CLC016 datasheet(10/20 Pages) NSC

10 / 20 page

Product Description (Continued)

The jitter transfer function is the small signal transfer func-

tion,

o/θi, and is given by:

where f

BW is the PLL bandwidth and fZ is a zero in the closed

loop transfer function.

The phase detector gain and VCO gain are fixed internally.

Selection of the external loop filter components defines the

overall jitter transfer function. Additionally, the filter compo-

nents control the acquisition performance of the PLL.

A Bode plot for the closed loop PLL jitter transfer function is

shown in

Figure 5.

At frequencies above f

BW (the PLL bandwidth) the jitter is at-

tenuated. At frequencies below f

BW the jitter is transmitted

through the PLL. A small amount of jitter peaking (

δ) occurs

at frequencies below f

BW. The amount of peaking increases

when f

Z moves closer to fBW.

Setting the Loop Bandwidth (Selecting R

BW)

The fractional loop bandwidth,

BW, is the ratio of fBW to the

data rate. The CLC016 is specified for operation with frac-

tional loop bandwidths ranging from 0.05% to 0.5%. For ex-

ample, if the loop bandwidth is 1 MHz and the data rate is

270 Mbps, then the fractional loop bandwidth is:

The fractional loop bandwidth is set by the loop component

BW:

where

ρ is the data transition density in average number of

data transitions per bit cell, and ranges in value from 0 to 1.

For example, if a pseudo-random data stream is used, the

value of

ρ is 1/2, and a data transition will occur once every

two bit cells on the average. The phase detector and VCO

gain set the constants in the equation.

If the value of R

BW is 500Ω and ρ = 1/2, the fractional loop

bandwidth is:

For a data rate of 270 Mbps this corresponds to a loop band-

width f

BW = 644 kHz. The jitter at frequencies above 644 kHz

will be attenuated by the PLL.

The equation may be rearranged to obtain R

BW as a function

of the desired fractional loop bandwidth:

Setting the Jitter Peaking Factor (Selecting C

The jitter peaking factor,

δ, is set by the ratio of the critical

frequencies f

Z and fBW. The ratio is defined as:

Figure 6 shows how the jitter peaking factor,

δ, varies with α.

For example, if the value of

α is 0.1, then the jitter peaking is

about 0.6 dB.

The approximation for the required value of

α to obtain a

given amount of jitter peaking is:

α ≅ δ(0.134 + 0.058δ)

The critical frequency f

Z is:

Select C

Z by the following equation:

DS100087-16

FIGURE 4. PLL Loop

DS100087-17

FIGURE 5. Closed-Loop Transfer Function

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