Astable
assembly using two reversers |
Astable
carried out with the integrated circuit 555 |
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Created it, 06/09/09
Update it, 06/09/20
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3. - THE CIRCUITS MULTIVIBRATORS OR OSCILLATING ASTABLES
3. 1. - DEFINITION AND FUNCTION
An oscillator is an electronic circuit which generates a periodic signal. These signals are of two types.
First of all, there are sinusoidal signals used in the techniques of radiocommunication. It is the carrier wave the radio operator signal and signal T.V. This type of wave is also generated in the synthetizers of musical notes, in the technology of the radar…
Then, the second type of signal which interests us more particularly here, is the signal rectangular, suitable for numerical technology.
Indeed, this signal is characterized only by transitions from a level H on a level L and vice versa to a frequency determined by the generating circuit. This circuit is generally called multivibrator astable. This circuit has two logical states L and H unstable. The exit rocks periodically of a logical state at the other complementary state. This is indicated on figure 43.
T is the period of the rectangular signal determined by the particular characteristics of the assembly.
The principal function of this signal in the logical circuits is to provide a clock called generally clock. This clock is necessary in the synchronous logical circuits where the changes of logical state into different point from the circuit are done either with the face going up, or with the face going down from the clock.
Currently, the circuits of clock meet in the computers, in the measuring apparatus of time, of the frequency, for the data transmission …
3. 2. - VARIOUS ASSEMBLIES OF ASTABLES
We will review a certain number of assemblies multivibrators.
3. 2. 1. - ASSEMBLY USING A TRIGGER OF SCHMITT
Figure 44 presents an oscillator using to you a trigger of family CMOS.
With the powering of the assembly, tension Vc is null and the exit is thus on the level H.
As it appears on figure 45, the condenser C takes care through resistance R and at the moment t1, Vc reached threshold VT+ of the trigger. The exit rocks and passes on the level L : the condenser undertakes its discharge through R and at the moment t2, Vc reached the only VT- of the trigger.
The exit passes by again thus on the level H and the phenomenon reproduces thus indefinitely. The period of oscillation T is defined by the relation:
Note :
ln is the symbol of the function Napierian logarithm. A computer allows calculation.
For R = 100 KW, C = 0,1 µF, Vcc = 5 volts, VT+ = 3,05 volts, VT- = 1,95 volt, one find :
T @ 8,9 ms and f oscillation @ 112 Hz.
This assembly is thus simple but presents a disadvantage. Indeed, thresholds VT+ and VT- are a function of the supply voltage Vcc ; this assembly thus does not have an absolute stability in frequency, but can have fluctuations related to the variations of tension Vcc. For a variation of Vcc of 5 volts to 15 volts, the frequency can vary from 4 to 5 %.
Nevertheless, this assembly can be employed for applications not requiring a great stability and a high degree of accuracy.
Moreover, the use of a controlled food improves the stability of the oscillating assembly appreciably.
3. 2. 2. - ASSEMBLY USING THREE REVERSERS
This assembly is indicated on figure 46.
It uses the fact that there is a travel time DT for each reverser. The chronogram indicated on figure 47 makes it possible to include / understand operation of it.
When the signal as in point VI (or V3) passes from the level L on the level H, it appears clearly that the corresponding exit rocks after a lapse of time equal to DT.
It is thus for the three reversers. The letter “a”
on figure 47, shows the evolution of item VI
at the V3 point. Entry VI
thus Re-rocks after 3 DT.
The period of the signal is worth 6 DT
and its frequency 
.
DT is expressed in seconds.
This circuit makes it possible to obtain an oscillator at high frequency because the travel times DT are relatively short. If one wants to reduce the frequency of oscillation, it is enough to add other reversers. Their number must remain odd.
If n is the number of reversers, the frequency of oscillation is worth 1 / 2nDT.
With this assembly, stability is always a function of the supply voltage, the temperature and the load located at its exit, therefore logical circuit that it must control.
It is possible to improve this Y connection integrating three passive components as illustrated on figure 48.
The chronogram relating to operation is located on figure 49.
