поискавой системы для электроныых деталей |
|
ML145146 датащи(PDF) 8 Page - LANSDALE Semiconductor Inc. |
|
ML145146 датащи(HTML) 8 Page - LANSDALE Semiconductor Inc. |
8 / 12 page LANSDALE Semiconductor, Inc. ML145146 www.lansdale.com Page 8 of 12 Issue 0 DESIGN CONSIDERATIONS CRYSTAL OSCILLATOR CONSIDERATIONS The following options may be considered to provide a refer- ence frequency to Motorola’s CMOS frequency synthesizers. The most desirable is discussed first. 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 of OSCin may be used. OSCout, an 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 and 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 ampli- tude signals (standard CMOS logic levels), DC coupling is used. OSCout, an unbuffered output, should be left floating. In general, the highest frequency 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 appropriate crystal may be used to provide a reference source frequency. A fundamental mode crystal, parallel resonant at the desired operating frequency, should be connected as shown in Figure 8. For VDD = 5.0 V, the crystal should be specified for a load- ing capacitance. CL, which does not exceed 32 pF for frequen- cies to approximately 8.0 MHz, 20 pF for frequencies in the area of 8.0 to 15 MHz, and 10 pF for higher frequencies. These are guidelines that provide a reasonable compromise between IC capacitance, drive capability, swamping variations stray in IC input/output capacitance, and realistic CL values. The shunt load capacitance, CL, presented across the crystal can be esti- mated to be: where Cin = 5.0pF (See Figure 9) Cout = 6.0pF (See Figure 9) Ca = 1.0pF (See Figure 9) CO = the crystal’s holder capacitance (See Figure 10) C1 and C2 = external capacitors (See Figure 8) The oscillator can be “trimmed” on–frequency by making a portion or 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 stabilization 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 10 The drive level specified by the crys- tal manufacturer is the maximum stress that a crystal can with- stand without damaging or excessive shift in frequency. R1 in Figure 8 limits the drive level. The use of R1 may not be nec- essary in some cases (i.e. R1 = 0 ohms). To verify that the maximum DC supply voltage does not overdrive the crystal, monitor the output frequency as a func- tion of voltage at OSCout. (care should be taken to minimize loading.) the frequency should increase very slightly as the dc supply voltage is increased. An overdriven crystal will decrease in frequency or become unstable with an increase in supply voltage. The operating supply voltage must be reduced or R1 must be increased in value if the overdrive condition exists. The user should note that the oscillator start–up time is propor- tional to the value of R1. Through the process of supplying crystals for use with CMOS inverters, many crystal manufacturers have developed expertise in CMOS oscillator design with crystals. Discussions with such manufacturers can prove very helpful. See Table 1. |
Аналогичный номер детали - ML145146 |
|
Аналогичное описание - ML145146 |
|
|
ссылки URL |
Конфиденциальность |
ALLDATASHEETRU.COM |
Вашему бизинису помогли Аллдатащит? [ DONATE ] |
Что такое Аллдатащит | реклама | контакт | Конфиденциальность | обмен ссыками | поиск по производителю All Rights Reserved©Alldatasheet.com |
Russian : Alldatasheetru.com | Korean : Alldatasheet.co.kr | Spanish : Alldatasheet.es | French : Alldatasheet.fr | Italian : Alldatasheetit.com Portuguese : Alldatasheetpt.com | Polish : Alldatasheet.pl | Vietnamese : Alldatasheet.vn Indian : Alldatasheet.in | Mexican : Alldatasheet.com.mx | British : Alldatasheet.co.uk | New Zealand : Alldatasheet.co.nz |
Family Site : ic2ic.com |
icmetro.com |