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Astable Multivibrators
©Paul GodinCreated February 2007
Oscillator Basics
Definitions
◊ Astable◊ No stable state◊ Produces alternate high/low states
◊ Astable Multivibrators are also known as:◊ Clocks◊ Oscillators
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Uses of Astables
◊ Provide edges for edge-triggered devices◊ flip-flops◊ counters◊ shift registers◊ Digital to Analog / Analog to Digital converters◊ microprocessors◊ communications, etc…
◊ Can provide sound for certain applications◊ practical audible sound in the 100Hz to 5kHz range◊ exercise caution when applying square waves
to speakers
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Period and Duty Cycle
◊ Duty cycle describes the ratio of the time in the high state versus the overall period of the pulse.
%100tt
tD.C.
LH
H
T
tH tL
Review
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Does Duty Cycle Matter?
◊ To an edge-triggered device, does the duty cycle affect its operation?
◊ If a 10% D.C. clock is applied to the following circuit, what is the output D.C.?
Review
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Square waves and speakers
◊ Cautions:◊ The average power for a square wave is higher than
for a sine wave with the same peak voltage. Speaker coil damage may result.
◊ A speaker is an electro-mechanical device. It is physically unable to produce the instantaneous motion of a square wave. Damage to the cone and physical structure may result.
◊ Speakers have a low impedance and likely represents a greater load than the driving circuit is capable of handling. Damage to the driving circuit may result.
!
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Speaker Interfaces
◊ Use cheap speakers!◊ Keep the output voltages low.◊ Use an output device that can handle the load.◊ Filter the output square waves
◊ Use an RC circuit in series.◊ Use an audio transformer.
Discussion in class
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Schmitt-Triggered Oscillators
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Oscillator Circuits
◊ Describe the output for the following device:
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Oscillator Parameters
◊ In the previous oscillator circuit:◊ What determines the output frequency?◊ What is the waveform of the output?◊ What determines the duty cycle?
◊ How can we slow the process down?
In-Class Discussion
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Controlling the Simple Oscillator
◊ The output frequency of the oscillator can be adjusted by adding an RC to the circuit:
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Schmitt Oscillator
◊ The separation between Vt+ and Vt- can be used to create an oscillating circuit.
◊ An RC network is used to control the oscillation rate by controlling the charge and discharge time of the capacitor voltage.
◊ Easy oscillator to build. Used where precise or accurate frequency isn’t necessary.◊ displays◊ visual effects
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Simple Oscillator Output
Vc: Charge/Discharge Cycle
Discharge Time
Charge Time
Oscillator Animationastable 1.13
Schmitt Trigger Oscillator Control
Schmitt Triggered Oscillators may be controlled by the use of RC circuits.
To achieve a specific frequency, the values of R and C may be calculated.
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Calculating Charge/Discharge Instantaneous Voltages
)1()( RC
t
initAppliedinit eEEEv
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RC Calculation Method
◊ Get the Vt+ and the Vt- from the Schmitt-triggered inverter.◊ Note the specifications are for an average device. Your
specific device’s values may be a little different.
◊ Calculate the Rise and Fall times using the charge/discharge formulas for RC circuits.
◊ Add the time low and the time high to get the period.
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Charge/Discharge Formula(for time)
Ev
RCt1
1ln
Where:v = change in charge of the capacitor (between VT+ and VT-)E = applied voltage to the capacitor ln = natural logt = time for the charge or discharge
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Determining “E”
Vcc
Gnd
Vt+
Vt-
E
Vcc
Gnd
Vt+
Vt- E
Capacitor is at VT-, about to start a charge cycle:
Capacitor is at VT+, about to start a discharge cycle
E = the value between the current capacitor charge
(VT-) and the applied voltage Vcc.
E = the value between the current capacitor charge
(VT+) and the applied voltage (Ground).
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Application of the Formula
◊ Determine the frequency and duty cycle for the following circuit, given:
Vt+= 1.6VVt- = 0.9V
R= 1kC= 1F
Assume Logic High = 5V andLogic Low = 0V
Ev
RCt1
1ln
EXAMPLE
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Solution
Solve for time High (also the rise time for the capacitor voltage):
v = Vt+ - Vt- = 1.6V - 0.9V = 0.7VE = Vcc - Vt- = 5V - 0.9V = 4.1V
s
VV
Fk
Ev
RCt
2.187
1.47.01
1ln11
1
1ln
Vcc
Gnd
Vt+
Vt-
E
EXAMPLE
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Solution
Solve for time Low (also the fall time for the capacitor voltage):
v = Vt+ - Vt- = 1.6V - 0.9V = 0.7VE = Vt+ - 0 = 1.6V - 0V = 1.6V
s
VV
Fk
Ev
RCt
4.575
6.17.01
1ln11
1
1ln
Vcc
Gnd
Vt+
Vt- E
EXAMPLE
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Solution
Solve for frequency:
T = tH+tL= 187.2s + 575.4s = 762.6s
f = 1/T= 1/762.6s = 1.311kHz
Solve for duty cycle:
DC= tH/(tH+tL)= 187.2s / 762.6s =24.5%
EXAMPLE
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Note: this formula will be given for tests/exams
Practice Problem
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◊ Determine the frequency and duty cycle for the following circuit, given:
Vt+= 1.4VVt- = 1.1V
R= 10kC= 1F
Assume Logic High = 5V andLogic Low = 0V
Ev
RCt1
1ln
The Simple Oscillator
◊ Advantages:◊ Easy to build◊ Fair range of frequency◊ Small footprint
◊ Disadvantages:◊ Unstable, as the frequency will vary with temperature
variations.◊ Difficult to predict values due to the range of Vt+ and
Vt- between different gates, even within the same IC package.
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Crystal Oscillators
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Crystal Oscillators
◊ Crystal Oscillators are commonly used in conjunction with microprocessors, communications circuits and other frequency-sensitive devices because of their:◊ reliability◊ stability◊ accuracy◊ ease of use
Symbol
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Crystal Oscillators
◊ A crystal oscillator is constructed from a piece of quartz crystal that is cut and shaped to the appropriate size.
◊ A property called piezoelectricity happens with quartz crystals.◊ If pressure is applied, it creates voltage◊ If voltage is applied, it physically vibrates
◊ When a voltage is applied to the crystal, it vibrates at a very specific frequency.
◊ Crystal oscillators commonly require small capacitors to aid with the back-and-forth voltage, and require a source of current.
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Crystal Oscillator Circuits
There are many different configurations for crystal oscillators. Following are some examples of basic circuits:
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Crystal Oscillator Circuits
There are many other ways to create a stable oscillation with crystals.
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Crystal Oscillator Circuits
◊ The following circuit will be used in lab:
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Crystal Oscillators
◊ Crystal Oscillators are often packaged in an oval-shaped metallic “can”.
◊ Those with 2 leads require external circuitry; those with 4 leads typically already possess the internal circuitry required to produce the oscillation (voltage and ground needs to be applied).
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Operation of the Simple Oscillator
0
1- Logic 0 read by input of inverter.
1
2-Output becomes logic 1.
5- The capacitor discharges to
VT-.
4-Output becomes logic 0.
0
3- Capacitor voltage increases to VT+. The gate senses a
logic 1 input.
1
Animated
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©Paul R. Godinprgodin°@ gmail.com
END
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