MC100EL1648
http://onsemi.com
9
VOLTAGE CONTROLLED MODE
The tank circuit configuration presented in Figure 11,
Voltage Controlled Varactor Mode, allows the VCO to be
tuned across the full operating voltage of the power supply.
Deriving from Figure 6, the tank capacitor, C, is replaced
with a varactor diode whose capacitance changes with the
voltage applied, thus changing the resonant frequency at
which the VCO tank operates as shown in Figure 3, tank
option #1. The capacitive component in Equation 1 also
needs to include the input capacitance of the device and
other circuit and parasitic elements.
Figure 11. Voltage Controlled Varactor Mode
50
70
90
110
130
150
170
190
024681
FREQUENCY (MHz)
V
in
, INPUT VOLTAGE (V)
Figure 12. Plot 1. Dual Varactor MMBV609,
V
IN
vs. Frequency
C
L
4 (3)
V
CC
3 (1) 2 (14)
V
IN
F
OUT
Tank #1
8 (10)
1 (12)
*
0.1 mF0.1 mF
5 (5)6 (7) 7 (8)
V
EE
0.1 mF 0.1 mF0.01 mF100 mF
**
1 KW
*Use high impedance probe (>1.0 MegW must be used).
**The 1200 W resistor and the scope termination imped-
ance constitute a 25:1 attenuator probe. Coax shall be
CT−070−50 or equivalent.
L = Micro Metal torroid #T20−22, 8 turns #30
Enameled Copper wire (@ 40 nH)
C = MMBV609
8 pin (14 pin) lead package
When operating the oscillator in the voltage controlled
mode with Tank Circuit #1 (Figure 3), it should be noted that
the cathode of the varactor diode (D), pin 8 (for 8 lead
package) or pin 10 (for 14 lead package) should be biased at
least 1.4 V above V
EE
.
Typical transfer characteristics employing the
capacitance of the varactor diode (plus the input capacitance
of the device, about 6.0 pF typical) in the voltage controlled
mode is shown in Plot 1, Dual Varactor MMBV609 V
in
vs.
Frequency. Figure 6, Figure 7, and Figure 8 show the
accuracy of the measured frequency with the different
variable capacitance values. The 1.0 kW resistor in Figure 11
is used to protect the varactor diode during testing. It is not
necessary as long as the dc input voltage does not cause the
diode to become forward biased. The tuning range of the
oscillator in the voltage controlled mode may be calculated
as follows:
f
max
f
min
+
C
D
(max) ) C
S
Ǹ
C
D
(min) ) C
S
Ǹ
Where
f
min
+
1
2p
ǒ
L(C
D
(max) ) C
S
Ǔ
Ǹ
Where
C
S
= Shunt Capacitance (input plus external
capacitance)
C
D
= Varactor Capacitance as a function of bias
voltage
Good RF and low−frequency bypassing is necessary on
the device power supply pins. Capacitors on the AGC pin
and the input varactor trace should be used to bypass the
AGC point and the VCO input (varactor diode),
guaranteeing only dc levels at these points. For output
frequency operation between 1.0 MHz and 50 MHz, a 0.1 mF
capacitor is sufficient. At higher frequencies, smaller values
of capacitance should be used; at lower frequencies, larger
values of capacitance. At high frequencies, the value of
bypass capacitors depends directly on the physical layout of
the system. All bypassing should be as close to the package
pins as possible to minimize unwanted lead inductance.
Several different capacitors may be needed to bypass
various frequencies.
