EL4584CSZ datasheet(10/15 Pages) INTERSIL

10 / 15 page

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

ϖn = loop filter bandwidth, and ζ = loop filter damping

factor.

1. Kd = 300µA/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

0.001

10.738

300

0.001

12.273

300

0.001

14.318

300

0.001

TABLE 4. XTAL VCO COMPONENT VALUES

(APPROXIMATE) (Continued)

FREQUENCY

(MHz)

Ω)

(pF)

(µF)

FIGURE 14. COLPITTS OSCILLATOR

π LC

--------------------------

(EQ. 5)

1C2CV

1C2

()

1CV

()

2CV

()

--------------------------------------------------------------------------

(EQ. 6)

dKVCO

------------------------ C

------- R

ξω

dKVCO

------------------------

(EQ. 7)

VCOfrequency

SYNCfrequency

–

----------------------------------------------------------

14.31818M

15.73426k

------------------------------

910

(EQ. 8)

EL4584

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