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Clock Oscillator Application Notes

The purpose of these application notes is to help customers in specifying Clock Oscillators. Background information about the type of Oscillators offered by ECS is included along with some common definitions and helpful formulas. The ECS Oscillator product line consists of Clock Oscillators, TCXOs, VCXOs, VCTCXOs and VCOs.

Clock Oscillator: The standard clock oscillator is the most common type of oscillator used and has applications in virtually every aspect of the electronics industry. The clock oscillator is used to establish a reference frequency used for timing purposes. A typical application is the sequencing of events in a computer.

A crystal controlled clock oscillator typically consists of an amplifier and a feedback network that selects a part of the amplifier output and returns it to the amplifier input. A simplified block diagram of such a circuit is shown below in (Fig 1).

 clock oscillator application notes figure 1
Figure 1) Simplified Block Diagram of a Crystal Controlled Clock Oscillator

The basic criteria for oscillation in an oscillator are: 1. The open loop gain must be greater than the losses around the oscillator loop and 2. The phase shift around the oscillator loop must be either 0 or 360 degrees.

An oscillator can be used to generate different types of waveforms. The most common types of waveforms produced by an oscillator are sinusoidal and square.

The Main parameters used in specifying a clock oscillator are listed below.

Logic TTL, HCMOS: In general, an HCMOS oscillator with drive TTL circuitry (not vice versa). The industry is moving away from the TTL logic as IC manufacturers are discontinuing the supply for many common TTL IC’s. Most ECS clock oscillators are HCMOS/TTL compatible.

Frequency Stability: The most common stabilities are 25, 50 and 100 PPM. Overall stability usually includes accuracy at 25°C, effects due to changes in operating temperature, input voltage, aging, shock and vibration. The ± 100PPM stability has been the most popular as it is sufficient to run microprocessors. The Telecommunications industry has been moving toward tighter and tighter stabilities. Stabilities beyond ± 100PPM are no longer offered in commercial (0-70°C) applications, since standard process controls achieve this stability as a minimum. Requesting 50 PPM is usually a little more expensive. Clock oscillators requiring 25 PPM can significantly affect the price. For tighter than 25 PPM stability applications, please consult the factory or consider a TCXO.

TCXOs (Temperature Compensated Crystal Oscillators)

Typically consist of tight tolerance quartz crystal, a temperature compensation network, an oscillator circuit and a variety of buffer and/or output stages determined by the output requirement. The crystal has a characteristic of changing frequency when a capacitor is inserted in series with the crystal unit as shown in (Fig. 2)


clock oscillator application notes figure 2

Figure 2) Load Capacitance Characteristics of Crystal Unit

Utilizing the above characteristics, frequency can be stabilized by inserting a temperature compensation circuit consisting of thermistors, resistors and capacitors in the oscillation look as shown in (Fig. 3). The temperature compensation network is used to sense the ambient temperature and “pull” the crystal frequency in a manner which reduces frequency vs. temperature effect of the quartz crystal.

clock oscillator application notes figure 3

Figure 3) Temperature Compensation Circuit

A TCXO is generally required when overall stability needs are greater than those of a clock oscillator. Also, the long-term aging effects of a TCXO are better than those of most clock oscillators.

Input Voltage: Most TCXOs are designed to operate at 5VDC, 3.3 VDC or a combination of both.

RF Output: A TCXO can be manufactured with various types of outputs: sine wave, clipped sine wave, TTL, HCMOS and ECL. Be sure to specify the desired output type, signal requirements and the load that the oscillator will be driving.

TCXOs also have a frequency adjustment feature which allow for readjustment of the oscillator to its center frequency to compensate for aging. This adjustment can be provided in the following ways.

1) A mechanical adjustment (internal trimmer) within the oscillator accessible via hole in the enclosure.

2) An electrical adjustment via a lead in the enclosure for either a remotely located potentiometer or a voltage. An oscillator using this technique is called a Temperature Compensated Voltage Controlled Crystal Oscillator or TCVCXO.

3) A combination of both mechanical and electrical adjustment.

VCXOs (Voltage Controlled Crystal Oscillator) are crystals controlled oscillators in which the output frequency can be adjusted by varying the external control voltage across a variable capacitor ( varactor diode) within the oscillator circuit. The associated change in frequency due to the change in control voltage is known as pullability. VCXOs are used widely in telecommunications, instrumentation and other electronic equipment where a stable but electrically tunable oscillator is required.

