LMX2542LQ2121 NSC [National Semiconductor], LMX2542LQ2121 Datasheet - Page 13

• Manufacturer Part Number: LMX2542LQ2121
• Description: PLLatinum Cellular and GPS Frequency Synthesizer System with Integrated VCO
• Manufacturer: NSC [National Semiconductor]

1.0 Functional Description
1.4 RF DIGITAL FILTERED LOCK DETECT
A digital filtered lock detect status genrated from the RF
phase frequency detector (PFD) is available on the LD pin
(Pin 19) when the RF_LD bit (R0[21]) is set to 1. The LD
output is therefore used to indicate the lock status of the RF
synthesizer system. Furthermore, the LD output can be
forced to GND at all times when the RF_LD bit is set to 0.
When used as a lock detect output, the two inputs to the
PFD, f
filter then compares the difference between the phases of
the inputs to the PFD to an RC generated delay of approxi-
mately 10 ns. This delay is represented by t
Figure 2 below. If the phase error is less than 10 ns (∆t
N
and f
R
, are first divided by 64. The lock detect digital
W
FIGURE 1. Lock Detect Flow Diagram
in Figure 1 and
(Continued)
<
t
W
)
13
for 4 consecutive PFD comparison cycles, the RF PLL enters
a locked state and the LD output is then forced HIGH. Once
the phase error becomes greater than 10 ns (∆t
PLL falls out of lock and the LD is forced LOW (∼GND). The
phase error in Figure 2 is measured on the leading edge. If
the phase difference between the two inputs to the PFD is
equal to 10 ns (∆t = t
unpredictable. Refer to Section 2.2.4 for further details on
how to program the digital filtered lock detect.
Note: f
is the PFD input from the programmable feedback divider (N
counter).
R
is the PFD input from the reference oscillator and f
20082406
W
), then the LD output becomes
>
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t
W
) the RF
N