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AD9858

Rev. A | Page 18 of 32

The maximum usable frequency in the fundamental range of

the DDS is typically between 40% and 45% of the Nyquist

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.

phase value before the change, but at the new tuning word’s

phase increment value (FTW). (Note that this is not the same as

phase-coherent over frequency changes; see Figure 31.)

REFERENCE SIGNAL

FREF = A

FOUT = 2A

FOUT = A

FOUT = 2A

FOUT = A

PHASE COHERENT

PHASE CONTINUOUS

WHERE

θ = PHASE OF OUTPUT SIGNAL, Φ = PHASE AT TIME OF FIRST FREQUENCY

TRANSITION, AND

Φ' = PHASE AT TIME OF SECOND FREQUENCY TRANSITION.

θ = 2θREFΦ

θ = 2θREF+Φ + Φ'

θ = 2θREF

θ = θREF

For a desired output frequency (FO) and sampling rate

(SYSCLK), the frequency tuning word (FTW) of the AD9858 is

calculated according to the following equation

(

) SYSCLK

FTW

N /

where N is the phase accumulator resolution in bits (32 in the

AD9858), FO is in Hz, and the FTW is a decimal number.

Once a decimal number has been 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.

Figure 31. The Difference between a Phase Continuous

Frequency Change and a Phase Coherent Frequency Change

Single-Tone Mode

Frequency-Sweeping 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 frequency tuning word

(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).

The AD9858 provides 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 a

frequency incremental quantity to the current value, 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 a register

known as the delta frequency tuning word (DFTW). The rate at

which the frequency is incremented is set by another register,

the delta frequency ramp rate word (DFRRW). Together these

two 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 will provide adequate

attenuation for the first image frequency component.

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