CMOS logic devices have been used many years oscillators. Many designe

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AND8141/D Unbuffered Gates Crystal Oscillators
CMOS logic devices have been used many years oscillators. Many designers have shied away from trying design their oscillator fear issues that hard foresee. issue determining whether oscillator will "start-up". Another issue size logic circuit. Whereas past, package gates inverters small SOIC package considered very small, today's wireless/handheld products require much smaller footprint board area than package. overcome issues with start-up size Semiconductor come with nearly perfect solution NL27WZU04. implies unbuffered gate. Standard inverters such NL17SZ04 consists pairs channel devices wired together form single inverter (Figure This structure makes excellent inverter with very strong output drive excellent digital characteristics. Unfortunately, this structure less reliable when used oscillator section enormous gain. device overcomes this objection makes ideal oscillator other (gain) application. Figure inverter simpler, with only standard "04" gate channel pair. With only section, device much more usable
application. reader might assume that with only stage, resulting inverter weak; however, this case. standard "04" inverter certainly stronger, "U04" certainly usable inverter. NL27WZU04 buffer drive load, when operating compared NL27WZ04 that drive unbuffered device faster than standard device, since inverter only needs through level logic, instead three. order unbuffered inverter amplifier oscillator, there needs bias established. simplest best solution feedback resistor from output input. large value resistor several megohms needed. output will forced maintain itself input threshold value. This point will only stable point. resistor value should small enough establish bias large enough permit inverter function with some gain. rest circuitry just Pierce crystal oscillator. This particular article will deal only with fundamental mode crystal operation. certainly possible have similar circuit function more overtone mode, however, this tricky this application will left reader.
Input
Output
Input
Output
Figure
Figure
Semiconductor Components Industries, LLC, 2003
December, 2003 Rev.
Publication Order Number: AND8141/D
AND8141/D
Final Circuit have laid board function MHz. NL27WZU04 inverters, used oscillator, second used driver. NL27WZU04 rated from 1.65 empirical test start-up might sweep supply voltage slowly, from zero volts note where oscillation begins. stable circuit will start-up about less function normally from with very little change frequency. circuit does start properly, then select value feedback resistor. shunt capacitor should match loading required crystal manufacturer, taking into account input device well. reader will also note that have included second resistor. Many designers leave this out; however, prevents damage crystal. value this resistor empirically been determined about Another empirical test this value, build test board select value, example, operating voltage desired level, e.g.
vary level ±20%. oscillator frequency should shift small amount should continuous without large abrupt shifts frequency. frequency shift small non-abrupt, value right. frequency jumps erratic with small shifts supply voltage, then increasing value 20-50%. Repeat this process until oscillator stable. Figure shows working schematic layout oscillator capable driving almost circuit that stable known have good startup characteristics. reader invited this starting point oscillator design, create design properly suited voltage, frequency drive requirements. shunt capacitance should value recommended crystal manufacturer, including shunt value NL27WZU04. resistor values Figure good starting points empirical tests should performed.
NL27WZU04 Test Point
Figure
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AND8141/D

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