Applications Of Oscillators Computer Science Essay

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An electronic oscillator is an electronic circuit that produces a repetitive electronic signal, often a sine wave or a square wave.

A low-frequency oscillator (LFO) is an electronic oscillator that generates an AC waveform at a frequency below ≈20 Hz. This term is typically used in the field of audio synthesizers, to distinguish it from an audio frequency oscillator. Oscillators designed to produce a high-power AC output from a DC supply are usually called inverters.

Types of electronic oscillator

There are two main types of electronic oscillator: the harmonic oscillator and the relaxation oscillator.

Harmonic oscillator

The harmonic, or linear, oscillator produces a sinusoidal output. The basic form of a harmonic oscillator is an electronic amplifier with the output attached to an electronic filter, and the output of the filter attached to the input of the amplifier, in a feedback loop. When the power supply to the amplifier is first switched on, the amplifier's output consists only of noise. The noise travels around the loop, being filtered and re-amplified until it increasingly resembles the desired signal.

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A piezoelectric crystal (commonly quartz) may take the place of the filter to stabilise the frequency of oscillation, resulting in a crystal oscillator.

There are many ways to implement harmonic oscillators, because there are different ways to amplify and filter. For example:

Armstrong oscillator

Hartley oscillator

Colpitts oscillator

Clapp oscillator

Delay line oscillator

Pierce oscillator (crystal)

Phase-shift oscillator

RC oscillator (Wien Bridge and "Twin-T")

Cross-coupled LC oscillator

VačkáÅ™ oscillator

Opto-Electronic Oscillator.

FET phase shift Oscillator.

Description of Various Harmonic oscillators:

The Armstrong oscillator (also known as Meissner oscillator) is named after the electrical engineer Edwin Armstrong, its inventor. It is sometimes called a tickler oscillator because the feedback needed to produce oscillations is provided using a tickler coil (T in the circuit diagram) via magnetic coupling between coil L and coil T. Assuming the coupling is weak, but sufficient to sustain oscillation, the frequency is determined primarily by the tank circuit (L and C in the illustration) and is approximately given by (2pi/LC). In a practical circuit, the actual oscillation frequency will be slightly different from the value provided by this formula because of stray capacitance and inductance, internal losses (resistance), and the loading of the tank circuit by the tickler coil.

This circuit is the basis of the regenerative receiver for amplitude modulated radio signals. In that application, an antenna is attached to an additional tickler coil, and the feedback is reduced, for example, by slightly increasing the distance between coils T and L, so the circuit is just short of oscillation. The result is a narrow-band radio-frequency filter and amplifier. The non-linear characteristic of the transistor or tube provides the demodulated audio signal.

The circuit diagram shown is a modern implementation, using a field-effect transistor as the amplifying element. Armstrong's original design used a vacuum tube triode.

Hartley oscillator- It is an electronic oscillator using two coils in serial, parallel to a single capacitor, forming the LC circuit that determines the frequency.

A Hartley oscillator is essentially any configuration that uses two series-connected coils and a single capacitor (see Colpitts oscillator for the equivalent oscillator using two capacitors and one coil). Although there is no requirement for there to be mutual coupling between the two coil segments, the circuit is usually implemented this way.

It is made up of the following:

Two inductors in series, which need not be mutual

One tuning capacitor

Advantages of the Hartley oscillator include:

The frequency may be varied using a variable capacitor

The output amplitude remains constant over the frequency range

Either a tapped coil or two fixed inductors are needed

Disadvantages include:

Harmonic-rich content if taken from the amplifier and not directly from the LC circuit.

Colpitts Oscillator-A Colpitts oscillator, named after its inventor Edwin H. Colpitts,[1] is one of a number of designs for electronic oscillator circuits using the combination of an inductance (L) with a capacitor (C) for frequency determination, thus also called LC oscillator. One of the key features of this type of oscillator is its simplicity (needs only a single inductor) and robustness. The left picture shows the schematic as used in the first publication, the right the schematic from the patent application.