The varactor diode is a semiconductor device that is designed to act as a variable capacitor when a voltage is applied to it. When used in series with crystal, as shown in (Fig. 4), changing the control voltage causes diode capacitance to change. This change in capacitance causes the total crystal load capacitance to change and subsequently causes a change in crystal frequency.

clock oscillator application notes figure 4

Figure 4) Typical VCXO Circuit

Due to the growing applications of VCXOs in digital data transmissions phase jitter (short-term stability) has become an important consideration. Phase jitter provides a precise way to establish when a phase transition occurs.

Definitions: The following definitions will aid you in understanding oscillator performance and terminology.

Nominal Frequency: The center or nominal output of a crystal oscillator.

Frequency Tolerance: The deviation from the nominal frequency in terms of parts per million (PPM) at room temperature. (25°C ±5°C)

Frequency Range: The frequency band that the oscillator type or model can be offered.

Frequency Stability: The maximum allowable frequency deviation compared to the measured frequency at 25°C over the temperature window, i.e. 0°C to +70°C. The typical stability for clock oscillators is ±0.01% (±100PPM).

Operating Temperature: Temperature range within which output frequency and other electrical, environmental characteristics meet the specifications.

Aging: The relative frequency change over a certain period of time. Typically aging for clock oscillators is ±5PPM over 1 year maximum.

Storage Temperature: The temperature range within which the unit is safely stored without damaging or changing the performance of the unit.

Supply Voltage: The maximum voltage which can safely be applied to the VCC terminal with respect to ground.

Input Voltage (VIN): The maximum voltage which can be safely applied to any input terminal of the oscillator.

Output HIGH Voltage (VOH): The minimum voltage at an output of the oscillator under proper loading.

Output LOW Voltage (VIH): The maximum voltage to guarantee threshold trigger at the input of the oscillator.

Supply Current: The Current flowing into Vcc terminal with respect to ground. Typically supply current is measured without load.

Symmetry of Duty Cycle: The symmetry of the output waveform at the specified level (at 1.4 V for TTL, at 1/2 Vcc for HCMOS, or 1/2 waveform peak level for ECL).

Rise Time (TR): Waveform rise time from Low to High transition measured at the specified level (20% to 80% for HCMOS, ECL and 0.4 V to 2.4 V for TTL).

Fall Time (TF): The waveform fall time from High to Low transition, measured at the specified level (80% to 20% for the HCMOS, ECL and 2.4 V to 0.4 V for TTL).

Load/Fan Out: The maximum load that the different families of oscillators can drive is defined as the output load driving capability. The load driving capability (fan-out) of each family of oscillators is specified in terms of the number of gates an oscillator can drive.

Jitter (short-term stability): The modulation in phase or frequency of the oscillator output.

HCMOS/TTL Compatible: The oscillator is designed with ACMOS logic with driving capability of TTL and HCMOS loads while maintaining minimum logic High of HCMOS.

Tri-State Enable: When the input is left OPEN or tied to logic “1” the normal oscillation occurs. When the input is grounded (tied to logic “0”, the output is HIGH IMPEDANCE state. The input has an internal pull-up resistor thus allowing the input to be left open.

Output Logic: The output of an oscillator is designed to meet various specified logic’s such as TTL, HCMOS, ECL, Sine, Clipped-Sine (DC cut).

Harmonic Distortion: The non-linear distortion due to unwanted harmonic spectrum component related with target signal frequency. Each harmonic component is the ratio of electric power against desired signal output electric power and is expressed in terms of dbc, i.e. -20 dBc. Harmonic distortion specification is important especially in sine output when a clean and less distorted signal is required.

Dual and Multiple Outputs: More than one signal is capable of being generated from a single oscillator. The signals may be related (usually a multiple or divisor of the signal produced by a single crystal).

Start-Up Time: The start up time of an oscillator is defined as the time an oscillator takes to reach its specified RF output amplitude.



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Quartz Crystal Design Parameters

Series vs. Parallel: “Series” resonant crystals are intended for use in circuits which contain no reactive components in the oscillator feedback loop. “Parallel” resonant crystals are intended for use in circuits which contain reactive components (usually capacitors) in the oscillator feedback loop. Such circuits depend on the combination of the reactive components and the crystal to accomplish the phase shift necessary to start and maintain oscillation at the specific frequency. Basic depictions of two such circuits are shown below.


