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參數(shù)資料
| 型號： | LTC6990IS6#TRPBF |
| 廠商： | LINEAR TECHNOLOGY CORP |
| 元件分類： | XO, clock |
| 英文描述： | VCO, 0.000488 MHz - 2 MHz |
| 封裝： | LEAD FREE, PLASTIC, TSOT-23, 6 PIN |
| 文件頁數(shù)： | 8/28頁 |
| 文件大小： | 310K |
| 代理商： | LTC6990IS6#TRPBF |
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LTC6990
16
6990f
APPLICATIONS INFORMATION
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 PRVCO
(
)
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
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) dene the VCO’s transfer function and
simplify the calculation of the the RVCO and RSET resis-
tors. 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, these parameters provide a useful and intuitive
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 xed 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.
Step 4: Calculate and Select RSET
The nal step is to calculate the correct value for RSET
using the following equation:
RSET =
1MHz 50k
NDIV f(0V) VSET KVCO
(
)
(3e)
Select the standard resistor value closest to the calculated
value.
Some applications require combinations of fOUT(MIN),
fOUT(MAX), VCTRL(MIN) and VCTRL(MAX) that are not achiev-
able. These applications result in unrealistic or unrealiz-
able (e.g. negative value) resistors. These applications
will require preconditioning of the VCTRL signal via range
scaling and/or level shifting to place the VCTRL into a range
that yields realistic resistor values.
Frequency Error in VCO Applications Due to VSET Error
As stated earlier, f(0V) represents the frequency for VCTRL
= 0V, which is the same value as would be generated by
a single resistor between SET and GND with a value of
RSET || RVCO. Therefore, f(0V) is not affected by error or
drift in VSET (i.e. ΔVSET adds no frequency error when
VCTRL = 0V).
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