General PLL Troubleshooting

The Synthesizer Section relies heavily on Phase-Lock Loops (PLL). Typically, faulty PLL’s are either locked at the wrong frequency or unlocked. The information below applies to troubleshooting these two classes of problems on a generalized PLL.

PLL Locked at Wrong Frequency

Numbers in the following text identify items in Figure 10-4.

nAny frequency errors at reference (1) will be multiplied by N/M on the PLL’s output.

w A sampler reference-frequency error (2) will be multiplied by its harmonic on the PLL’s output.

nA mixer reference-frequency error (3) produces the identical error on the PLL’s output.

nIf divider input or output frequencies (4) are wrong, check for incorrect divide numbers and data controlling the dividers.

P R E T U N E

A L T E R N A T E

P A T H

UP DOWN

R E F E R E N C E

I NPUT

S K 1 1 0 4

-Figure 10-4. PLL Locked at Wrong Frequency

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 10-5.

Synthesizer Section lo-13