Unlocked
PLL
An unlocked PLL can be caused by problems inside or outside the PLL. Troubleshoot this
problem by working backward from the oscillator as described in the steps below. Numbers in
the following text identify items in Figure 11-6.
1. The loop integrator output voltage (1) should be attempting to tune the oscillator to the
correct frequency:
The voltage at (1) should increase as the frequency increases on all of the PLLs:
I
PLL
I
Measurement Point
1
I
YTO PLL
1
A14J23
pin 1 (YTO ERROR)
1
Reference PLL A155502 pin 3 (LO3 ERR)
Sampler PLL
A15J200
pin 13 (OFL ERR)
1
Fractional N PLL
1
A14TP13
(INTEGRATOR)
1
UP
;:-;-
2
PRETUNE
OUTPUT
REF
DOWN
L--_
spl30e
Figure 11-6. Unlocked PLL
2. If the integrator output voltage changes in the manner described in step 1, the problem is
external to the PLL. For example, the reference frequency could be faulty. If the integrator
output voltage appears incorrect, confirm that the pulses out of the phase detector (2) are
attempting to tune the oscillator in the correct direction.
3. If the phase detector output is bad, check the inputs to the detector (3). One input should
be higher in frequency than the other; this should match the phase detector outputs.
Synthesizer Section 1 l-15