AD9858SLASHTLPCBZ datasheet(17/32 Pages) AD

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AD9858

Rev. C | Page 17 of 32

Single-Tone Mode

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.

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).

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.

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.

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.

For a desired output frequency (fOUT) and sampling rate (SYSCLK),

the FTW of the AD9858 is calculated by

FTW = (fOUT × 2N)/SYSCLK

where:

N is the phase accumulator resolution in bits (32 in the AD9858).

SYSCLK is in Hertz.

FTW is a decimal number.

40ns

80ns

TIME

120ns

160ns

DELTA FREQUENCY RAMP RATE WORD (≥8ns)

8ns

16ns

TIME

24ns

32ns

DELTA FREQUENCY TUNING WORD

Figure 32. Frequency vs. Time Plots for a Given Sweep Profile

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