MC12149

4

MOTOROLA RF/IF DEVICE DATA

Figure 2. MC12149 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 k

Ω 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

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 contributions 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 components.

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

following equations:

Equation 2

Ci

+ C1

CV

C1

) CV

) Cp

Equation 3

C

+ Ci

Cb

Ci

) Cb

L =

Lp + LT

Equation 4

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 varactor 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 actual frequency of the tank.

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

a simple spreadsheet based on Equation 1 through

Equation 4 to calculate the frequency of operation for various

varactor/inductor selections before determining the initial

starting condition for the tank.

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

inductor (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 MC12149, 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 frequencies 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 general, 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 operating

100 MHz below their series resonance point. As the desired

frequency of operation increases, the values of the C1 and

Cb capacitors will decrease since the series resonance point

Freescale Semiconductor, Inc.

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