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The frequency is generally determined by the inductance and the two capacitors at the bottom of the drawing.

A Colpitts oscillator is the electrical dual of a Hartley oscillator. Fig. 1 shows the basic Colpitts circuit, where two capacitors and one inductor determine the frequency of oscillation. The feedback needed for oscillation is taken from a voltage divider made by the two capacitors, where in the Hartley oscillator the feedback is taken from a voltage divider made by two inductors (or a tapped single inductor).

As with any oscillator, the amplification of the active component should be marginally larger than the attenuation of the capacitive voltage divider, to obtain stable operation. Thus, using the Colpitts oscillator for a variable frequency oscillator VFO is best done by using a variable inductance for tuning, instead of tuning one of the two capacitors. If tuning by a variable capacitor is needed, it should be a third one connected in parallel to the inductor (or in series as in the Clapp oscillator).

Fig. 2 shows an often preferred variant, where the inductor is also grounded (which makes circuit layout easier for higher frequencies). Note that the feedback energy is now fed into the connection between the two capacitors. The amplifier provides a current, not a voltage amplification.

Fig 3. shows a working example with component values. Instead of bipolar junction transistors, other active components like field effect transistors or vacuum tubes, capable of producing gain at the desired oscillation frequency, could be used.

The Clapp oscillator- It is one of several types of electronic oscillator constructed from a transistor (or vacuum tube) and a positive feedback network, using the combination of an inductance (L) with a capacitor (C) for frequency determination, thus also called LC oscillator.

Referring the notional circuit in the figure, the network comprises a single inductor and three capacitors, with two capacitors (C1 and C2) forming a voltage divider that determines the amount of feedback voltage applied to the transistor input. The Clapp oscillator is a Colpitts oscillator with an additional capacitor placed in series with the inductor. The oscillation frequency in hertz (cycles per second) for the circuit in the figure, which uses a field-effect transistor (FET), is

frequency oscillator (VFO). In a Colpitts VFO, the voltage divider contains the variable capacitor (either C1 or C2). This causes the feedback voltage to be variable as well, sometimes making the Colpitts circuit less likely to achieve oscillation over a portion of the desired frequency range. This problem is avoided in the Clapp circuit by using fixed capacitors in the voltage divider and a variable capacitor (C0) in series with the inductor.

Delay line oscillator- It is a form of electronic oscillator that uses a delay line as its principal timing element.

By inverting the output of the delay line and feeding that signal back to the input of the delay line, the circuit is caused to oscillate. Properly designed, the simplest style of the circuit will oscillate with a period of two times the delay period of the delay line. By the use of additional taps from the delay line additional outputs can be derived that are correlated in frequency with the main output but vary in phase.

The delay line may be realized with a physical delay line (such as an LC network or a transmission line). A ring oscillator uses a delay line formed from the gate delay of a cascade of logic gates. The timing of a circuit using a physical delay line is usually much more accurate; it is also easier to get such a circuit to oscillate in the desired mode.

The delay line oscillator may be allowed to free run or it may be gated for use in asynchronous logic.

Since the optical cavity is a delay line, a laser can be regarded as a special case of the delay-line oscillator.

The Pierce oscillator is a type of electronic oscillator circuit particularly well-suited for implementing crystal oscillator circuits. Named for its inventor, George W. Pierce (1872-1956),[1][2] the Pierce oscillator is a derivative of the Colpitts oscillator. Virtually all digital IC clock oscillators are of Pierce type, as the circuit can be implemented using a minimum of components: a single digital inverter, two resistors, two capacitors, and the quartz crystal, which acts as a highly selective filter element. The low manufacturing cost of this circuit, combined with the outstanding frequency stability of the quartz crystal, give it an advantage over other designs in many consumer electronics applications.

Biasing resistor

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R1 acts as a feedback resistor, biasing the inverter in its linear region of operation and effectively causing it to function as a high gain inverting amplifier. To see this, assume the inverter is ideal, with infinite input impedance and zero output impedance; this resistor forces the input and output voltages to be equal. Hence the inverter will neither be fully on nor off, but in the transition region where it has gain.