Load Capacitance: This refers to capacitance external to the crystal, contained within the feedback loop of the oscillator circuit. If the application requires a “parallel” resonant crystal, the value of load capacitance must be specified. If the application requires a “series” resonant crystal, load capacitance is not a factor and need not be specified. Load capacitance is the amount of capacitance measured or computed across the crystal terminals on the PCB.

Frequency Tolerance: Frequency tolerance refers to the allowable deviation from nominal, in parts per million (PPM), at a specific temperature, usually +25° C.

Aging: Aging refers to the cumulative change in frequency experienced by a crystal unit over time. The rate of frequency change is fasted during the first 45 days of operation. The most common factors affecting aging include drive level, internal contamination, crystal surface change, ambient temperature, wire fatigue and frictional wear. All of these problems can be minimized by proper circuit design which allows for low operating temperatures, minimum drive levels and static pre-aging.

Pullability: Pullability refers to the change in frequency of a crystal unit, either from the natural resonant frequency (Fr) to a load resonant frequency (FL), or from one load resonant frequency to another. See Figure C. The amount of pullability exhibited by a given crystal unit at a given value of load capacitance is a function of the shunt capacitance (Co) and the motional capacitance (C1) of the crystal unit.


If pullability is a factor in design, collaboration with our engineers is advisable; bandwidth can be controlled to some extent, during fabrication, by varying the crystal parameters. An approximation of the pulling limits for standard crystals can be obtained from the following formula:

equation one

The exact limits also depend upon the Q of the crystal as well as associated stray capacitances. Pullability can be approximately doubled by modified crystal fabrication and by adding capacitance or inductance external to the crystal. If the Co and C1 are known then the pulling in ppm between two capacitances can be obtained using the following formula.

equation two

To obtain AVERGE pulling per pF about a known load capacitance use the following formula.

equation three

Equivalent Circuit: The equivalent circuit, shown in Figure B is an electrical depiction of the quartz crystal unit when operating at frequency of natural resonance. The CO, or shunt capacitance, represents the capacitance of the crystal electrodes plus the capacitance, of the holder leads. R1, C1 and L1 compose the “motion arm” of the crystal and are referred to as the motional parameters. The motional inductance (L1) represents the vibrating mass of the crystal unit. The motional capacitance (C1), represents the elasticity of the quartz and the resistance (R1), represents bulk losses occurring within the quartz.


Impedance/Reactance Curve: A crystal has two frequencies of zero phase, as illustrated in Figure D. The first, or lower of the two, is Series Resonant Frequency, denoted as (ꬵs). At this point, the crystal appears resistive in the circuit, impedance is at a minimum and current flow is maximum. As the frequency is increased beyond the point of series resonance, the crystal appears inductive in the circuit. When the reactances of the motional inductance and shunt capacitance cancel, the crystal is at the Frequency of Anti-resonance, denoted as (fa). At this point, impedance is maximized and current flow is minimized.

Shock Characteristics: Although crystals are designed to handle normal shock in handling, shock impulses (such as half sine, square, sawtooth and complex combinations) can occur in the field. Because crystals are relatively delicate, they should be isolated from equipment to minimize shock damage. But, avoid over specification, since the elastic properties of the materials and the degree of isolation afforded by the equipment can decrease the destructive potential of a shock.

Quality factor (Q): The “Q” value of a crystal unit is a measure of the units relative quality, or efficiency of oscillation. The maximum attainable stability of a crystal unit is dependent on the “Q” value. In Figure D the separation between the series and parallel frequencies is called the bandwidth. The smaller the bandwidth, the higher the “Q” value, and the steeper the slope of the reactance. Changes in the reactance of external circuit components have less effect (less “pullability”) on a high “Q” crystal, therefore such a part is more stable.


Calculation of Load Capacitance: if the circuit configuration is as shown in Figure A for the parallel version, the load capacitance may be calculated by means of the following equation:

equation four

C stray includes the pin to pin input and output capacitance of the microprocessor chip at the Crystal 1 and Crystal 2 pins, plus any parasitic capacitances. As a rule of thumb, C stray may be assumed to equal 5.0 pF. Therefore, if CL1 = CL2 = 50pF, CL =30pF.
Trim Sensitivity: Trim sensitivity is a measure of the incremental fractional frequency change for an incremental change in the value of the load capacitance. Trim sensitivity (S) is expressed in terms of PPM/pF and is calculated by the following equation:

equation five

Where (Ct) is the sum of Co and CL.

