Agilent Technologies 856290216 service manual Unlocked PLL, Synthesizer l-15

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

PRETUNE

OUTPUT

UP

2

;:-;-

D O W N

L--_

REF

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