AD9546SLASHPCBZ datasheet(83/205 Pages) AD

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Data Sheet

AD9546

Rev. 0 | Page 83 of 205

Because the threshold parameter specifies a fractional period

(δp), it relates to fractional frequency (δf) as follows:

δp = −δf/(1 + δf)

(1)

For example, consider the case where the expected maximum

relative deviation of the input reference frequency for REFAA

is 75 × 10−6 (75 ppm). That is, δf = 75 × 10−6.

From Equation 1, δp = −74.994375 × 10−6, or −74.994375 ppm,

or −74994.375 ppb. Because the threshold parameter is an

unsigned number with integer units of ppb, threshold = 74,994.

Therefore, program Register 0x042C to Register 0x042E with the

24-bit value corresponding to 74,994 (0x 01 24F2 hexadecimal).

To properly verify small values of the threshold parameter, the

reference monitor requires a very precise timing reference. The

reference monitor uses time stamps from the TDC to assess the

input signal parameters. Because the underlying precision of

the TDC relates to the system clock, small threshold values

require a suitably precise system clock or a suitably

compensated system clock (see the System Clock

Compensation section).

Hysteresis

The hysteresis parameter applies to a faulted reference, whereas

the threshold parameter applies to a nonfaulted reference. The

reference monitor makes fault and unfault decisions based on

how reference period samples relate to tREF, threshold, and

hysteresis. To specify the hysteresis parameter for a particular

reference, use Bits[2:0] of the register address associated with

the reference per Table 53.

Table 53. Hysteresis Register Address

Reference Input

REFA

0x040F

REFAA

0x042F

REFB

0x044F

REFBB

0x046F

Auxiliary REF0

0x048F

Auxiliary REF1

0x04AF

Auxiliary REF2

0x04CF

Auxiliary REF3

0x04EF

The programmed value of the hysteresis parameter defines a

hysteresis scale factor (SF) per Table 54.

Table 54. Hysteresis Selection Table

Bits[2:0] (Decimal)

Scale Factor (SF)

0.03125

0.0625

0.125 (default)

0.25

0.5

0.75

0.875

The SF value constitutes a fractional portion of the value of

threshold and defines the amount of hysteresis.

Hysteresis (ppb) = Threshold × SF

For example, given threshold = 74,994 ppb and SF = 0.0625,

determine the hysteresis.

Hysteresis (ppb) = 74,994 ppb × 0.0625 = 4,687

The reference monitor compares reference period samples

against the tREF, threshold, and SF parameters to make fault and

unfault decisions per Figure 61.

BASE PERIOD =

tREF × R

THRESHOLD

TIME

MAXIMUM DEVIATION FROM

THE BASE PERIOD FOR A

FAULTED REFERENCE TO

BECOME UNFAULTED

MINIMUM DEVIATION FROM

THE BASE PERIOD FOR AN

UNFAULTED REFERENCE

TO BECOME FAULTED

THRESHOLD × (1 − SF)

THRESHOLD × SF

BASE

PERIOD

Figure 61. Reference Monitor Hysteresis

Jitter Tolerance

Use the jitter tolerance parameter to define the maximum rms

jitter the reference monitor accepts before indicating an

excessive jitter condition. Each reference input has a dedicated

address ranges shown in Table 55. The jitter tolerance parameter

is an unsigned 16-bit integer with units of nanoseconds

(10−9 sec) up to a maximum of approximately 65.5 µs.

Table 55. Jitter Tolerance Address Ranges

Reference Input

REFA

0x0413 to 0x0414

REFAA

0x0433 to 0x0434

REFB

0x0453 to 0x0454

REFBB

0x0473 to 0x0474

Auxiliary REF0

0x0493 to 0x0494

Auxiliary REF1

0x04B3 to 0x04B4

Auxiliary REF2

0x04D3 to 0x04D4

Auxiliary REF3

0x04F3 to 0x04F4

For example, given an expected jitter limit of 75 ns rms, then

jitter tolerance = 75 (0x004B hexadecimal).

When jitter tolerance = 0, the reference monitor does not include

its estimation of jitter as part of the excess noise status decision.

However, even when jitter tolerance = 0, the reference monitor

continues to perform jitter estimation to continue making out

of tolerance decisions.

AD9546/PCBZ 数据表(PDF) 83 Page - Analog Devices

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