AD9546SLASHPCBZ datasheet(165/205 Pages) AD

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Data Sheet

AD9546

Rev. 0 | Page 165 of 205

ensure that the internally generated time stamps precisely mark

the zero-crossing point of the accumulator.

The internal time stamps generated by the auxiliary NCO relate

to the internal time scale of the AD9546.

Because the internal time stamps truly mark the rollover period

of the auxiliary NCO accumulator as well as relate to the

internal time scale, the time stamps can serve as an input

reference source to either of the DPLLs. This feature allows the

AD9546 to have a completely self contained reference source to

the DPLLs (that is, no external reference source required).

Because the auxiliary NCO time stamps are an available

reference source, the user has access to them through the

UTSPs (see the User Time Stamp Processor (UTSP) section).

For example, suppose the user programs auxiliary NCO 0 for

fAUX = 5 kHz. To read the time stamps from auxiliary NCO 0,

the user can program UTSP 0 to use auxiliary NCO 0 as both a

time stamp source and as a time scale for UTSP 0. Then,

reading the first time stamp yields a reference value related to

the time scale of auxiliary NCO 0. However, the next time

stamp has a value of 1 unit greater than the first, because the

time scale has units of 200 μs (per fAUX). Unfortunately, this

method of time stamp conversion by UTSP 0 is not conducive

to external use.

However, by relating the time stamps to an appropriate time

scale, the time stamp conversion can become meaningful. To

do so, program auxiliary NCO 1 for fAUX = 1 Hz. Because fAUX =

1 Hz the time scale for auxiliary NCO 1 carries units of 1 sec.

Next, program UTSP 0 to use the time scale from auxiliary

NCO 1 instead of the time scale from auxiliary NCO 0 (but

keep auxiliary NCO 0 as the time stamp source to UTSP 0).

Then, reading the first time stamp yields a reference value

(related to the time scale of auxiliary NCO 1) and the next time

stamp corresponds to a value precisely 200 μs later than the

first with subpicosecond precision.

AUXILIARY NCO PULSE OUTPUT

In addition to generating time stamps, the auxiliary NCOs also

produce a physical output signal (see Figure 111). The output

signal is effectively a single pulse commensurate with the

rollover of the internal phase accumulator of the auxiliary

NCO. The auxiliary NCO quantizes the pulse timing such that

the peak-to-peak pulse jitter is approximately 1.4 ns.

The user can route the output pulse signal to any one of the Mx

pins by defining the desired Mx pin as an output and selecting

the desired auxiliary NCO as the output source (see the Status

and Control Pins section).

The user programs the duration of the pulse via a significand

(S) value and exponent (E) value. To program S, use Bits[3:0] of

auxiliary NCO 1). To program E, use Bits[7:4] of the same

registers.

Pulse Width = (1/fACCUM) × (1 + S × 2(E + 5))

For example, given fS = 2.4 GHz, S = 6, and E = 2, then

Pulse Width = (1/fACCUM) × (1 + S × 2(E + 5))

= (96/fS) × (1 + 6 × 2(2 + 5))

= (96/(2.4 × 109)) × (1 + 6 × 2(2 + 5))

= (96/(2.4 × 109)) × (1 + 6 × 27)

= 30.76 μs

However, given a desired pulse duration (tP), one must

determine the appropriate E and S values for a given system

clock PLL VCO frequency (fS). To do so, let

K = (tP × fS)/3072 – 1/32

Next, determine E as

E = ceil(log(K/15)/log(2))

where:

0 ≤ E ≤ 15.

E = ceil(x) means E = x if x is an integer. Otherwise, E is the

nearest integer to x in the positive direction.

Finally, determine S as

S = round(K/2E)

where:

0 ≤ S ≤ 1.

round(x) means round x to the nearest integer.

When the computation of E or S yields a result outside the

specified range, use the appropriate limit. For example, if the

computation of E yields a result of –2, then use E = 0.

The K, E, and S values for tP = 100 μs and fS = 2.4 GHz are K =

78.09375, E = 3, and S = 10.

E = 3 and S = 10 are within their respective range limits.

Because E and S must be integers, the resulting pulse duration

is quantized. Given the K, E, and S results for a desired pulse

period of tP = 100 μs, the actual pulse period is

tP = (96/fS) × (1 + S × 2(E + 5))

= 102.44 μs

Because the output frequency (fAUX) of the auxiliary NCO and

the output pulse width (tP) are independently programmable

parameters, the user must ensure

tP ≤ 1/fAUX

Although the user has the option of generating pulses

commensurate with tagged time stamps (see the Tagged

Auxiliary NCO Time Stamps section), the tP ≤ 1/fAUX limit on tP

still applies (that is, based on fAUX and not the tag frequency).

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