At the moment t0, the exit S is on the level L and the entry of reverser 1 is on the level H. the potential of the V1 point thus will decrease and as soon as this potential reaches the threshold of swing of reverser 1, that is to say for Vcc / 2, the three reversers will rock in chain. The exit of reverser 2 passes from the level H on the level L at moment t1 is a voltage drop of - Vcc and the V1 point are found with the potential (Vcc / 2) -Vcc = -Vcc / 2.
However, the exit passed on the level H, therefore the potential of this V1 point will grow until + Vcc / 2 (moment t2) where the three reversers will commutate simultaneously. The V1 point is found (Vcc / 2) + Vcc = 3 / 2 Vcc.
Finally, one attends a series of loads and discharges of the condenser C and each time the V1 point crosses the threshold of swing of reverser 1, the state of the exit changes.
As an indication, the frequency of oscillation is given by the formula :
This oscillator is insensitive with the variations of the supply voltage Vcc and its stability in frequency is of as much better than its frequency is low. Indeed, the frequency depends mainly on the three components R1, R2 and C.
3. 2. 3. - ASSEMBLY ASTABLE USING TWO REVERSERS
This assembly is presented at figure 50. The frequency of
oscillation is given by the formula 
.
The value of R2 resistance must be at least
ten times higher than that of R1.
In addition, the values of C and R1 should not be too low, because reverser 2 cannot provide a very high current in exit.
It is always possible to put a variable R1 resistance. This makes it possible to adjust the frequency of exit of the oscillator.
It is also possible to vary the cyclic report/ratio  of the rectangular signal.
Figure 51 represents this cyclic
report/ratio Â.
The following assembly indicated on
figure 52 makes it possible to vary this cyclic report/ratio Â.
The chronogram located on figure 53 makes it possible to clarify the operation of this oscillator.
At moment t1, the exit S is on the level L and point A on the level H.
The point B is then with the potential - Vcc / 2, as we will see it at the end of this reasoning, Vcc being the supply voltage of the assembly.
The condenser C thus discharges through the diode D2 and part of the P1 potentiometer since point A is with the potential + Vcc and the point B with the potential - Vcc / 2.
At the moment t2, the condenser C is completely discharged.
The point B is with the potential 0 volt.
Potential of the point B continuous to increase since point A is always with the potential + Vcc.
The condenser C takes care now through the same D2 diode until moment t3.
Moment t1 at moment t3, only led the D2 diode, the D1 diode being polarized in reverse.
It is the same current IL which discharges the condenser C initially then charges it in the second time.
At moment t3, the point B is with the potential + Vcc / 2, therefore reverser 1 rocks as well as reverser 2.
Point A passes to potential 0 and the exit S to the potential + Vcc.
Potential of the exit S a increased instantaneously by + Vcc, therefore the potential of the point B makes in the same way and thus passes to + 3 / 2 Vcc.
Moment t3 with t5, same phenomena that those described above reproduce ; but this time, it is the D1 diode which lead and the D2 diode which is polarized in reverse.
From t3 with t4, the condenser C discharges, then of t4 with t5, C takes care.
When the potential of the point B arrives at the threshold of swing of the reverser + Vcc / 2, the two reversers rock.
The exit S which was with + Vcc master key with the potential 0 volt, is a negative face of - Vcc which is transmitted completely to the point B by the condenser C. This point B which was with a potential from + Vcc / 2 master key thus with :
We returned to the starting point of our explanation and a new cycle can start again.
While varying the position of the cursor of the P1 potentiometer, the time-constants of load and discharge of C (that relating to period t3 - t1 and that relating to the period t5 - t3) vary.
Thus, the cyclic report/ratio Â
varies.
3. 2. 4. -
This circuit already used in a pseudo-monostable assembly can be it to constitute an oscillator. Its diagram is indicated on figure 54.
Switch I is closed when Q is on the level L and open when Q is on the level H, like to the powering.
The operation of the oscillator is represented in the form of chronogram on figure 55.
With the powering, the condenser C takes care through resistances in series RA and RB since switch I is open (Q is on the level H).
The truth table of figure 56 indicates the operation of rocker RS to you.
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When tension Vc reached 2 / 3 Vcc, the entry R passes on the level H, therefore the exit Q passes on the level L. This closes switch I.
The condenser C discharges
through resistance RB. The constant of
discharge is worth q1 = RB C. When Vc
reached threshold 1 / 3 Vcc, the entry S
pass on the level H and Q
passes by again on the level H.
the
condenser C is reloaded with a time-constant
q2 = (RA + RB) C
and the cycle continues thus indefinitely.