Resonator

The crystal in combination with C1 and C2 forms a pi network band-pass filter, which provides a 180 degree phase shift and a voltage gain from the output to input at approximately the resonant frequency of the crystal. To understand the operation of this, it can be noted that at the frequency of oscillation, the crystal appears inductive; thus it can be considered a large inductor with a high Q. The combination of the 180 degree phase shift (i.e. inverting gain) from the pi network and the negative gain from the inverter results in a positive loop gain (positive feedback), making the bias point set by R1 unstable and leading to oscillation.

Isolation resistor

A second resistor could be used between the output of the inverter and the crystal to isolate the inverter from the crystal network. This would also add additional phase shift to C1

A phase-shift oscillator is a simple sine wave electronic oscillator. It contains an inverting amplifier, and a feedback filter which 'shifts' the phase of the amplifier output by 180 degrees at the oscillation frequency.

The filter produces a phase shift that increases with frequency. It must have a maximum phase shift of considerably greater than 180° at high frequencies, so that the phase shift at the desired oscillation frequency is 180°.

The most common way of achieving this kind of filter is using three identical cascaded resistor-capacitor filters, which together produce a phase shift of zero at low frequencies, and 270 degrees at high frequencies. At the oscillation frequency each filter produces a phase shift of 60 degrees and the whole filter circuit produces a phase shift of 180 degrees.

One of the simplest implementations for this type of oscillator uses an operational amplifier (op-amp), three capacitors and four resistors, as shown in the diagram.

The mathematics for calculating the oscillation frequency and oscillation criterion for this circuit are surprisingly complex, due to each R-C stage loading the previous ones. The calculations are greatly simplified by setting all the resistors (except the negative feedback resistor) and all the capacitors to the same values. In the diagram, if R1 = R2 = R3 = R, and C1 = C2 = C3 = C, then:

and the oscillation criterion is:

Rfeedback = 29(R)

Relaxation oscillator

A relaxation oscillator produces a non-sinusoidal output, such as a square wave or saw tooth. The oscillator contains a nonlinear component such as a transistor that periodically discharges the energy stored in a capacitor or inductor, causing abrupt changes in the output waveform.

Square-wave relaxation oscillators are used to provide the clock signal for sequential logic circuits such as timers and counters, although crystal oscillators are often preferred for their greater stability. Triangle-wave or sawtooth oscillators are used in the timebase circuits that generate the horizontal deflection signals for cathode ray tubes in analogue oscilloscopes and television sets. In function generators, this triangle wave may then be further shaped into a close approximation of a sine wave.

Types of relaxation oscillator circuits include:

multivibrator

ring oscillator

delay line oscillator

rotary traveling wave oscillator.

Multivibrator- A multivibrator is an electronic circuit used to implement a variety of simple two-state systems such as oscillators, timers and flip-flops. It is characterized by two amplifying devices (transistors, electron tubes or other devices) cross-coupled by resistors and capacitors.

There are three types of multivibrator circuit:

astable, in which the circuit is not stable in either state-it continuously oscillates from one state to the other. Due to this, it does not require a input (Clock pulse or other).

monostable, in which one of the states is stable, but the other is not-the circuit will flip into the unstable state for a determined period, but will eventually return to the stable state. Such a circuit is useful for creating a timing period of fixed duration in response to some external event. This circuit is also known as a one shot. A common application is in eliminating switch bounce.

bistable, in which the circuit will remain in either state indefinitely. The circuit can be flipped from one state to the other by an external event or trigger. Such a circuit is important as the fundamental building block of a register or memory device. This circuit is also known as a latch or a flip-flop.

In its simplest form the multivibrator circuit consists of two cross-coupled transistors. Using resistor-capacitor networks within the circuit to define the time periods of the unstable states, the various types may be implemented. Multivibrators find applications in a variety of systems where square waves or timed intervals are required. Simple circuits tend to be inaccurate since many factors affect their timing, so they are rarely used where very high precision is required.

