LMX2542 datasheet(13/22 Pages) NSC

13 / 22 page

1.0 Functional Description (Continued)

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

N and fR, are first divided by 64. The lock detect digital

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

W in Figure 1 and

Figure 2 below. If the phase error is less than 10 ns (

∆t < t

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 > t

W) the RF

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

W), then the LD output becomes

unpredictable. Refer to Section 2.2.4 for further details on

how to program the digital filtered lock detect.

Note: f

R is the PFD input from the reference oscillator and fN

is the PFD input from the programmable feedback divider (N

counter).

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FIGURE 1. Lock Detect Flow Diagram

LMX2542 Datasheet(PDF) 13 Page - National Semiconductor (TI)