Solder Reflow of Surface Mount Devices: Mounting of SMD units is typically accomplished by means of solder reflow, as indicated in Figure E either by infrared heat or by vapor phase. The following graphs depicts the recommended times and temperatures for each of the two methods.


Soldering Characteristics: A variety of methods can be used to solder ECS products to P.C.B.s and substrates:
• Wave or Dual Wave
• Hot Air or Convection Flow
• Vapor Phase Reflow
• Infrared Reflow
• Bubble Solder Immersion
• Other (laser, etc.)

chart one

Due to the natural characteristics of material, some of our products cannot withstand heat shock. Extreme temperatures can cause tin (Sn) plating from the inside of the enclosure to reach its melting point, depositing solder on the quartz element. This can cause the component to oscillate at a lower frequency or fail completely. In other cases, solder contact can degrade, resulting in an open circuit. These problems can be avoided by preheating the components and board, and following the recommended soldering process time/temperature profiles noted above.

Note: It is important to check with your ECS factory representative before subjecting any crystal components to extreme environmental conditions.

Useful Crystal Equations:

chart two

Field Vibration: There are two basic types of vibration, periodic and random. Typically, vibration in the field produces complex waves of motion which can affect the output of quartz crystals. Most failures due to vibration occur as a direct result of mechanically amplified resonances, as higher acceleration levels are reached by resonant areas, resulting in higher potential for damage. All factors influencing vibrations should be thoroughly evaluated by using a prototype. Structural system, component location, mounting and encapsulation should all be considered to maximize stability. Remember that crystals are designed to withstand normal handling vibration; added ruggedizing may adversely affect desirable qualities such as stability tolerance or aging.


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Principles of Operation for Ceramic Resonators

Equivalent Circuit Constants: Fig.1.2 shows the symbol for a ceramic resonator. The impedance and phase characteristics measured between the terminals are shown in Fig.1.5. This figure illustrates that the resonator becomes inductive in the frequency range between the frequency fr (resonant frequency), which provides the minimum impedance, and the frequency fa (anti-resonant frequency), which provides the maximum impedance. It becomes capacitative in other frequency ranges. This means that the mechanical osciallation of a two-terminal resonator can be replaced with an equvalent circuit consisting of a combination of series and parallel resonant circuits with an inductor L, a capacitor C, and a resistor R. In the vicinity of the resonant frequency, the equivalent circuit can be expressed as shown in Fig.1.4. The fr and fa frequencies are determined by the piezoelectric ceramic material and its physical parameters. The equivalent circut constants can be determined from the following formulas:

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Considering the limited frequency range of fr<f<fa, the impedance is given as Z=Re+jwLe (Le<=0) as shown in Fig.1.5. The ceramic resonator should operate as an inductor Le (H) having the loss Re (). Fig 1.1 shows comparisons for equivalent circuit constants between a ceramic resonator and a quartz crystal resonator. Note there is a large difference in capacitance and Qm which results in the difference of oscillating conditons when actually operated. The table in the appendix shows the standard values of equivalent circuit constants for each type of ceramic resenator. Higher harmonics for other modes of oscillation exist other than the desired oscillation mode. These other oscillaiton modes exist because the ceramic resonator uses mechanical resonance. Fig.1.6 shows these characteristics.

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Basic Oscillating Circuits

Generally, the oscillating circuits can be grouped into the following three types:

1. Positive feedback

2. Negative resistance element

3. Delay of transfer time or phase in the case of ceramic resonators, quartz crystal resonators, and LC oscillators, positive feedback is the circuit of choice.

Among the positive feedback oscillation circuits using LC, the tuning type anti-coupling oscillation circuit, by Colpitts and Hartley, are typically used. See Fig. 1.7.

In Fig.1.7 a transistor, which is the most basic amplifier, is used.

The oscillation frequencies are approximately the same as the resonance frequency of the circuit consisting of L, CL1, and Cl2 in the Colpitts circuit or consisting of L1, L2, and C in the Hartley circuit. These frequencies can be represented by the following formulas.