Before the advent of low-cost integrated circuits, chains of multivibrators found use as frequency dividers. A free-running multivibrator with a frequency of one-half to one-tenth of the reference frequency would accurately lock to the reference frequency. This technique was used in early electronic organs, to keep notes of different octaves accurately in tune. Other applications included early television systems, where the various line and frame frequencies were kept synchronized by pulses included in the video signal.

A ring oscillator is a device composed of an odd number of NOT gates whose output oscillates between two voltage levels, representing true and false. The NOT gates, or inverters, are attached in a chain; the output of the last inverter is fed back into the first.

Because a single inverter computes the logical NOT of its input, it can be shown that the last output of a chain of an odd number of inverters is the logical NOT of the first input. This final output is asserted a finite amount of time after the first input is asserted; the feedback of this last output to the input causes oscillation.

A circular chain composed of an even number of inverters cannot be used as a ring oscillator; the last output in this case is the same as the input. However, this configuration of inverter feedback can be used as a storage element; it is the basic building block of static random access memory, or SRAM.

A real ring oscillator only requires power to operate; above a certain threshold voltage, oscillations begin spontaneously. To increase the frequency of oscillation, two methods may be used. Firstly, the applied voltage may be increased; this increases both the frequency of the oscillation and the power consumed, which is dissipated as heat. The heat dissipated limits the speed of a given oscillator. Secondly, a smaller ring oscillator may be fabricated; this results in a higher frequency of oscillation given a certain power consumption.

Delay Line Oscillator- A delay line oscillator is a form of electronic oscillator that uses a delay line as its principal timing element.

By inverting the output of the delay line and feeding that signal back to the input of the delay line, the circuit is caused to oscillate. Properly designed, the simplest style of the circuit will oscillate with a period of two times the delay period of the delay line. By the use of additional taps from the delay line additional outputs can be derived that are correlated in frequency with the main output but vary in phase.

The delay line may be realized with a physical delay line (such as an LC network or a transmission line). A ring oscillator uses a delay line formed from the gate delay of a cascade of logic gates. The timing of a circuit using a physical delay line is usually much more accurate; it is also easier to get such a circuit to oscillate in the desired mode.

The delay line oscillator may be allowed to free run or it may be gated for use in asynchronous logic.

Since the optical cavity is a delay line, a laser can be regarded as a special case of the delay-line oscillator.

Applications of Oscillators:

Crystal oscillator-

A crystal oscillator is a timing device that consists of a crystal and an oscillator circuit, providing an output waveform at a specific frequency. When a crystal is placed into an amplifier a small amount of energy is fed back to the crystal, which causes it to vibrate. These vibrations act to stabilize the frequency of the oscillator circuit.

Target Frequency

The Target Frequency of an oscillator is the desired output frequency of an oscillator, specified in MHz or kHz

(megahertz or kilohertz) @ 25°C. A Frequency Tolerance should be specified along with the Target Frequency.

Overall Frequency Tolerance

Overall Frequency Tolerance is the allowable frequency deviation from the Target Frequency, specified as a maximum frequency deviation in ppm (parts per million). The deviation is specified "inclusive" of a set of operating conditions such as Operating Temperature Range, Supply Voltage, Output Load and Aging.

Operating Temperature Range

The Operating Temperature Range is the specified range to which the device will be exposed during oscillation. All specifications such as Overall Frequency Tolerance, Symmetry and Supply Current will be met within the Operating Temperature Range and is specified as a maximum and a minimum temperature in °C.

Storage Temperature Range

The Storage Temperature Range is the absolute limits of temperature to which the device will be exposed in a non oscillation state, without being damaged, and is specified as a maximum and a minimum temperature in °C.

Supply Current (Icc)

Supply Current is the amount of current consumption by an oscillator from the power supply, and is usually specified as a maximum current in milliamps (mA).

