GENNUM CORPORATION

522 - 75 - 05

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Fig. 22 Model of the GS9025A

2.9.1

Transfer Function

be approximated as:

Equation 1

where

and

N is the divider modulus

D is the data density (=0.5 for NRZ data)

This

response

has

1

zero

three

poles

The bode plot for this transfer function is plotted in

Figure 23.

Fig. 23 Transfer Function Bode Plot

The

3dB

bandwidth

of

the

transfer

function

is

approximately:

2.9.2

Transfer Function Peaking

There are two causes of peaking in the PLL transfer function

given by Equation 1.

The first is the quadratic:

which has:

and

This response is critically damped for Q = 0.5.

Thus, to avoid peaking:

or

Therefore,

To reduce the high frequency content on the loop filter, keep

The second is the zero-pole combination:

LOOP

FILTER

VCO

PHASE

DETECTOR

2

π Kƒ

+

-

Ns

Ø

o

Ø

i

-------

sC

1

+

sC

L

R

LF

---------

–

⎝⎠

⎛⎞

1

+

----------------------------------------------------------------

1

s

2

C

L

R

LF

---------

1

++

---------------------------------------------------------

=

L

N

DI

--------------------

=

w

Z

1

C

-----------------------

=

w

P1

1

C

L

R

LF

---------

–

---------------------------------------

=

w

BW

R

LF

L

---------

=

w

P2

1

C

-----------------------

=

FREQUENCY

w

3dB

w

BW

12

w

BW

w

P2

------------

–

w

⁄

()

2

12

w

BW

w

P2

------------

–

----------------------------------

+

----------------------------------------------------------------------

w

BW

0.78

------------

≈

=

s

2

C

L

R

LF

---------

1

++

w

o

1

C

--------------------

=

QR

LF

R

LF2

L

------------

=

R

LF

C

LF2

L

-------------

1

2

---

<

1

R

--------------------------

L

R

LF

---------

4

>

sC

1

+

sC

1

R

LF

---------

–

⎝⎠

⎛⎞

1

+

----------------------------------------------------------

s

w

Z

-------

1

+

s

w

P1

----------

1

+

--------------------

=