Download V. Oscillators

V. Oscillators
1. Introduction to oscillators
2. Relaxation oscillators
3. The classic timer chip: the 555

CMOS 555s
4. Voltage-controlled oscillators
5. Quadrature oscillators
6. Quartz-crystal oscillators

Voltage controlled crystal oscillator - VCXO
1. Introduction to oscillators
Oscillators are waveform generators.
A device without an oscillator either does not do anything or expects to be driven by
something else, which probably contains an oscillator.
The following applications of oscillators are the most common, important:

Signal generators (souse of radio, TV signal).

Function generators (sine, triangle, linear etc.).

Pulse generators (series of pulses for clocks, counters and computers).

Souse of regular oscillations. It is necessary in any cyclical measuring process and in any
instruments which periodic state or waveform (digital multimeter, oscilloscope, counter,
timer, calculator or computer).
Different application of oscillators needs different circuit design. Demand may be made on
stability of oscillation, on accuracy or on adjustability of generator.
2. Relaxation oscillators
To charge – to fill, to load, to take in the correct amount of electricity.
Charging and discharging a capacitor can make a very simple kind of oscillator. The capacitor
is charging through a resistor and after the voltage reaches a level it is rapidly discharging.
Oscillator based on this principle is known as relaxation oscillator. It is inexpensive and
simple and when carefully designed, can be quite stable in frequency. The basic circuit is
shown in Fig.1.
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Charging capacitor C
0.5V
Fig.1. Op-amp relaxation oscillator. Classic RC design.
The principle of operation:
Applying the power, for the first time, causes the output goes to positive saturation (or
negative, it does not matter).

C begins charging towards V+ through R. It causes that on the inverting input (-) appears
voltage of growing value.

In the same time, (+) input has got the voltage of constant value 0.5V+.

As long as the difference between (-) and (+) input is positive, nothing changes:
VNONINV  VINV  0 gives V+ level on the output, it is positive saturation.

The capacitor still is charging. When it reaches 0.5V+ on the inverting input (-) its output
goes negative saturation: VNONINV  VINV  0 gives V- level on the output.

The above process repeats with the period T  2.2RC .
Nowadays in relaxation oscillators are used op-amp and specially designed ICs (Integrated
Circuits). Sometimes we need an oscillator of very low noises. The appropriate solution is in
Fig. 2.
B)
A)
Fig.2. Low noise oscillators. A) Uses a pair of CMOS (Complementary Metal-OxideSemiconductor) inverters. B) An improved A solution, more complicated.
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The output noises are really small. When A circuit is running at 100kHz, disturbances
measured at 100kHz100Hz are dumped at about 85dB with respect to the 100kHz: How
many times are dumped? 20 log X  85dB , log X  4.25 , X  10 4.25  17 783 times noises are
weaker than the signal.
3. The classic timer chip: the 555
The previous relaxation generators were designed from R, C and op-amp or a form of digital
logic (see Fig.2, inverters). The next, higher level is the use of ICs generators or oscillators.
They are specially design ICs. The most popular are the 555 and its successors. See Fig.3.
Fig.3. Simplified 555 scheme.
Some symbols in the figure belong to the digital world. We will talk about it later. The
principle of operation of 555 is as follows:

It has 2 inputs: the trigger (terminal 2) and the threshold (terminal 6).

An input level below



1
1
VCC activates the trigger. The trigger input is active below VCC .
3
3
2
An input level above VCC activates the threshold. The threshold input is active above
3
2
VCC .
3
The output (terminal 3) goes High=HVCC or it goes Low=Lground level.
The output goes high H when the 555 receives the trigger input, and stays there until the
555 gets the threshold input.

Then the output goes low L and the discharge transistor Q1 is turned on.
Fig.4 shows the process.
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Turning on and
off the
discharge
transistor Q1
H
L
Fig.4. The 555 connected as an oscillator.
When the output is H, Q1 is turned off (key is opened) and capacitor C begins charging
through RA+RB towards Vcc=10V. When it has reached 2/3Vcc, the threshold causes the
output to go L. This causes that Q1 goes on (key is closed) and the capacitor begins
discharging
through
RB
toward
ground.
The
cycle
repeats
with
the
period
T  0.693  R A  2R B   C
This way we get rectangular wave on the output. Its duty cycle is always greater then 50%,
because C is charging through RA+RB (slower) and discharging through RB only (quicker).
Definition of duty cycle:
ti
period T
duty cycle 
ti
T
The 555:




Runs with a single positive supply (from 4.5V to 16V).
Gives good frequency stability (close to 1%).
Can be used for generation of a single pulse of arbitrary width as well as a bunch of other
signals.
Is small and inexpensive.