Supply Voltage (Vdd)

Supply Voltage is the DC input voltage range recommended for operation of an oscillator, and is usually specified as a DC voltage with a percentage tolerance. For example: 5.0 Vdc, ±10% is a typical specification. All specifications such as Overall Frequency Tolerance, Symmetry and Supply Current will be met within the specified Supply Voltage range.

Crystal oscillators are used in many electronic applications. Some of these applications include:

Military & Aerospace

Communications

Electronic warfare

Fuses

Guidance systems

IFF (Identification Friend or Foe)

Navigation

Radar

Sensors

Sonobouys

Research and Measurement

Astronomy and Geodesy

Celestrial navigation

Instruments

Medical Instrument/Devices

Space tracking

Industrial

Aviation

Communications

Computers

CRT displays

Digital systems

Disk drives

Instrumentation

Marine

Mobile/cellular/portable

Modems

Navigation

Radio, telephone & pager

Sensors

Tagging/identification

Telecommunications

Utilities

Automotive

Engine control, stereo, clock

Trip computer

GPS

Consumer

Cable TV systems

CB & amateur radio

Cellular & cordless

Color TV

Home computers

Pacemakers and other medical devices

Phones, pagers

Radio & hi-fi equipment

Toys & games

VCR & video camera

Watches & clocks

CMOS oscillators

Several square wave oscillators can be built using CMOS logic elements. These circuits offer the following advantages:

Guaranteed startability

Relatively good stability with respect to power supply

variations

Operation over a wide supply voltage range (3V to 15V)

Operation over a wide frequency range from less than 1

Hz to about 15 MHz

Low power consumption (see AN-90)

Easy interface to other logic families and elements including

TTL Oscillators

Several RC oscillators and two crystal controlled oscillators are described. The stability of the RC oscillator will be sufficient for the bulk of applications; however, some applications will probably require the stability of a crystal. Some applications that require a lot of stability are:

1. Timekeeping over a long interval. A good deal of stability is required to duplicate the performance of an ordinary wrist watch (about 12 ppm). This is, of course, obtainable with a crystal. However, if the time interval is short and/or the resolution of the timekeeping device is relatively large, an RC oscillator may be adequate. For example: if a stopwatch is built with a resolution of tenths of seconds and the longest interval of interest is two minutes, then an accuracy of 1 part in 1200 (2 minutes x 60 seconds/minute x 10 tenth/second) may be acceptable since any error is less than the resolution of the device.

2. When logic elements are operated near their specified limits. It may be necessary to maintain clock frequency accuracy within very tight limits in order to avoid exceeding the limits of the logic family being used, or in which the timing relationships of clock signals in dynamic MOS memory or shift register systems must be preserved.

LOGICAL OSCILLATORS

Before describing any specific circuits, a few words about logical oscillators may clear up some recurring confusion. Any odd number of inverting logic gates will oscillate if they are tied together in a ring as shown in Figure 1. Many beginning logic designers have discovered this (to their chagrin) by inadvertently providing such a path in their designs. However, some people are confused by the circuit in Figure 1 because they are accustomed to seeing sinewave oscillators implemented with positive feedback, or amplifiers with non-inverting gain. Since the concept of phase shift becomes a little strained when the inverters remain in their linear region for such a short period, it is far more straightforward to analyze the circuit from the standpoint of ideal switches with finite propagation delays rather than as amplifiers with 180° phase shift. It then becomes obvious that a "1" chases itself around the ring and the network oscillates.

ECL Oscillators

ECL is designed to drive 50 ohm transmission lines. ECL oscillators are generally used above 50MHz and frequently above 100MHz. Because of the speed of these signals and the fact that ECL provides no clamping circuits, good RF techniques must be used in handling these signals.

Power supply:

NEL assumes a typical system power supply operating on -5.2V ±5% will ramp from -0.52V to - 4.68V in approximately 10 mSec (practical limit around this nominal is 2 to 5 mSec). The +5.0V ±5% versions would be measured from +0.5V to +4.5V. The charge curve is assumed to be similar to the exponential curve typical of the capacitive charge of a linear ramp. Deviations in voltage, voltage tolerance, ramp time, or ramp shape (including steps or non-monotonicity) must be communicated to the oscillator manufacturer. One reason for this is that there are different ways of exciting the crystal during power up and changing the above characteristics could change the method of excitation which, in turn, could affect start up.

