AZ12010

November 2006 * REV - 5

www.azmicrotek.com

6

Loop Filter Design

The combination of the phase detector, amplifier, VCO and divider form a second-order phase-locked loop. Proper

selection of the loop components is important to obtain stable, low jitter operation.

The loop bandwidth (or natural frequency,

loop’s response to a disturbance. The value of

ζ is typically 0.7 to ensure the fastest step response consistent with no

ringing. However in many oscillator application

chosen as a compromise between settling time, VCO jitter and reference feedthrough. These values can be computed by

the following equations:

1

1

τ

ω

φ

VCO

n

K

K

N

=

2

2

n

ω

τ

1

1

1

C

R

=

τ

1

2

2

C

R

=

τ

=

φ

K

Phase Detector Gain (20.3 radians/V)

=

VCO

K

VCO Gain (radians/sec/volt)

=

N

Frequency Divisor value (32)

The component definitions are shown in the figure below. R3 should be equal to R1 to minimize integrator offsets.

CHARGE

PUMP

INTREF INTSUM

INTOUT

CPREF CPOUT

INTEGRATOR

R2

C1

R1

R3

EXTERNAL

VCSO

CONTROL

VOLTAGE

Figure 1 Charge Pump and Integrator