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LANSDALE Semiconductor, Inc.

ML145145

CRYSTAL OSCILLATOR CONSIDERATIONS

The following options may be considered to provide a refer-

ence frequency to Motorola’s CMOS frequency sytnthesizers.

Use of a Hybrid Crystal Oscillator

Commercially available temperature–compensated crystal

oscillators (TCXOs) or crystal–controlled data clock oscilla-

tors provide very stable reference frequencies. An oscillator

capable of sinking and sourcing 50 µA at CMOS logic levels

may be direct or DC coupled to OSCin. In general, the highest

frequency capability is obtained utilizing a direct–coupled

square wave having a rail–to–rail (VDD to VSS) voltage

swing. If the oscillator does not have CMOS logic levels on the

outputs, capacitive or AC coupling to OSCin may be used.

OSCout, and unbuffered output, should be left floating.

For additional information about TCXOs and data clock

oscillators, please consult the latest version of the eem

Electronic Engineers Master Catalog, the Gold Book, or simi-

lar publications.

Design an Off–Chip Reference

The user may design an off–chip crystal oscillator using ICs

specifically developed for crystal oscillator applications, such

as the ML12061 MECL device. The reference signal from the

MECL device is ac coupled to OSCin. For large amplitude sig-

OSCout, an unbuffered output, should be left floating. In gen-

eral, the highest freqency capability is obtained with a

direct–coupled square wave having rail–to–rail voltage swing.

Use of the On–Chip Oscillator Circuitry

The on–chip amplifier (a digital inverter) along with an appro-

priate crystal may be used to provide a reference source fre-

quency. A fundamental mode crystal, parallel resonant at the

desired operating frequency, should be connected as shown in

Figure 7.

For VDD = 5.0 V, the crystal should be specified for a load-

ing capactitanc. CL, which does not exceed 32 pf for frequen-

cies to approximately 8.0 to 15 MHz and 10 pF for higher fre-

quencies. These are guidelines that provide a reasonable com-

promise between IC capacitance, drive capability, swamping

c–variations in stray and IC input/output capacitance, and real-

istic CL values. The shunt load capacitance. CL, presented

across the crystal can be estimated to be:

where

Cin = 5.0 pf (see Figure 8)

Cout = 6.0 pf (see Figure 8)

Ca = 1.0 pf (see Figure 8)

CO = the crystal’s holder capacitance (see Figure 9)

C1 and C2 = external capacitors (see Figure 7)

The oscillator can be “trimmed” on–frequency by making a

portion of all of C1 variable. The crystal and associated com-

ponents must be located as close as possible to the OSCin and

OSCout pins to minimize distortion, stray capacitance, stray

inductance and startup stablilization time. In some cases, stray

capacitance should be added to the value for Cin and Cout.

Power is dissipated in the effective series resistance of the

crystal Re, in Figure 9. The drive level specified by the crystal

manufacturer is the maximum stress that a crystal can with-

stand without damage or excessive shift in frequency. R1 in

Figure 7 limits the drive level. The use of R1 may not be nec-

essary in some cases (i.e., R1 = 0 Ω)

To verify that the maximum dc supply voltage does not over-

drive the crystal, monitor the output frequency as a function of

voltage at OSCout. (Care should be taken to minimize load-

ing.) The frequency should increase very slightly as the dc sup-

ply voltage is increased. An overdriven crystal will decrease in

frequency or become unstable with an increase in supply volt-

age. The operating supply voltage must be reduced or R1 must

be increased in value if the overdriven condition exists. The

user should note that the oscillator start–up time is proportion-

al to the value of R1.

Through the process of supplying crystals for use with

CMOS inverters, many crystal manufactureres have developed

expertise in CMOS oscillator design with crystals. Discussions

with such manufacturers can prove very helpful (see Table 1).

nals (standard CMOS logic levels), DC coupling is used.

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