Load/Trace Length:

In all cases 50 ohm transmission line and 50 ohm load to -2.0 volts at the end is recommended. All electrical parameters for the devices are specified with a 50 ohm load. Deviation from this can significantly change symmetry, logic levels, and transition times(Tr & Tf). In addition, it also affects proper starting of the oscillator during power up. The length of the signal path is not critical when using proper transmission line theory. But, signal loss must be considered. When operating with a +5.0 volt power supply transmission lines are still recommended. The load should be the Thevenin equivalent of 50 ohms using an 81 ohm pull up and 130 ohm pull down to provide the proper bias. It is common to use higher impedances (as high as 530 ohms) in many applications. Although this is not recommended, these higher impedance loads can be used if lead lengths are kept to less than 2 inches. Also, note that the load impedance will greatly affect symmetry. In these cases the signal conductor should still be routed using good RF techniques. Connection of other active devices to the output of the oscillator should not be done (such as a wired configuration). All of these guidelines apply to both outputs in the case of a dual output oscillator. Under no circumstances should an unused output be left unterminated. The oscillator manufacturer should be consulted whenever the design deviates from the standard 50 ohm load condition. Other loads cause reflections, bias changes, and ringing which may affect the operation of the oscillator.

Schmidt Trigger Oscillator

Figure illustrates an oscillator made from a single Schmitt trigger. Since the MM74C14 is a hex Schmitt trigger, this oscillator consumes only one sixth of a package. The remaining

5 gates can be used either as ordinary inverters like the MM74C04 or their Schmitt trigger characteristics can be used to advantage in the normal manner. Assuming these five inverters can be used elsewhere in the system, Figure 6 must represent the ultimate in low gate count oscillators. Voltage V1 is depicted in Figure 7 and changes between the two thresholds of the Schmitt trigger. If these thresholds were constant percentages of VCC over the supply voltage range, the oscillator would be insensitive to variations in VCC. However, this is not the case. The thresholds of the Schmitt trigger vary enough to make the oscillator exhibit a good deal of sensitivity to VCC. Applications that do not require extreme stability or that have access to well regulated supplies should not be bothered by this sensitivity to VCC. Variations in threshold can be expected to run as high as four or five percent when VCC varies from 5V to 15V.

TCXOs - Temperature Compensated Crystal Oscillators

Temperature compensated crystal oscillators typically employ a thermistor network to generate a correction voltage which reduces the frequency variation over temperature. The correction voltage is usually applied to a varactor diode in the crystal circuit such that the crystal frequency may be varied by a small amount. TCXO stability can approach 0.1 PPM but several problems must be addressed. A TCXO that resides at one temperature extreme for an extended period of time may exhibit a frequency shift when returned to normal room temperature. Usually this hysterisis or "retrace" error is temporary but a seemingly permanent offset is common. Retrace errors are usually less than about 0.1 PPM but can be much higher, especially if the mechanical tuning device (trimmer capacitor or potentiometer) is shifting. This hysterisis makes the manufacture of TCXOs with specifications approaching 0.1 PPM quite difficult. The high precision crystals found in oven oscillators exhibit less retrace but they are unsuitable for use in TCXOs because they often exhibit activity dips at temperatures below the designed oven temperature and SC-cuts and high overtone types cannot be tuned by a sufficient amount to compensate for the frequency excursion.

TCXOs may exhibit temporary frequency drift when the ambient temperature changes if the thermistor network does not have the same thermal time-constant as the crystal.

Adjusting the mechanical tuning (to compensate for aging) may change the electrical tuning sensitivity in some designs causing the thermistor network to under-correct or over-correct.

Warm-up may not be as fast as expected if internal heat sources slowly warm the thermistor network and crystal.

