? semiconductor components industries, llc, 2003 november, 2003 ? rev. 3 1 publication order number: and8053/d and8053/d designing crystal oscillators with modern cmos gates prepared by: fred zlotnick applications manager introduction designing a crystal oscillator with modern cmos gates is difficult with the limited information available. the designer has many families to choose from: 4000 series, hcmos, lcx, and vhc, for starters. the selection of the logic family depends on the system voltage and frequency of operation. designing a crystal oscillator using gates instead of buying a pre?packaged device that includes all the active components provides the following benefits: ? save money ? control the parameters that are critical to the design in order to save money, the designer has to be willing to undertake the design and understand the tradeoffs. by doing this, he also is able to take control of critical parameters such as frequency, accuracy, and stability. this article will only be concerned with the fundamental mode operation. today's crystals can be purchased with fundamental modes up to about 30 mhz. these techniques also work well for ceramic resonators. v cc figure 1. crystal oscillator using unbuffered cmos gate theory of operation: in nearly all cases, using a gate as an oscillator entails causing the gate to go into class aao or biased between the on and off state. this is accomplished by placing a large resistor, between 10 and 20 m from the output to the input. the mode of operation is the apierceo crystal oscillator (amateur radio handbook, 1968). this circuit can be considered a grounded source transistor, with capacitors to ground. the input (gate) and output (drain) are afloating.o this circuit has been around in vacuum tube form for many years, and is just a modern version of the circuit. to implement this circuit, an aunbufferedo inverter is usually used (see figure 1). devices such as the mc74vhcu04 or mc74lcxu04, available from on semiconductor, will provide stable operation from below 1 mhz to above 20 mhz, from 2?5 volts operation. since there are six active inverters in each 14?pin package, the designer is encouraged to use at least one of the devices as a buffer. the unbuffered gate, such as a vhcu04, contains just one p?channel, and one n?channel device. since there is just one complementary pair, this is only one transistor gate delay. these devices are fabricated in 0.6 cmos and have significant class a gain for any crystal application up to 100 mhz or more. if the designer needs a circuit to operate between 7 and 18 volts, the mc14049ub from the 4000 series of metal gate cmos makes a good choice. the designer has to be careful not to make demands on the circuit that exceed the maximum frequency capabilities of this family (~ 4?10 mhz). in all cases, the designer needs to be aware that it is possible to damage the crystal with over?active excitation. by using the unbuffered inverters and a series resistor (see figure 2), in the vhc or metal gate series, the designer can be confident that there will not be too much drive. a test to determine whether appropriate drive is being obtained is described in the high?speed cmos data book, dl129/d. the designer builds a prototype of the circuit and applies v cc through a variable supply. if the frequency shifts very slightly higher when the supply voltage is varied, the design is valid. if the frequency decreases or becomes unstable, a resistor should be added to the circuit. application note http://onsemi.com
and8053/d http://onsemi.com 2 1?25 mhz 12 m 1 2 34 osc out 12 p 12 p v cc figure 2. complete crystal oscillator circuit nl27wzu04 10 k 5 6 the above circuit will serve as a good starting point for a design. the capacitor values may need to be altered to suit the crystal. this same design could be done using standard 14 pin devices, e.g. mc74hcu04 ? just remember to tie off unused inputs. on semiconductor and are registered trademarks of semiconductor components industries, llc (scillc). scillc reserves the right to mak e changes without further notice to any products herein. scillc makes no warranty, representation or guarantee regarding the suitability of its products for an y particular purpose, nor does scillc assume any liability arising out of the application or use of any product or circuit, and specifically disclaims any and all liability, including wi thout limitation special, consequential or incidental damages. atypicalo parameters which may be provided in scillc data sheets and/or specifications can and do vary in different application s and actual performance may vary over time. all operating parameters, including atypicalso must be validated for each customer application by customer's technical experts. scillc does not convey any license under its patent rights nor the rights of others. scillc products are not designed, intended, or authorized for use as components in systems intended for surgical implant into the body, or other applications intended to support or sustain life, or for any other application in which the failure of the scillc product could create a sit uation where personal injury or death may occur. should buyer purchase or use scillc products for any such unintended or unauthorized application, buyer shall indemnify and hold scillc and its officers, employees, subsidiaries, af filiates, and distributors harmless against all claims, costs, damages, and expenses, and reasonable attorney fees arising out of, direct ly or indirectly, any claim of personal injury or death associated with such unintended or unauthorized use, even if such claim alleges that scillc was negligent regarding the design or manufacture of the part. scillc is an equal opportunity/affirmative action employer. this literature is subject to all applicable copyright laws and is not for resale in any manner. publication ordering information n. american technical support : 800?282?9855 toll free usa/canada japan : on semiconductor, japan customer focus center 2?9?1 kamimeguro, meguro?ku, tokyo, japan 153?0051 phone : 81?3?5773?3850 and8053/d literature fulfillment : literature distribution center for on semiconductor p.o. box 5163, denver, colorado 80217 usa phone : 303?675?2175 or 800?344?3860 toll free usa/canada fax : 303?675?2176 or 800?344?3867 toll free usa/canada email : orderlit@onsemi.com on semiconductor website : http://onsemi.com order literature : http://www.onsemi.com/litorder for additional information, please contact your local sales representative.
|
