LTC6906 datasheet(16/30 Pages) AD

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LTC6990

Rev. D

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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)

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) – 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, 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.

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

GND

SET

OUT

V+

DIV

0.1µF

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

(

)

K VCO =

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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