Phase noise may be inferior to oven designs and the noise may change over temperature.

TCXOs are preferred to oven oscillators in low power applications and when a warm-up period is not acceptable. The only warm-up time is the time required for the components to reach thermal equilibrium and the total current consumption can be very low - often determined by the output signal power requirements.

Older TCXO designs employ from one to three thermistors to flatten the crystal temperature frequency curve. Newer designs employ digital logic or a microprocessor to derive a correction voltage from values or coefficients stored in memory. Some designers are requesting non-compensated oscillators fully characterized over temperature so that a system microprocessor can provide the required correction voltage. These oscillators are designed so that the crystal has a long thermal time constant to reduce short-term instability and the electrical tuning is trimmed to a specified sensitivity. Each oscillator is given a "curve" number much the way crystals are graded and the external micro measures the ambient temperature calculates a correction voltage for the specific curve. The calculation may be an interpolation between points in a look-up table or a computation using a cubic equation with temperature and the oscillator's "angle" as inputs.

VCXOs - Voltage-Controlled Crystal Oscillators

Voltage controlled crystal oscillators typically employ a varactor diode to vary the frequency of oscillation by application of a tuning voltage. Tuning ranges vary from a fraction of a ppm to hundreds of ppm. Design considerations include tuning linearity, tuning bandwidth, tuning range, output level stability over tuning, post-tuning drift, phase noise, and stability.

Linearity is usually specified as a percentage of the total tuning deviation from a straight line fit to the tuning curve. A 10% linearity specification would allow a deviation of 100 Hz away from the best straight line fit for an oscillator with a 1 kHz tuning range. The potential problem is that this deviation can occur rapidly at one end of the range as shown below. The slope is significantly lower at the top of the curve despite the fact that the oscillator meets a fairly tight linearity specification. For PLL systems where the tuning slope impacts the loop stability, it may be appropriate to specify the minimum and maximum tuning slope at all points on the curve in addition to the percent linearity.

Tuning bandwidth is primarily set by the circuitry connecting the tuning voltage to the varactor. A high tuning bandwidth is usually easy to accomplish but beware of substantial phase shift caused by sudden roll-off. Try to specify a bandwidth about three to ten times higher than required by the PLL. Alternately, specify the roll-off characteristic for an additional decade.

The tuning range should be kept as low as possible to reduce the oscillator's susceptibility to noise on the tuning line. Remember to make allowances for temperature drift and long-term aging combined.

It is common for wide-pull VCXOs to change output level over tuning by a few dB. Make sure to specify the output level stability if it is important.

Post-tuning drift is relatively minor in VCXOs but it is not zero. A small amount of drift may be seen after tuning as the crystal and varactor reach a new thermal equilibrium.

Phase noise and stability are usually degraded by the circuit modifications necessary to achieve large tuning ranges and the tuning voltage must be low noise.

Calculating the effects of tuning port noise on oscillator phase noise:

The spectral density, Sy(f), of the phase noise of an oscillator due to noise on the input is simply calculated by squaring the product of the tuning sensitivity and the voltage noise density. For example, consider a 10 MHz oscillator with a tuning sensitivity of 0.5 ppm and a white noise voltage on the tuning line of 100 nV/root-Hz:

Sy(f) = (0.5E-6 x 100E-9)^2 = 25E-28

The phase noise spectral density is found by multiplying the frequency spectral density, Sy(f), by the carrier frequency squared and dividing by the offset frequency squared. In the example, the phase noise at 100 Hz offset would be calculated as follows:

S(100) = 10MHz^2 x 25E-28 / 100^2 = 25E-18 = -166 dBc

Where S(100) is the phase noise spectral density at 100 Hz offset.

Note that the square of the offset frequency is in the denominator so the phase noise due to white tuning noise rolls off at 20dB per decade.

