ns
160ns
AD9858