The period T of the
rectangular signal, as well as the cyclic report/ratio Â,
are given by the following formulas :
T = 0,7 (2 RB +
RA) C
Â
= (RA + RB) / (RA + 2 RB)
It is thus possible to vary these two parameters by modifying
the respective values of the three components RA, RB
and C.
Almost all the assemblies seen previously use networks RC.
These assemblies can have an insufficient stability for certain
achievements. Oscillators with quartz are thus used when a high stability is
necessary for an assembly.
This criterion of stability intervenes in the clocks measuring
the time in which a stability of 10-6, is 1
second over approximately 13 days, is usually reached.
In the digital circuits working at their speed limit, a great
stability is also necessary not to exceed the standards of operation of the
integrated circuits.
Another example is provided by the communication systems to
microprocessors where a very good stability is necessary.
a) Quartz crystals
The quartz of silica (Si02)
is crystallized in the hexagonal system.
There are three axes of symmetry in the crystalline structure of
quartz as shown in the figure 57.
It is :
optical axis ZZ' passing
by the tops. mechanical axis XX'
passing by the edges. electric axis YY'
perpendicular to the faces of the hexagon. In electronics, quartz is
represented by the symbol of the figure 58-a.
The figure 58-b shows the aspect of the case usually used for quartz.
Let us see now their physical properties.
b) The piezoelectric effect.
In the electric oscillators, one uses a quartz plate cut in the
crystal according to one of the axes seen previously.
When one applies an alternating voltage at the boundaries of
this plate, this one becomes deformed and enters in mechanical vibration.
It is noticed that the amplitude of the mechanical vibrations is
maximum for a certain frequency of the alternating voltage: this
constitutes the frequency of resonance of the quartz blade which depends mainly
on the axis chosen for the size, dimensions, and the thickness of this plate.
When the quartz crystal is with resonance, it behaves as a tuned
circuit which would have like structure that represented on figure 59.
Here some typical values of the elements of this circuit.
L = 3 H Cs = 0,05 pF R = 2 kW Cm = 10 pF The Cm capacity is due to
the assembly (from where its name The circuit series L, Cs, R
is the circuit equivalent to the crystal itself. Notice the low value of the
condenser Cs and on the other hand, the
important value of inductance L compared to
those obtained with windings. It is this last characteristic which gives to
quartz a factor quality Q = (L One can define two distinct frequencies of resonance for this
crystal:
Figure 60 gives the curve of the impedance of a quartz according
to the frequency of the signal applied on its terminals.
c) Oscillators with quartz.
One can thus imagine two types of oscillators according to
whether resonance series or parallel resonance is used.
However, the oscillators with resonance series are more precise
and more stable because they rigorously oscillate on the frequency of the
crystal itself.
On the other hand, the oscillators using parallel resonance are
dependant on the value of the capacity of assembly Cm
and other stray capacities of the assembly. Those, possibly put in parallel on
quartz, can vary the frequency of the oscillator.
These is thus the first that we will generally retain for the
use in the logical assemblies or with microprocessors.
Figure 61 gives a very simple example of it.
Any oscillator consists of an
amplifier and a reaction of the output signal in phase with the input signal.
Here, the two reversers in series
play the part of an amplifier not reverser whereas the reaction is operated by
the quartz which, at its frequency of resonance series, behaves like a simple
resistance and thus does not bring any dephasing.
The condenser C
and resistances are used to make oscillate the assembly with starting until
quartz enters in resonance.
At the point medium of two
resistances the rectangular signal of exit is taken.
This theory thus finishes with this
assembly using a quartz.
The next theory will be devoted to
the numbering systems and the various codes used in numerical electronics.
3. 2. 5. - OSCILLATING ASSEMBLIES USING A
QUARTZ
the frequency of resonance series due to the crystal itself and for which the
impedance is minimal bus L enters in
resonance with Cs.
the frequency of parallel resonance due to the parallelization on the circuit R,
L, Cs of the capacity of assembly Cm.
For this frequency of parallel resonance that one sometimes improperly calls
frequency of car resonance, the impedance at
the boundaries of quartz passes a maximum as for any parallel circuit LC.
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