CGS410 datasheet(13/18 Pages) NSC

13 / 18 page

30 Circuit Operation (Continued)

TLF11919 – 16

FIGURE 3-14 Common Video Application

Figure 3-15 shows the Pierce Oscillator configuration when

using an external crystal The feedback resistor placed be-

tween XTLIN and XTLOUT biases the input The additional

resistor in the diagram serves to limit the amount of power

dissipated by the crystal This value is based upon crystal

drive specifications In most circumstances this resistor is

not required The two capacitors off the crystal leg serve to

form the crystal tank These components combined with

the electrical function of the crystal form the additional 180

degree phase shift required for oscillation

TLF11919 – 18

FIGURE 3-15 Crystal Configuration

Component values depend on the crystal manufacturer

specifications CXI and CXO will typically range from 10 pF to

30 pF Resistor RXL limits the amount of current flow into

the external crystal tank RXL is usually between 100X and

600X In many instances this component may be eliminat-

ed The feedback resistor (RFB) biases the internal inverter

so proper oscillation can take place Recommended values

are from 10k to 100k

In systems where the lowest possible phase noise is re-

quired a high-Q external VCO may be implemented An

example is illustrated in

Figure 3-16 Here the CGS410 pro-

vides the phase comparison the first stage charge pump

output and the user programmable divider circuitry In these

types of configurations EXTCLK can be driven with a small

sinusoidal input EXTCLK is capacitively coupled while the

VCO is DC terminated

An external OP-AMP such as

National Semiconductor’s LM324 provides the additional

VCO voltage input range required In this example the OP-

AMP is biased by the resistor divider In most instances the

voltage present on the OP-AMP ‘‘a’’ input is half the OP-

AMP source voltage The OP-AMP feedback consists of a

CRC network which provides the voltage input characteris-

tics of the VCO

TLF11919 – 19

FIGURE 3-16 CGS410 Using an External

Loop Filter and VCO

The designer may drive the CGS410 XTLIN in a variety of

configurations In most instances XTLIN is capacitively cou-

pled to remove any DC effects from the source Typical ca-

pacitor values will vary depending on the frequency and de-

sired waveform at the XTLIN input In most instances this

value ranges from several hundred pF up to approximately

001f (See

Figure 3-17 )

TLF11919 – 20

FIGURE 3-17 External XTLIN Drive Options

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