For more information www.linear.com/LTC6990 APPLICATIONS INFORMATION Frequency Modula" />
參數(shù)資料
型號： SC-KIT-TBX
廠商： Linear Technology
文件頁數(shù)： 8/30頁
文件大?。?/td> 0K
描述： TIMERBLOX SAMPLE KIT
設(shè)計(jì)資源： TimerBlox Designer
特色產(chǎn)品： TimerBlox?
標(biāo)準(zhǔn)包裝： 1
系列： TimerBlox®
主要目的： 計(jì)時(shí)，時(shí)鐘振蕩器
嵌入式： 否
已用 IC / 零件： LTC6990，LTC6991，LTC6992-1，LTC6993-2，LTC6994-1
已供物品： 裸板，樣品 IC
LTC6990
16
6990fc
For more information www.linear.com/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 resistor.
As shown in Figure 11, voltage VCTRL sources/sinks a cur-
rent through RVCO to vary the ISET current, which in turn
modulatestheoutputfrequencyasdescribedinEquation(2).
fOUT =
1MHz 50k
NDIV RVCO
1
+
RVCO
RSET
VCTRL
VSET
(2)
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.
KVCO =
fOUT(MAX) fOUT(MIN)
VCTRL(MAX) 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,theseparametersprovideausefulandintuitive
way to look at the VCO application. The f(0V) parameter is
the output frequency when VCTRL is at 0V. Viewed another
way, it is the fixed output frequency when the RVCO and
RSETresistorsareinparallel.KVCOisactuallythefrequency
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.
RVCO =
1MHz 50k
NDIV VSET KVCO
(3d)
Select the standard resistor value closest to the calculated
value.
Figure 11. Voltage Controlled Oscillator
6990 F08
LTC6990
OE
GND
SET
OUT
V+
DIV
C1
0.1F
R1
R2
RSET
V+
RVCO
VCTRL
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)– KVCO VCTRL
f(0V) =
1MHz 50k
NDIV RSET RVCO
(
)
KVCO =
1MHz 50k
NDIV VSET RVCO
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
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