ACS8514 Semtech Corporation, ACS8514 Datasheet - Page 17

• Manufacturer Part Number: ACS8514
• Description: Synchronous Equipment Timing Source Partner IC for 2nd T4 Dpll, Accurate Monitoring & Input Extender
• Manufacturer: Semtech Corporation

Crystal Frequency Calibration
The absolute crystal frequency accuracy is less important
than the stability since any frequency offset can be
compensated by adjustment of register values in the IC.
This allows for calibration and compensation of any crystal
frequency variation away from its nominal value. ±50 ppm
adjustment would be sufficient to cope with most crystals,
in fact the range is an order of magnitude larger due to
the use of two 8-bit register locations. The setting of the
conf_nominal_frequency register (addr 3Ch, 3Dh) allows
for this adjustment. An increase in the register value
increases the output frequencies by 0.0196229 ppm for
each LSB step.
The default register value (in decimal) = 39321 (9999
hex) = 0 ppm offset. The minimum to maximum offset
range of the register is 0 to 65535 dec, giving an
adjustment range of -771 ppm to +514 ppm of the output
frequencies, in 0.0196229 ppm steps.
Example: If the crystal was oscillating at 12.800 MHz
+ 5 ppm, then the calibration value in the register to give a
- 5 ppm adjustment in output frequencies to compensate
for the crystal inaccuracy, would be:
39321 - (5/0.0196229) = 39066 (dec) = 989A (hex).
Output Wander & Jitter
Wander and jitter present on the output depends on::
Jitter and Wander Transfer
The T4 DPLL has a programmable jitter transfer
characteristic. This is set by the T4 DPLL bandwidth
(register 66). The -3 dB jitter transfer attenuation point
can be set to 18, 35 or 70 Hz. The wander and jitter
transfer characteristic is shown in Figure 4 .
The monitor DPLL has an effective bandwidth of 0.1 to 70
Hz. The setting of bandwidth for this PLL is mainly used to
control how quickly the DPLL follows the input source
during input phase and frequency measurements. Since
the output clock from the monitor DPLL is not accessible,
it’s transfer characteristic is not measurable.
Revision 3.00 April 2007 © Semtech Corp.
ADVANCED COMMS & SENSING
The magnitudes of wander and jitter on the selected
input reference clock (in Locked mode)
The internal wander and jitter transfer characteristic (in
Locked mode). See below.
The wander on the local oscillator clock (when the T4
DPLL is free running or holding its frequency).
Page 17
FINAL
Wander on the local oscillator clock will not have a
significant effect on the T4 DPLL output clock when
locked, since the bandwidth is set high enough so that the
DPLL can compensate quickly enough for any frequency
changes in the crystal.
In Free-run or frequency holdover wander on the crystal is
more significant. Variation in crystal temperature or supply
voltage both cause drifts in operating frequency, as does
ageing. These effects must be limited by careful selection
of a suitable component for the local oscillator.
Input Wander and Jitter Tolerance
The ACS8514 is compliant to the requirements of all
relevant standards, principally ITU Recommendation
G.825
GR253, G812, G813 and ETS 300 462-5 (1997) in terms
of jitter tolerance.
All reference clock inputs have a tight frequency tolerance
but a generous jitter tolerance. Using either lock8k mode
or direct lock mode and the multi UI phase detector, the
jitter tolerance limits can set to exceed all tolerance
requirements. When the multi UI phase detector is used,
the DPLLs can tolerate and track up to +/- 8191 UI. This
limit is programmable (see register 74h).
Pull-in, hold-in and pull-out ranges are shown in Table 7.
Table 7 Input Reference Freq range
Notes
(i) The frequency acceptance and generation range will be ±4.6
(ii) The fundamental acceptance range and generation range is
G.703
G.783
G.823
GR-1244-
CORE
ppm around the required frequency when the external crystal
frequency accuracy is within a tolerance of ±4.6 ppm.
±9.2 ppm with an exact external crystal frequency of 12.800
MHz. This is the default DPLL range; the range is also
programmable from 0 to 80 ppm in 0.08 ppm steps.
Spec.
:
[19]
[15]
[6]
[9]
[13]
, ANSI DS1.101-1999
±16.6 ppm
Acceptance
Frequency
Monitor
Range
ACS8514 SETS Buddy
±4.6 ppm
(Note 0)
±9.2 ppm
(Note (i))
Acceptance
Frequency
(Pull-in)
Range
[1]
, Telcordia GR1244,
±4.6 ppm
(Note 0)
±9.2 ppm
(Note (i))
Acceptance
Frequency
(Hold-in)
Range
DATASHEET
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±4.6 ppm
(Note 0)
±9.2 ppm
(Note (i))
Acceptance
Frequency
(Pull-out)
Range