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In a ceramic resonator oscillator, the inductor is replaced by a ceramic resonator, taking advantage of the fact that the resonator becomes inductive between resonant and anti-resonant frequencies. The most commonly used circuit is the Colpitts circuit.

The operating principle of these oscillation circuits can be seen in Fig.2.1. Oscillation occurs when the followin conditions are satisfied. Loop Gain: G= a : B> 1 Phase amount:

In a Colpitts circuit, an inversion of 180 is used, and it is inverted more than =180 with L and C in the feedback circuit. The operation with a ceramic resonator can be considered as the same.

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Typical Oscillation Circuit: The most common oscillator circuit for a ceramic resonator is a Colpitts circuit. The design of the circuit varies with the application and the IC to be used, etc. Although the basic configurations of the circuit is the same as that of a crystal controlled oscillator, the difference in mechanical Q results from a difference in circuit constants. Some typical examples follow.

Design Considerations: It is becoming more common to configure the oscillation circuit with a digital IC, using an inverter gate. Fig.3.1 on the following page shows the configuration of a basic oscillation circuit with a CMOS inverter. INV.1 operates as an inverting amplifier for the oscillating circuit. INV.2 is used as a waveform shaper and also acts as a buffer for the output. The feedback resistance Rf provides negative feedback around the inverter so that oscillation will start when power is applied. If the value of Rf is too large and the insulation resistance of the input inverter is too low, then oscillation will stop due to the loss of loop gain. Also, if Rf is too great, noise from other circuits can be introduced into the oscillation circuit. Obviously, if Rf of 1M is generally used with a ceramic resonator. Damping resistor Rd has the following function although it is sometimes omitted. It makes the coupling between the inverter and the feedback circuit loose; thereby decreasing the load on the output side of the inverter. In addition, the phase of the feedback circuit is stabilized. It also provides a means of reducing the gain at higher frequencies, thus preventing the possibility of spurious oscillation.

Loading Capacitance: Load capacitance CL1 and CL2 provides a phase lag of 180. These values should be properly selected depending on the application, the IC used, and the frequency. If CL1 and CL2 are lower values than necessary, the loop gain at high frequencies is increased, which in turn increases the probability of spurious oscillation. This is particularly likely around 4-5MHz where the thickness vibration mode lies.

 Screen Shot CER 7

This clearly shows that the oscillation frequency is influenced by the loading capacitance. Caution should be taken in defining its value when a tight tolerance for oscillation frequency is required.

CMOS Inverter: A CMOS inverter can be used as the inverting amplifier, the one-stage type of the 4069 CMOS group is the most useful. Because of the excessive gain, ring oscillation or CR oscillation is a typical problem when using the three-stage buffer type inverterm such as the 4049 group. ECS employs the RCA CD4O69UBE as a CMOS standard circuit, as shown in Fig.3.2.

HCMOS Inverter Circuit: Recently, the high speed CMOS (HCMOS) is increasingly being used for circuits allowing high speed and low power consumption for microprocessors. There are two types if HCMOS inverters: the un-buffered 74HCU series and the 74HC series with buffers. The 74HCU system is optimum for ceramic resonators. See Fig.3.3.

TTL Inverter Circuit: The value of load capacitance CL1 and CL2 should be greater than those of CMOS due to impedance matching. In addition, the feedback resistance Rf should be as small as several  K. Note that the bias resistance Rd is required to properly determine the DC operating point.

Frequency Correlation: The oscillator circuits shown on the following page are ECS standard test circuits. The inverters used in these circuits are widely accepted as the industry standard because their characteristics are representative of those found in microprocessors within the same family (CMOS/HCMOS/TTL). Naturally, applications will differ in what IC is used, and as can be expected, oscillator circuit characteristics will vary from IC to IC. Usually, this variation is negligible and a ceramic resonator part number can be selected simply by classifying the processor as CMOS, HCMOS or TTL. Given that the standard ECS ceramic resonators are 100% frequency sorted to the test circuits on the following page, it is relatively easy to correlate the frequency of oscillation of our standard circuit that of a customer specified circuit. For example, if the microprocessor being used is a Motorola 6805 at a frequency of 4MHz, then the correct ECS part number would be ZTA4.OMG (frequency sorted to the CD4O69UBE CMOS test circuit). Circuit parameters should be selected as below:

Screen Shot CER 8

 By actually setting up this circuit as well as the standard test circuit shown in Fig.3.1 below, it is possible to establish the average shift that can be expected when using the ZTA5.OMG with a 6805 processor. The actual data is shown below:

Screen Shot CER 9

From this data, it is possible to predict that the standard ZTA4.00MG resonator will have an approximate +0.06% frequency shift from the original 4.00MHz +0.5% initial tolerance. This is of course a negligible shift and will not affect circuit performace in any way.

