MC12147 View Datasheet(PDF) - Motorola => Freescale
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MC12147 Datasheet PDF : 13 Pages
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MC12147
is a function of the capacitance value. To simplify the
selection of C1 and Cb, a table has been constructed based
on the intended operating frequency to provide
recommended starting points. These may need to be altered
depending on the value of the varactor selected.
Frequency
200 – 500 MHz
500 – 900 MHz
900 – 1200 MHz
C1
47 pF
5.1 pF
2.7 pF
Cb
47 pF
15 pF
15 pF
The value of the Cb capacitor influences the VCO supply
pushing. To minimize pushing, the Cb capacitor should be
kept small. Since C1 is in series with the varactor, there is a
strong relationship between these two components which
influences the VCO sensitivity. Increasing the value of C1
tends to increase the sensitivity of the VCO.
The parasitic contributions Lp and Cp are related to the
MC12147 as well as parasitics associated with the layout,
tank components, and board material selected. The input
capacitance of the device, bond pad, the wire bond,
package/lead capacitance, wire bond inductance, lead
inductance, printed circuit board layout, board dielectric, and
proximity to the ground plane all have an impact on these
parasitics. For example, if the ground plane is located directly
below the tank components, a parasitic capacitor will be
formed consisting of the solder pad, metal traces, board
dielectric material, and the ground plane. The test fixture
used for characterizing the device consisted of a two sided
copper clad board with ground plane on the back. Nominal
values where determined by selecting a varactor and
characterizing the device with a number of different tank/
frequency combinations and then performing a curve fit with
the data to determine values for Lp and Cp. The nominal
values for the parasitic effects are seen below:
Parasitic Capacitance
Parasitic Inductance
Cp
4.2 pF
Lp
2.2 nH
These values will vary based on the users unique circuit
board configuration.
Basic Guidelines:
1. Select a varactor with high Q and a reasonable
capacitance versus voltage slope for the desired
frequency range.
2. Select the value of Cb and C1 from the table above .
3. Calculate a value of inductance (L) which will result in
achieving the desired center frequency. Note that L
includes both LT and Lp.
4. Adjust the value of C1 to achieve the proper VCO
sensitivity.
5. Re–adjust value of L to center VCO.
6. Prototype VCO design using selected components. It
is important to use similar construction techniques and
materials, board thickness, layout, ground plane
spacing as intended for the final product.
7. Characterize tuning curve over the voltage operation
conditions.
8. Adjust, as necessary, component values – L,C1, and
Cb to compensate for parasitic board effects.
9. Evaluate over temperature and voltage limits.
10. Perform worst case analysis of tank component
variation to insure proper VCO operation over full
temperature and voltage range and make any
adjustments as needed.
Outputs Q and QB are open collector outputs and need a
inductor to VCC to provide the voltage bias to the output
transistor. In most applications, dc–blocking capacitors are
placed in series with the output to remove the dc component
before interfacing to other circuitry. These outputs are
complementary and should have identical inductor values for
each output. This will minimize switching noise on the VCC
supply caused by the outputs switching. It is important that
both outputs be terminated, even if only one of the outputs is
used in the application.
Referring to Figure 2, the recommended value for L2a and
L2b should be 47 nH and the inductor components
resonance should be at least 300 MHz greater than the
maximum operating frequency. For operation above 1100
MHz, it may be necessary to reduce that inductor value to 33
nH. The recommended value for the coupling capacitors
C6a, C6b, and C7 is 47 pF. Figure 2 also includes decoupling
capacitors for the supply line as well as decoupling for the
output inductors. Good RF decoupling practices should be
used with a series of capacitors starting with high quality 100
pF chip capacitors close to the device. A typical layout is
shown below in Figure 3.
The output amplitude of the Q and QB can be adjusted
using the CNTL pin. Refering to Figure 1, if the CNTL pin is
connected to ground, additional current will flow through the
current source. When the pin is left open, the nominal current
flowing through the outputs is 4 mA. When the pin is
grounded, the current increases to a nominal value of 10 mA.
So if a 50 ohm resistor was connected between the outputs
and VCC, the output amplitude would change from 200 mV
pp to 500 mV pp with an additional current drain for the
device of 6 mA. To select a value between 4 and 10 mA, an
external resistor can be added to ground. The equation below
is used to calculate the current.
) ) (200 136 Rext) 0.8V
+ ) Iout(nom)
200 (136 Rext)
Figure 4 through Figure 13 illustrate typical performance
achieved with the MC12147. The curves illustrate the tuning
curve, supply pushing characteristics, output power, current
drain, output spectrum, and phase noise performance. In
most cases, data is present for both a 750 MHz and 1200
MHz tank design. The table below illustrates the component
values used in the designs.
Component 750MHz Tank 1200MHz Tank Units
R1
5000
5000
Ω
C1
5.1
2.7
pF
LT
4.7
1.8
nH
CV
3.7 @ 1.0 V
3.7 @ 1.0 V
pF
11 @ 4.0 V
11 @ 4.0 V
Cb
100*
15
pF
C6, C7
47
33
pF
L2
47
47
nH
* The value of Cb should be reduced to minimize pushing.
MOTOROLA RF/IF DEVICE DATA
5
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