AZ12000

AZ12001

January 2005 * REV - 3

www.azmicrotek.com

10

Internal VCO

The internal VCO is designed for reliable, low jitter operation up to 800 MHz. It operates as a single terminal negative

impedance type circuit.

The tank circuit should have a minimum Q of 12 for reliable operation. The series combination of CV and C1 resonate

with L1 to set the operating frequency. The VCO control voltage is isolated through an inductor or resistor and changes

the varactor capacitance based on that control voltage. Note that the CPPOL pin should be tied high for internal VCO

operation since the tank frequency decreases with increasing control voltage.

External VCO

When VCOSEL is high, the internal VCO is disabled and the EXTVCO, EXTVCO

¯¯¯¯¯¯¯¯ pair is enabled. That input pair is

sine wave and PECL compatible.

The CPPOL pin sets the frequency slope polarity based on the operation of the external VCO. When CPPOL is low, the

charge pump generates pulses for an integrator and loop filter assuming the VCO frequency goes lower as the integrator

output voltage increases. When CPPOL is high, pulses are generated for a VCO in which the frequency goes higher as

the integrator output voltage increases.

VCO

L1

C2

TANK

L2

VCOV

CC

TANK

CV

VARACTOR

VCO

CONTROL

VOLTAGE

R1

Figure 3 Internal VCO with Tank