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Trang chủ  http://www.lansdale.com
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Figure 1. Simplified Schematic
Q3
Q4
Q1 Q2
Q5
Q6
VCC
Q
QB
VCC
GND
VREF
CNTL
136Ω
200Ω
1000Ω
Q2
TANK
VREF
LANSDALE Semiconductor, Inc.
ML12149
OPERATIONAL CHARACTERISTICS
A simplified schematic of the ML12149 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 mini-
mize interaction between the VCO outputs and the oscillator tank
transistor pair, a buffer is incorporated into the circuit. This differ-
ential buffer is realized by the Q3 and Q4 transistor pair. The dif-
ferential 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 10 mA. 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 cur-
rent source thus increasing the output amplitude of the Q/QB out-
put pair. If less than 10mA of current is needed, a resistor can be
added to ground which reduces the amount of current.
The Q/QB outputs drive an additional differential buffer which
generate the Q2 output signal. To minimize current, the circuit is
realized as an emitter–follower buffer with an on chip pull down
resistor. This output is intended to drive the prescaler input of the
PLL synthesizer block.
APPLICATION INFORMATION
Figure 2 illustrates the external components necessary for the
proper operation of the VCO buffer. The tank circuit configura-
tion in this figure allows the VCO to be tuned across the full
operating voltage of the power supply. This is very important in
3.0 V 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
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 cir-
cuit which determines the VCO operating frequency. This is
described in 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 ele-
ments. 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 oper-
ating point.
Equation 1
f=
o
1
2
LC
Vcc line without compromising the tuning range of the VCO.
Issue B


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