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Circuits for Various IC/LSI:

Ceramic resonators are being used in a wide range of applications in combination with various kinds of IC's by making good use of the previously mentioned features. The following are a few examples of acutal applications.

Applications of Microprocessors: Ceramic resonators are optimum as a stable oscillating element for various kinds of microprocessors: 4 bit, 8 bit, and 16 bit. As the general frequency tolerance required for the reference clock of microprocessors is +2% - 3%, standard units meet this requirement. Ask your ECS or LSI manufacturers about circuit constants because they vary with frequency and the LSI circuit being used. Fig.A shows an application with a 4 bit microprocessor, and Fig.B shows an application with an 8 bit microprocessor.

Remote Control IC: Remote controls have increasingly become a common feature. Oscillation frequency is normally 400-500 KHz, with 455KHz being the most popular. This 455KHz is divided by a carrier signal generator so that approximately 38KHz of carrier is generated.

Screen Shot CER 14

VCO (Voltage Controlled Oscillator) Circuits: VCO circuits are used in TV's and audio equipment because the signals need to be processed in synchronization with pilot signals transmitted from broadcasting stations. Oscillation circuits, such as LC and RC were originally used; however, ceramic resonators are now used since they require no adjustment and have superior stability over the older type of circuits. Resonators for VCO applications are required to have a wide variable frequency.

Miscellaneous: Other than the above mentioned uses, ceramic resonators are widely used with IC's for voice synthesis and clock generation. For general timing control applications, oscillation frequency is usually selected by the user based on the IC manufacturer's recommended operating frequency range. The selection of this frequency with a given IC will dictate what circuit values and which ceramic resonator will be appropriate. Please contact your local ECS sales representative when selecting a ceramic resonator part number. As mentioned earlier, there are many applications for ceramic resonators. Some of the more application specific oscillator circuits require that unique ceramic resonators be developed for that application and IC.

Oscillation Rise Time

Oscillation rise time means the time when oscillation develops from a transient area to a steady area at the time the power to the IC is activated. WIth a ceramic resonator, it is defined as the time to reach 90% of the oscillation level under steady conditions as shown in Fig.6.1. Rise time is primarily a function of oscillating circuit design. Generally, smaller loading capacitance, a higher frequency ceramic resonator, and a smaller size of ceramic resonator will cause a faster rise time. The effect of load capacitance becomes more apparent as the capacitance of the resonator decreases. Fig6.2 shows an actual measurement of rise time against load capacitance (CL) and supply voltage. It is noteworthy that the rise time is one or two decades faster for a ceramic resonator than for a quartz crystal. (This point is graphically illustrated in Fig.6.3)

Starting Voltage: Starting voltage means the minimum supply voltage at which an oscillating circuit can operate. Starting voltage is affected by all circuit elements. It is determined mostly by the characteristics of the IC. Fig.6.4 shows an example of an actual measurement for the starting voltage characteristics against the loading capacitance.

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The following describes the general characteristics of oscillation in the basic circuit. Contact ECS International for detailed characteristics of oscillation with specific kinds of IC's and LSI's. The stability against temperature change ia +0.3 to 0.5% within a range of -20 C to +80 C, although it varies slightly depending on the ceramic material. Influences of load capacitance (CL1, CL2) on the oscillation frequency is relatively high as can be calculated from the formula for fosc. The fosc varies by approximately + 0.1% because of the capacitance deviation of +0.1% in the working voltage range. The fosc also varies with the characteristics of the IC.

Supply Voltage Variation Characteristics: See Fig.1 below for an example of an actual measurement of stability for a given oscillation frequency.

Oscillation Level: Below are examples of actual measurements of the oscillation level against temperature, supply voltage, and load capacitance (CL1, CL2). The oscillating level is required to be stable over a wide temperature range, and temperature characteristics be as flat as possible. This change is linear with supply voltage unless the IC has an internal constant voltage power source.

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