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Figure 2. ML12149 Typical External Component Connections

8

7

6

5

1

2

3

4

VCO

1. This input can be left open, tied to ground, or tied with a resistor to ground, depending

on the desired output amplitude needed at the Q and QB output pair.

2. Typical values for R1 range from 5.0 kΩ to 10 kΩ.

Q2

Q

GND

QB

VCC

CNTL

TANK

VREF

C2a

C3a

Note 1

Cb

LT

CV

R1

Vin

C2a

C3a

VCC Supply

C7

C6a

C6b

L2b

L2a

To Prescaler

VCO Output

VCO Output

C1

ML12149

LANSDALE Semiconductor, Inc.

A simplified linear approximation of the device, package, and

typical board parasitics has been developed to aid the designer in

selecting the proper tank circuit values. All the parasitic contribu-

tions have been lumped into a parasitic capacitive component and a

parasitic inductive component. While this is not entirely accurate, it

gives the designer a solid starting point for selecting the tank com-

ponents. Below are the parameters used in the model.

Cp Parasitic Capacitance

Lp Parasitic Inductance

LT Inductance of Coil

C1 Coupling Capacitor Value

Cb Capacitor for decoupling the Bias Pin

CV Varactor Diode Capacitance (Variable)

The values for these components are substituted into the follow-

ing equations:

From Figure 2, it can be seen that the varactor capacitance (CV) is

in series with the coupling capacitor (C1). This is calculated in

Equation 2. For analysis purposes, the parasitic capacitances (CP) are

treated as a lumped element and placed in parallel with the series

combination of C1 and CV. This compound capacitance (Ci) is in

series with the bias capacitor (Cb) which is calculated in Equation 3.

The influences of the various capacitances; C1, CP, and Cb, impact

the design by reducing the variable capacitance effects of the varac-

tor which controls the tank resonant frequency and tuning range.

Now the results calculated from Equation 2, Equation 3 and

Equation 4 can be substituted into Equation 1 to calculate the actu-

al frequency of the tank.

To aid in analysis, it is recommended that the designer use a sim-

ple spreadsheet based on Equation 1 through Equation 4 to calcu-

late the frequency of operation for various varactor/inductor selec-

tions before determining the initial starting condition for the tank.

The two main components at the heart of the tank are the induc-

tor (LT) and the varactor diode (CV). The capacitance of a varactor

diode junction changes with the amount of reverse bias voltage

applied across the two terminals. This is the element which actually

“tunes” the VCO. One characteristic of the varactor is the tuning

ratio which is the ratio of the capacitance at specified minimum

and maximum voltage points. For characterizing the ML12149, a

Matsushita (Panasonic) varactor – MA393 was selected. This

device has a typical capacitance of 11 pF at 1.0 V and 3.7 pF at 4.0

V and the C–V characteristic is fairly linear over that range.

Similar performance was also acheived with Loral varactors. A

multi–layer chip inductor was used to realize the LT component.

These inductors had typical Q values in the 35 to 50 range for fre-

quencies between 500 and 1000 MHz.

Note: There are many suppliers of high performance varactors

and inductors and Motorola can not recommend one vendor over

another.

The Q (quality factor) of the components in the tank circuit has a

direct impact on the resulting phase noise of the oscillator. In gen-

eral, the higher the Q, the lower the phase noise of the resulting

oscillator. In addition to the LT and CV components, only high

quality surface–mount RF chip capacitors should be used in the

tank circuit. These capacitors should have very low dielectric loss

(high–Q). At a minimum, the capacitors selected should be operat-

ing 100 MHz below their series resonance point. As the desired fre-

quency of operation increases, the values of the C1 and Cb capaci-

tors will decrease since the series resonance point is a function of

Legacy Applications Information

Equation 2

Ci =

x

+

x

+

C1

CV

C1

CV

Cp

Equation 3

C= Ci

Cb

Ci

Cb

L =

Lp + LT

Equation 4

Issue B