AD9549 datasheet(47/78 Pages) AD

47 / 78 page

Preliminary Technical Data

AD9549

Rev. PrA | Page 47 of 78

The frequency slew limiter sets a boundary on the rate of

change of the output frequency of the DDS. The frequency slew

limiting constant, KSLEW, is a 48-bit value stored in the I/O

























round

SLEW

Where PIO is the value stored in the I/O Register Map for the

"P"-Divider, fS is the DAC sample rate, and δf/δt is the desired

frequency slew rate limitation. For example, suppose that

fS=1GHz, PIO=9 and δf/δt=5kHz/second, then:

() ()

721









⋅













round

SLEW

The resulting slew rate can be calculated as:













SLEW

The above example yields: δf/δt=5.003kHz/s

FREQUENCY ESTIMATOR

The AD9549 has a frequency estimation function that will

automatically set the DDS output frequency so that the

feedback frequency (fDDS/S) and the prescaled reference

frequency (fREF_IN/R) are matched within an error tolerance (ε0).

It’s primary purpose is to allow the PLL to quickly lock when

the reference frequency is not known. The error tolerance is

defined as a fractional error and is controlled by a 16-bit

programmable value (K) via the I/O Register Map.

The precision of any frequency measurement is dependent on

two factors:

the timing resolution of the measurement device (δt)

the duration of the measurement (Tmeas)

The frequency estimator uses fS as its measurement reference, so

δt=1/f

(i.e., δt=1ns for a 1GHz DAC sample rate). The

duration of the measurement is controlled by K, which

establishes a measurement interval that is K cycles of the

measured signal such that Tmeas=KR/fREF_IN.

The frequency estimator uses a 17-bit counter to accumulate

the number δt periods within the measurement interval. The

finite capacity of the counter puts an upper limit on the

duration of the measurement, which is constrained to

Tmax=217/fS. If fS=1GHz, then this equates to ~131µs. The fact

that the measurement time is bounded by Tmax means there is a

limit to the largest value of K (KMAX) that can be used without

causing the counter to overflow. The value of KMAX is given by:

( )

65535

floor

MAX =

where

R is the modulus of the feedforward divider and fR is the input

reference frequency.

The measurement error (ε) associated with the frequency

estimator depends on the choice of the measurement interval

parameter (K). These are related by:

()

1 −

−

floor

With a specified fractional error (ε0), only those values of K for

which ε ≤ ε0 result in a frequency estimate that meets the

requirements. A plot of ε versus K (for a given ρ) takes on the

general form shown below.

216

ε bounded

by envelope

ε < ε

0 for all K > K1

ε > ε

0 for all K < K0

ε < ε

0 for some K

0 < K < K1)

Figure 34: Frequency Estimator

ε vs. K

An iterative technique is necessary to determine the exact

values of K0 and K1. However, a closed form exists for a

conservative estimate of K0 (KLO) and K1 (KHI):

( )

[

]

= ceil

( )

[

]

= ceil

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