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Clock Generator Module (CGMC)

Acquisition/Lock Time Specifications

MC68HC908GR8 — Rev 4.0

Technical Data

MOTOROLA

Clock Generator Module (CGMC)

125

7.9 Acquisition/Lock Time Specifications

The acquisition and lock times of the PLL are, in many applications, the

most critical PLL design parameters. Proper design and use of the PLL

ensures the highest stability and lowest acquisition/lock times.

7.9.1 Acquisition/Lock Time Definitions

Typical control systems refer to the acquisition time or lock time as the

reaction time, within specified tolerances, of the system to a step input.

In a PLL, the step input occurs when the PLL is turned on or when it

suffers a noise hit. The tolerance is usually specified as a percentage of

the step input or when the output settles to the desired value plus or

minus a percentage of the frequency change. Therefore, the reaction

time is constant in this definition, regardless of the size of the step input.

For example, consider a system with a 5 percent acquisition time

tolerance. If a command instructs the system to change from 0 Hz to

1 MHz, the acquisition time is the time taken for the frequency to reach

1MHz

±50 kHz. Fifty kHz = 5% of the 1-MHz step input. If the system is

operating at 1 MHz and suffers a –100-kHz noise hit, the acquisition time

is the time taken to return from 900 kHz to 1 MHz

±5 kHz. Five kHz = 5%

of the 100-kHz step input.

Other systems refer to acquisition and lock times as the time the system

takes to reduce the error between the actual output and the desired

output to within specified tolerances. Therefore, the acquisition or lock

time varies according to the original error in the output. Minor errors may

not even be registered. Typical PLL applications prefer to use this

definition because the system requires the output frequency to be within

a certain tolerance of the desired frequency regardless of the size of the

initial error.

7.9.2 Parametric Influences on Reaction Time

Acquisition and lock times are designed to be as short as possible while

still providing the highest possible stability. These reaction times are not

constant, however. Many factors directly and indirectly affect the

acquisition time.

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