MC12147D 查看數據表（PDF） - Motorola => Freescale

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MC12147D

Low Power Voltage Controlled Oscillator Buffer

Motorola => Freescale 

MC12147

OPERATIONAL CHARACTERISTICS

A simplified schematic of the MC12147 is found in

Figure 1. The oscillator incorporates positive feedback by

coupling the base of transistor Q2 to the collector of transistor

Q1. In order to minimize interaction between the VCO

outputs and the oscillator tank transistor pair, a buffer is

incorporated into the circuit. This differential buffer is realized

by the Q3 and Q4 transistor pair. The differential buffer drives

the gate which contains the primary open collector outputs, Q

and QB. The output is actually a current which has been set

by an internal bias driver to a nominal current of 4mA.

Additional circuitry is incorporated into the tail of the current

source which allows the current source to be increased to

approximately 10mA. This is accommodated by the addition

of a resistor which is brought out to the CNTL pin. When this

pin is tied to ground, the additional current is sourced through

the current source thus increasing the output amplitude of the

Q/QB output pair. If less than 10 mA of current is needed, a

resistor can be added to ground which reduces the amount of

current.

APPLICATION INFORMATION

Figure 2 illustrates the external components necessary for

the proper operation of the VCO buffer. The tank circuit

configuration in this figure allows the VCO to be tuned across

the full operating voltage of the power supply. This is very

important in 3V applications where it is desirable to utilize as

much of the operating supply range as possible so as to

minimize the VCO sensitivity (MHz/V). In most situations, it is

desirable to keep the sensitivity low so the circuit will be less

susceptible to external noise influences. An additional benefit

to this configuration is that additional regulation/ filtering can

be incorporated into the VCC line without compromising the

tuning range of the VCO. With the AC–coupled tank

configuration, the Vtune voltage can be greater than the VCC

voltage supplied to the device.

There are four main areas that the user directly influences

the performance of the VCO. These include Tank Design,

Output Termination Selection, Power Supply Decoupling,

and Circuit Board Layout/Grounding.

The design of the tank circuit is critical to the proper

operation of the VCO. This tank circuit directly impacts the

main VCO operating characteristics:

1) Frequency of Operation

2) Tuning Sensitivity

3) Voltage Supply Pushing

4) Phase Noise Performance

The tank circuit, in its simplest form, is realized as an LC

circuit which determines the VCO operating frequency. This

is described in Equation 1.

+ Ǹ fo

1

2p LC

Equation 1

In the practical case, the capacitor is replaced with a

varactor diode whose capacitance changes with the voltage

applied, thus changing the resonant frequency at which the

VCO tank operates. The capacitive component in Equation 1

also needs to include the input capacitance of the device and

other circuit and parasitic elements. Typically, the inductor is

realized as a surface mount chip or a wound–coil. In addition,

the lead inductance and board inductance and capacitance

also have an impact on the final operating point.

Figure 1. Simplified Schematic

VCC

Q

QB

VREF

TANK

Q1 Q2

Q3 Q4

Q5 Q6

VREF

136Ω

200Ω

CNTL

GND

MOTOROLA RF/IF DEVICE DATA

3

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