CMOS 555s
MOS = Metal-Oxide-Semiconductor.
CMOS= Complementary MOS, the electronic structure based on the pair of complementary
transistors n-MOS and p-MOS.
n-MOS, p-MOS= technological structure metal-oxide-semiconductor with n-type and p-type
channel.
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The classic 555 has few drawbacks, disadvantages (see Table 5.3): high supply voltage, high
supply current, high trigger current, inability to run with very low supply voltage. Using some
of 555 CMOS successors allow avoiding these disadvantages.
Le us have a look at Table: the CMOS are able to operate at very low supply voltage – down
to 1V! Are able to operate at low supply current, therefor they are economical (not wasteful),
have low power consumption. They are also able to run at higher frequency than the original
555s.
In Fig.5 is shown the modification of oscillator from Fig.4. The latter gives the output with
duty cycle>50%.
Fig.5. Low duty cycle oscillator.
The above gives low duty cycle positive pulses. The capacitor is charging rapidly through
external diode discharging slowly through internal transistor.
Up to now we have discussed rectangular oscillators. In Fig.6 is shown a way to generate a
triangle wave with CMOS 555.
Fig.6. Triangle generator.
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There are some other interesting timer chips:

The 322 timer from National Semiconductor

The 74HC4060

The Exar 2243

The Intersil ICM7242 and so on...
4. Voltage-controlled oscillators
There are another IC oscillators: Voltage Controlled Oscillators (VCOs). Examples are:
NE566, LM331, 8038, 2206 and the series 74LS624-9. Some of them are able to generate
signals which frequency ranges from 1 to 1000, i.e. fmin:fmax=1:1000. Different chips have
different features: some give output frequency up to 20MHz, some faster give output
frequency up to 200MHz and even to GHz range. The LM331 (Fig.7) is an example of
voltage-to-frequency conversion.
Fig.7. Typical V/F converter IC
Output frequency f depends on input voltage Vin. The output frequency range is 10Hz104Hz.
5. Quadrature oscillators
Sometimes we need a device able to generate a simultaneous pair of sine waves, which are
900 out of phase (phase shift is equal to 900) and which have equal amplitudes, like sine and
cosine. We say that such signals are in quadrature. How to obtain a cosine if we have a sine
A sint signal. The obvious way is to apply an integrator or differentiator:
ut    A sin tdt  A
1
d
cos t or ut   A sin t   A cos t

dt
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In both the cases the phase shift is 900, which is desired, but the amplitudes are not equal.
There are methods, which help to combat these inconveniences. By use of amplifiers and
inversion we are able to change the value and the sign of the cosine.
Another ways of getting quadrature oscillators is to design the following:




Analog trigonometric-function generator.
State-variable generator.
Phase sequence filters.
Quadrature square waves, etc.
6. Quartz-crystal oscillators
They are used when high stability and high accuracy of output frequency is required.
For ordinary RC oscillators we can attain stability at about 0.1% and initial predictability at
about 5-10%. It this good enough. It depends on the application. Very often it is not good
enough. It is enough for instance for multiplexed display in pocket calculator. It this case
multidigit numerical display is driven by lighting one digit after another with frequency 1kHz
typically. Only 1 digit is lit at any time, but your eye sees the whole display. In this case it
does not matter how precisely and accurately it is done – we can not see it, distinguish it
anyway.
It is a little better for LC oscillators – achievable frequency stability is 0.01%. It is good
enough for radio frequency receivers and TV sets.
For better, than the above, stability (0.001%) a crystal oscillator is required. For this purpose
a piece of quartz is used (glass or silicon dioxide or other chemicals). The quartz is so called
piezoelectric: a strain (causing mechanical deformation) generates a voltage and a voltage
causes a strain (mechanical deformation). In another words: applied electric field causes
acoustic waves in the quartz and in turn it causes a voltage at the crystal's surface. By plating
some contacts on the crystal surface we get electronic element. It can be modeled as RLC
circuit, pretuned (initially tuned) to some f0 frequency. Its equivalent circuit is shown in Fig.8
Fig.8. Equivalent circuit of a quartz oscillator.
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In Fig.9 we have some crystal oscillator circuits.
Fig.9. Various crystal oscillators.

A - is classic Pierce oscillator with FET transistor. The oscillator consists of L, C and
crystal.
 B – Colpitts oscillator. There is no L in the circuit.
 C – The crystal works as feedback element.
 D and E – generate logic output.
 E – uses very convenient MC12060 and MC 12061 from MOTOROLA. These chips are
designed to work with crystals in range of frequency 100kHz – 20MHz.
The above give sine wave and square-wave signal.
An alternative (if a square wave is needed) is the use of complete crystal oscillator modules
available as IC-sized metal packages. They come in lots of standard frequencies (1, 2, 4, 5, 6,
8, 10, 16 and 20MHz) as well as frequencies commonly used in microprocessor systems (e.g.
14.31818MHz used for video boards). Typical accuracy is only 0.01%=100ppm=100x10-6,
but you get it cheap (2-5$) and they are guaranteed to oscillate, which is not by any means
assured when you wire your own oscillator.
In general crystals are available from about 10kHz to about 10MHz and even to 250MHz.
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You can order the crystal for any desired frequency – you just phone to the company and
order any crystal you need. Apart of that, most commonly used frequencies are available off
the shelf. Many of them (used in TV color, digital wristwatches, etc.) are available for less
then a dollar. Low cost of crystals causes that very often crystals are used instead of RC
oscillators.

Voltage controlled crystal oscillator - VCXO
They have good-to-excellent stability (typically 10ppm – 100ppm from center frequency) and
good tunability. There are also so called TCXO – Temperature Compensated crystal
oscillators. They have temperature compensation, which causes that their frequency stability,
over the range 0-50C0, is as good as a few ppm up to 0.1ppm. Are more expensive.
The VCXO's and the TCXO's are available as a complete module from many companies
(manufacturers): Bliley, CTS Knights, Motorola, Reeves Hoffman, Vectron etc.
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CMOS 555 successors
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