MC12147 View Datasheet(PDF) - Motorola => Freescale
Part Name
Description
View to exact match
MC12147 Datasheet PDF : 13 Pages
| |||
MC12147
Figure 2. MC12147 Typical External Component Connections
VCC Supply
C3a
C2a
VCC
1
C3a
C2a
CNTL
Note 1
2
R1
C1
TANK
Vin
3
LT VREF
VCO
CV
4
Cb
8
Q L2a
7
GND
6
QB
5
C6a
VCO Output
L2b
C6b
VCO Output
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.
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:
+ ) ) Ci C1 CV Cp
C1 CV
Equation 2
+ ) C Ci Cb
Ci Cb
Equation 3
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 MC12147, a Matsushita
(Panasonic) varactor – MA393 was selected. This device has
a typical capacitance of 11 pF at 1V and 3.7 pF at 4V 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
35–50 range for frequencies between 500 and 1000MHz.
Note: There are many suppliers of high performance
varactors and inductors an 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
4
MOTOROLA RF/IF DEVICE DATA
Share Link: 