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| 型号： | AD9522-0/PCBZ |
| 厂商： | Analog Devices Inc |
| 文件页数： | 31/84页 |
| 文件大小： | 0K |
| 描述： | BOARD EVAL FOR AD9522-0 CLK GEN |
| 设计资源： | AD9522 Eval Board Schematic AD9522 BOM |
| 标准包装： | 1 |
| 主要目的： | 计时，时钟发生器 |
| 嵌入式： | 否 |
| 已用 IC / 零件： | AD9522-0 |
| 主要属性： | 12 LVDS/24 CMOS 输出，2.8 GHz VCO |
| 次要属性： | I²C & SPI 接口 |
| 已供物品： | 板 |
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AD9522-0
Rev. 0 | Page 37 of 84
Digital Lock Detect (DLD)
By selecting the proper output through the mux on each pin, the
DLD function is available at the LD, STATUS, and REFMON pins.
The digital lock detect circuit indicates a lock when the time
difference of the rising edges at the PFD inputs is less than a
specified value (the lock threshold). The loss of a lock is indicated
when the time difference exceeds a specified value (the unlock
threshold). Note that the unlock threshold is wider than the
lock threshold, which allows some phase error in excess of the
lock window to occur without chattering on the lock indicator.
The lock detect window timing depends on the value of the
CPRSET resistor, as well as three settings: the digital lock
detect window bit (0x018[4]), the antibacklash pulse width
bit (0x017[1:0], see Table 2), and the lock detect counter
(0x018[6:5]). The lock and unlock detection values in Table 2
are for the nominal value of CPRSET = 5.11 kΩ. Doubling the
CPRSET value to 10 kΩ doubles the values in Table 2.
A lock is not indicated until there is a programmable number of
consecutive PFD cycles with a time difference less than the lock
detect threshold. The lock detect circuit continues to indicate a
lock until a time difference greater than the unlock threshold
occurs on a single subsequent cycle. For the lock detect to work
properly, the period of the PFD frequency must be greater than
the unlock threshold. The number of consecutive PFD cycles
required for lock is programmable (0x018[6:5]).
Note that it is possible in certain low (<500 Hz) loop bandwidth,
high phase margin cases that the DLD can chatter during acqui-
sition, which can cause the AD9522 to automatically enter and exit
holdover. To avoid this problem, it is recommended that the
user make provisions for a capacitor to ground on the LD pin so
that current source digital lock detect (CSDLD) mode can be used.
Analog Lock Detect (ALD)
The AD9522 provides an ALD function that can be selected for
use at the LD pin. There are two operating modes for ALD.
N-channel open-drain lock detect. This signal requires a
pull-up resistor to the positive supply, VS. The output is
normally high with short, low going pulses. Lock is
indicated by the minimum duty cycle of the low going pulses.
P-channel open-drain lock detect. This signal requires a
pull-down resistor to GND. The output is normally low with
short, high going pulses. Lock is indicated by the minimum
duty cycle of the high going pulses.
The analog lock detect function requires an RC filter to provide a
logic level indicating lock/unlock. The ADIsimCLK tool can be
used to help the user select the right passive component values
for ALD to ensure its correct operation.
AD9522
ALD
LD
R1
C
VOUT
R2
VS = 3.3V
072
19
-06
7
Figure 44. Example of Analog Lock Detect Filter Using
an N-Channel Open-Drain Driver
Current Source Digital Lock Detect (CSDLD)
During the PLL locking sequence, it is normal for the DLD
signal to toggle a number of times before remaining steady
when the PLL is completely locked and stable. There may be
applications where it is desirable to have DLD asserted only
after the PLL is solidly locked. This is possible by using the
current source digital lock detect function.
The current source lock detect provides a current of 110 μA when
DLD is true and shorts to ground when DLD is false. If a capacitor
is connected to the LD pin, it charges at a rate determined by the
current source during the DLD true time but is discharged nearly
instantly when DLD is false. By monitoring the voltage at the
LD pin (top of the capacitor), LD = high happens only after the
DLD is true for a sufficiently long time. Any momentary DLD
false resets the charging. By selecting a properly sized capacitor,
it is possible to delay a lock detect indication until the PLL is
stably locked and the lock detect does not chatter.
To use current source digital lock detect, do the following:
Place a capacitor to ground on the LD pin.
Set 0x01A[5:0] = 0x04.
Enable the LD pin comparator (0x01D[3] = 1).
The LD pin comparator senses the voltage on the LD pin, and the
comparator output can be made available at the REFMON pin
control (0x01B[4:0]) or the STATUS pin control (0x017[7:2]). The
internal LD pin comparator trip point and hysteresis are given in
Table 17. The voltage on the capacitor can also be sensed by an
external comparator connected to the LD pin. In this case,
enabling the on-board LD pin comparator is not necessary.
The user can asynchronously enable individual clock outputs only
when CSDLD is high. To enable this feature, set the appropriate bits
in the enable output on the CSDLD registers (0x0FC and 0x0FD).
AD9522
LD
REFMON
OR
STATUS
C
VOUT
110A
DLD
LD PIN
COMPARATOR
07
21
9-
06
8
Figure 45. Current Source Digital Lock Detect
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