Datasheet

AD9557 Data Sheet
Rev. B | Page 30 of 92
In the automatic modes, a fully automatic priority-based algorithm
selects which reference is the active reference. When programmed
for an automatic mode, the device chooses the highest priority
valid reference. When both references have the same priority,
REFA gets preference over REFB. However, the reference position
is used only as a tie-breaker and does not initiate a reference switch.
The following list gives an overview of the five operating modes:
• Automatic revertive mode. The device selects the highest
priority valid reference and switches to a higher priority
reference if it becomes available, even if the reference in use
is still valid. In this mode, the user reference is ignored.
• Automatic non-revertive mode. The device stays with the
currently selected reference as long as it is valid, even if
a higher priority reference becomes available. The user
reference is ignored in this mode.
• Manual with automatic fallback mode. The device uses the
user reference for as long as it is valid. If it becomes invalid,
the reference input with the highest priority is chosen in
accordance with the priority-based algorithm.
• Manual with holdover mode. The user reference is the
active reference until it becomes invalid. At that point,
the device automatically goes into holdover.
• Manual mode without holdover. The user reference is the
active reference, regardless of whether or not it is valid.
The user also has the option to force the device directly into
holdover or free run operation via the user holdover and user
freerun bits. In free run mode, the free run frequency tuning
word register defines the free run output frequency. In holdover
mode, the output frequency depends on the holdover control
settings (see the Holdover section).
Phase Build-Out Reference Switching
The AD9557 supports phase build-out reference switching,
which is the term given to a reference switchover that completely
masks any phase difference between the previous reference and
the new reference. That is, there is virtually no phase change
detectable at the output when a phase build-out switchover occurs.
DIGITAL PLL (DPLL) CORE
DPLL Overview
A diagram of the DPLL core of the AD9557 appears in Figure 36.
The phase/frequency detector, feedback path, lock detectors,
phase offset, and phase slew rate limiting that comprise this second
generation DPLL are all digital implementations.
The start of the DPLL signal chain is the reference signal, f
R
,
which is the frequency of the reference input. A reference prescaler
reduces the frequency of this signal by an integer factor, R + 1,
where R is the 20-bit value stored in the appropriate profile register
and 0 ≤ R ≤ 1,048,575. Therefore, the frequency at the output
of the R divider (or the input to the time-to-digital converter
(TDC)) is
1+
=
R
f
f
R
TDC
DIGITAL
LOOP
FILTER
÷N1
R DIVIDER
(20-BIT)
24-BIT/24-BIT
RESOLUTION
FRAC1/
MOD1
17-BIT
INTEGER
TUNING
WORD
CLAMP
AND
HISTORY
×2
FREE RUN
TW
+
30-BIT NCO
DPFD
SYSTEM
CLOCK
FROM APLL
FROM
REF
INPUT
MUX
TO APLL
09197-136
Figure 36. Digital PLL Core
A TDC samples the output of the R divider. The TDC/PFD
produces a time series of digital words and delivers them to the
digital loop filter. The digital loop filter offers the following
advantages:
• Determination of the filter response by numeric
coefficients rather than by discrete component values
• The absence of analog components (R/L/C), which
eliminates tolerance variations due to aging
• The absence of thermal noise associated with analog
components
• The absence of control node leakage current associated
with analog components (a source of reference feed-
through spurs in the output spectrum of a traditional
analog PLL)
The digital loop filter produces a time series of digital words at
its output and delivers them to the frequency tuning input of
a sigma-delta (Σ-Δ) modulator (SDM). The digital words from
the loop filter steer the DCO frequency toward frequency and
phase lock with the input signal (f
TDC
).
The DPLL includes a feedback divider that causes the digital
loop to operate at an integer-plus-fractional multiple. The
output of the DPLL is






++×=
MOD1
FRAC1
N1ff
TDCDPLLOUT
)1(
_
where N1 is the 17-bit value stored in the appropriate profile
registers (Register 0x0715 to Register 0x0717 for REFA). FRAC1
and MOD1 are the 24-bit numerators and denominators of the
fractional feedback divider block. The fractional portion of the
feedback divider can be bypassed by setting FRAC1 to 0, but
MOD1 should never be 0.
The DPLL output frequency is usually 175 MHz to 200 MHz for
optimal performance.
TDC/PFD
The phase-frequency detector (PFD) is an all-digital block. It
compares the digital output from the TDC (which relates to the
active reference edge) with the digital word from the feedback
block. It uses a digital code pump and digital integrator (rather
than a conventional charge pump and capacitor) to generate the
error signal that steers the DCO frequency toward phase lock.