AD9517-4/PCBZ Analog Devices Inc, AD9517-4/PCBZ Datasheet - Page 36

• Manufacturer Part Number: AD9517-4/PCBZ
• Description: BOARD EVALUATION AD9517-4
• Manufacturer: Analog Devices Inc

Specifications of AD9517-4/PCBZ

• Design Resources: High Performance, Dual Channel IF Sampling Receiver (CN0140)
• Main Purpose: Timing, Clock Generator
• Utilized Ic / Part: AD9517-4
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

AD9517-4
DIGITAL LOCK DETECT (DLD)
By selecting the proper output through the mux on each pin,
the DLD function can be made available at the LD, STATUS,
and REFMON pins. The DLD 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 three settings:
the digital lock detect window bit (Register 0x018[4]), the
antibacklash pulse width setting (Register 0x017[1:0], see Table 2),
and the lock detect counter (Register 0x018[6:5]). A lock is not
indicated until there is a programmable number of consecutive
PFD cycles with a time difference that is 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 (Register 0x018[6:5]).
Analog Lock Detect (ALD)
The AD9517 provides an ALD function that can be selected for
use at the LD pin. There are two versions of ALD, as follows:
•
•
The analog lock detect function requires an R-C filter to
provide a logic level indicating lock/unlock.
Current Source Digital Lock Detect (DLD)
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 made is possible by using
the current source lock detect function. The current source lock
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.
Figure 49. Example of Analog Lock Detect Filter
Using an N-Channel Open-Drain Driver
ALD
AD9517-4
LD
R1
V
S
= 3.3V
R2
C
V
OUT
Rev. B | Page 36 of 80
detect function is set when it is selected as the output from the
LD pin control (Register 0x01A[5:0]).
The current source lock detect provides a current of 110 μA
when DLD is true, and it shorts to ground when DLD is false.
If a capacitor is connected to the LD pin, it charges at a rate that
is 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), it is possible to
get a logic high level only after the DLD has been 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.
The voltage on the capacitor can be sensed by an external
comparator connected to the LD pin. However, there is an
internal LD pin comparator that can be read at the REFMON
pin control (Register 0x01B[4:0]) or the STATUS pin control
(Register 0x017[7:2]) as an active high signal. It is also available
as an active low signal (REFMON, Register 0x01B[4:0] and
STATUS, Register 0x017[7:2]). The internal LD pin comparator
trip point and hysteresis are listed in Table 16.
External VCXO/VCO Clock Input (CLK/ CLK )
CLK is a differential input that can be used as an input to drive
the AD9517 clock distribution section. This input can receive
up to 2.4 GHz. The pins are internally self-biased, and the input
signal should be ac-coupled via capacitors.
The CLK/ CLK input can be used either as a distribution-only
input (with the PLL off), or as a feedback input for an external
VCO/VCXO using the internal PLL when the internal VCO is
not used. The CLK/ CLK input can be used for frequencies up
to 2.4 GHz.
CLK
CLK
V
COMPARATOR
S
AD9517-4
Figure 51. CLK Equivalent Input Circuit
Figure 50. Current Source Lock Detect
LD PIN
110µA
2.5kΩ
5kΩ
5kΩ
DLD
2.5kΩ
LD
REFMON
OR
STATUS
CLOCK INPUT
STAGE
C
V
OUT