LTC6990IDCB-TRPB View Datasheet(PDF) - Analog Devices
Part Name
Description
MFG CO.
LTC6990IDCB-TRPB Datasheet PDF : 30 Pages
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LTC6990
APPLICATIONS INFORMATION
Frequency Modulated Operation (Voltage-Controlled
Oscillator)
Operating the LTC6990 as a voltage-controlled oscillator
in its simplest form is achieved with one additional resis-
tor. As shown in Figure 11, voltage VCTRL sources/sinks
a current through RVCO to vary the ISET current, which
in turn modulates the output frequency as described in
Equation (2).
fOUT
=
1MHz • 50k
NDIV •RVCO
•
⎛
⎝⎜
1+
RVCO
RSET
–
VCTRL
VSET
⎞
⎠⎟
(2)
V+
VCTRL
OE
OUT
LTC6990
GND
V+
RVCO
SET
RSET
DIV
6990 F08
V+
C1
0.1µF R1
R2
Figure 11. Voltage Controlled Oscillator
Equation (2) can be re-written as shown below, where
f(0V) is the output frequency when VCTRL = 0V, and KVCO
is the frequency gain. Note that the gain is negative (the
output frequency decreases as VCTRL increases).
fOUT = f(0V) – K VCO • VCTRL
f(0V)
=
1MHz • 50k
NDIV
•
(RSET
R
VCO
)
K VCO
=
1MHz • 50k
NDIV • VSET • R VCO
The design procedure for a VCO is a simple four step
process. First select the NDIV value. Then calculate the
intermediate values KVCO and f(0V). Next, calculate and
select the RVCO resistor. Finally calculate and select the
RSET resistor.
Step 1: Select the NDIV Frequency Divider Value
For best accuracy, the master oscillator frequency should
fall between 62.5kHz and 1MHz. Since fMASTER = NDIV •
fOUT, choose a value for NDIV that meets the following
conditions
62.5kHz ≤
fOUT(MIN)
NDIV ≤
1MHz
fOUT(MAX )
(3a)
The 16:1 frequency range of the master oscillator and
the 2:1 divider step-size provides several overlapping fre-
quency spans to guarantee that any 8:1 modulation range
can be covered by a single NDIV setting. RVCO allows the
gain to be tailored to the application, mapping the VCTRL
voltage range to the modulation range.
Step 2: Calculate KVCO and f(0V)
KVCO and f(0V) define the VCO’s transfer function and sim-
plify the calculation of the the RVCO and RSET resistors.
Calculate these parameters using the following equations.
K VCO
=
fOUT(MAX )
VCTRL(MAX )
–
–
fOUT(MIN)
VCTRL(MIN)
(3b)
f(0V) = fOUT(MAX) + KVCO • VCTRL(MIN)
(3c)
KVCO and f(0V) are not device settings or resistor values
themselves. However, beyond their utility for the resistor
calculations, these parameters provide a useful and intui-
tive way to look at the VCO application. The f(0V) param-
eter is the output frequency when VCTRL is at 0V. Viewed
another way, it is the fixed output frequency when the
RVCO and RSET resistors are in parallel. KVCO is actually
the frequency gain of the circuit.
With KVCO and f(0V) determined, the RVCO and RSET values
can now be calculated.
Step 3: Calculate and Select RVCO
The next step is to calculate the correct value for RVCO
using the following equation.
R
VCO
=
1MHz • 50k
NDIV • VSET • K VCO
(3d)
Select the standard resistor value closest to the calculated
value.
Rev. D
16
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