Silicon-Spread Spectrum Oscillators

Automotive Advantages

Benefits of the SS approach go well beyond its efficacy in meeting certain FCC and regulatory requirements for EMI compliance. The benefits perceived for EMI compliance depend mostly on the bandpass specification of your measurement technique. SS techniques do minimize concentrations of peak energy, and the resulting distribution of this energy into the noise floor does reduce the need for filtering and shielding, but they can provide other benefits as well.

The increasing number of high-performance multimedia, audio, video, and wireless systems deployed in today's automobiles compels designers to pay special attention to any unwanted RF energy present at frequencies to which these subsystems are sensitive. For high-quality radios and wireless data systems, the elimination of RF energy peaks can determine whether a system is usable or not.

For years, radios have utilized a method known as frequency parking to avoid interference from power-supply switching noise. Such radios actually communicate with the power supply, commanding it to alter its switching frequency as necessary to shift energy peaks out of the tuner's input band. With the increasing number of interference sources in a modern automobile, however, you cannot always anticipate how the systems will work together. The situation is further complicated by the use of antenna diversity systems, and by restrictions on the placement of new subsystems.

Other benefits of the SS oscillator can be found in digital audio and in the factory-installed, hands-free interface. These systems commonly use a codec to increase audio quality by providing a digital interface to the cell phone or other telematic interface. The use of a dithered (spread-spectrum) oscillator as clock source to the codec eliminates the generation of annoying idle tones during otherwise silent intervals. This technique is also common in multimedia applications that incorporate switched-capacitor codecs. Besides eliminating the idle tones, an SS oscillator pushes energy peaks into the noise floor, which reduces (for example) the possibility of landing on channels used by a frequency-hopping wireless network.

Virtually all subsystems in the next-generation automobile are likely to include areas in which SS-clocking techniques can provide significant benefits in performance and EMI compliance. For that purpose, vendors such as Maxim/Dallas offer all-silicon oscillators that have reliable startup characteristics and are not affected by vibration. They are cost competitive with ceramic resonators, and cover a range from kilohertz to over sixty megahertz.

General Considerations

Controlling EMI remains a challenge for the electronics designer. A look at the origin of EMI often shows the largest contributor as a digital system clock, which follows for several reasons: the clock often has the highest frequency in the system, it is usually a periodic square wave, and clock traces are often the longest traces in the system. The frequency spectrum for such a signal consists of a fundamental tone and lower-amplitude harmonic tones, whose amplitudes diminish with increasing frequency.

Other signals in the system (those on the data and address buses) are updated at the same frequency as the clock, but they occur at irregular intervals and are generally uncorrelated with each other. The result is a broadband noise spectrum of much lower amplitude than that of the clock. The total energy in this spectrum is much larger than the clock energy, but it has little effect on the EMI tests. Those tests look at the highest spectral amplitudes; not the total radiated energy.

You can control EMI with filtering, shielding, and good PCB layout. But filtering and shielding add cost, and a precise layout takes time. Another approach is to attack the noise source itself-most commonly, the clock oscillator. You can easily lower the amplitudes of the fundamental and overtones by making the clock frequency vary with time. Because the energy of the clock signal remains constant, a varying frequency that broadens the overtones necessarily lowers their amplitudes.

A simple way to generate such a clock is to modulate a voltage-controlled oscillator (VCO) with a triangle wave. The resulting spectrum becomes broader as the triangle-wave amplitude increases. How fast should this triangle wave repeat? A slow sweep (in the audible range) can couple through power supplies to analog subsystems. A sweep that's too fast, on the other hand, may confuse the digital circuitry.

Conclusion

A large number of oscillator applications can be implemented with the extremely simple, reliable, inexpensive and versatile CMOS oscillators described in this note. These oscillators consume very little power compared to most other approaches. Each of the oscillators requires less than one full package of CMOS inverters of the MM74C04 variety.

Frequently such an oscillator can be built using leftover gates of the MM74C00, MM74C02, M74C10 variety. Stability superior to that easily attainable with TTL oscillators is readily attained, particularly at lower frequencies. These oscillators are so versatile, easy to build, and inexpensive that they should find their way into many diverse designs.