參數(shù)資料
型號: EL4584CSZ-EVAL
廠商: Intersil
文件頁數(shù): 2/15頁
文件大?。?/td> 0K
描述: EVALUATION BOARD FOR EL4584
標(biāo)準(zhǔn)包裝: 1
主要目的: 計(jì)時(shí),PLL
嵌入式:
已用 IC / 零件: EL4584CSZ
主要屬性: 適用于高達(dá) 36 MHz 的視頻
次要屬性: 5V 電源,< 2ns 抖動(dòng)(VCXO)
已供物品:
10
FN7174.3
May 9, 2008
The above oscillators are arranged as Colpitts oscillators,
and the structure is redrawn here to emphasize the split
capacitance used in a Colpitts oscillator. It should be noted
that this oscillator configuration is just one of literally
hundreds possible, and the configuration shown here does
not necessarily represent the best solution for all
applications. Crystal manufacturers are very informative
sources on the design and use of oscillators in a wide variety
of applications, and the reader is encouraged to become
familiar with them.
C1 is to adjust the center frequency, C2 DC isolates the
control from the oscillator, and V1 is the primary control
device. C2 should be much larger than CV so that V1 has
maximum modulation capability. The frequency of oscillation
is given by Equations 5 and 6:
Choosing Loop Filter Components
The PLL, VCO, and loop filter can be represented in Figure 15:
FIGURE 15.
Where:
Kd = phase detector gain in A/rad
F(s) = loop filter impedance in V/A
KVCO = VCO gain in rad/s/V
N = internal or external divisor
It can be shown that for the loop filter shown in Equation 7:
Where
n = loop filter bandwidth, and ζ = loop filter damping
factor.
1. Kd = 300A/2πrad = 4.77e-5A/rad for the EL4584.
2. The loop bandwidth should be about HSYNC
frequency/20, and the damping ratio should be 1 for
optimum performance. For our example,
n = 15.734kHz/20 = 787Hz≈5000rad/S.
3. N = 910 from Table 2.
4. KVCO represents how much the VCO frequency changes
for each volt applied at the control pin. It is assumed (but
probably is not) linear about the lock point (2.5V). Its
value depends on the VCO configuration and the varactor
transfer function CV = F(VC), where VC is the reverse
bias control voltage, and CV is varactor capacitance.
Since F(VC) is nonlinear, it is probably best to build the
VCO and measure KVCO about 2.5V. The results of one
such measurement are shown in the following. The slope
of the curve is determined by linear regression
techniques and equals KVCO. For our example,
KVCO = 6.05 Mrad/S/V.
FIGURE 16. FOSC vs VC, LC VCO
5. Now we can solve for C3, C4, and R3. We choose
R3 = 30kΩ for convenience.
6. Notice R2 has little effect on the loop filter design. R2
should be large, around 100k, and can be adjusted to
compensate for any static phase error t
θ at lock, but if
made too large, will slow loop response. If R2 is made
17.734
300
15
0.001
10.738
300
15
0.001
12.273
300
15
0.001
14.318
300
15
0.001
TABLE 4. XTAL VCO COMPONENT VALUES
(APPROXIMATE) (Continued)
FREQUENCY
(MHz)
R1
(k
Ω)
C1
(pF)
C2
(F)
FIGURE 14. COLPITTS OSCILLATOR
F
1
12
π LC
T
--------------------------
=
(EQ. 5)
C
T
C
1C2CV
C
1C2
()
C
1CV
()
C
2CV
()
++
--------------------------------------------------------------------------
=
(EQ. 6)
C
3
K
dKVCO
N
ω
2
n
------------------------ C
4
,
C
3
10
------- R
3
,
2N
ξω
n
K
dKVCO
------------------------
==
=
(EQ. 7)
N
VCOfrequency
H
SYNCfrequency
----------------------------------------------------------
14.31818M
15.73426k
------------------------------
910
==
=
(EQ. 8)
EL4584
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