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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING Pulse extenders. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Beginner radio amateur In information transmission systems, in order to reduce the influence of random fluctuations, as well as to control automation devices, it is often necessary to obtain wider pulses of a certain duration from short pulses. This task is easily implemented using a waiting multivibrator (single vibrator). A single vibrator is a trigger circuit that generates a single pulse under the action of an external control signal. This implies that the generated pulse exceeds the duration of the trigger. As a rule, one of two methods of pulse formation is used: analog or digital. The simplest is analog - the process of recharging the capacitor is used.
Rice. 1.9 Wide pulse shaper using a Schmitt trigger An example of such a circuit is shown in Fig. 1.9. For the correct operation of this one-shot, it is necessary that the duration of the input trigger pulse be large enough for the capacitor to be completely discharged. After the end of the trigger pulse, the capacitor is charged through the resistor to the supply voltage. In this case, as soon as the voltage reaches Upor, the element D2.1 will switch. In this case, the duration of the output pulse (ti) depends on the ratings of the installed capacitance and resistor in the timing circuit. A simplified formula allows you to roughly calculate the pulse duration:
Rice. 1.10. Areas of acceptable signal levels at the input of MOS microcircuits Taking into account the spread of voltage values of the switching threshold (Uthr), the pulse duration can take values from tmin=0,4RC to tmax=1,11RC. Usually, single vibrators use LE from one housing (crystal). In this case, the Unop scatter turns out to be insignificant and ti=0,69RC can be taken. This ratio is used to determine the pulse duration in most circuits, fig. 1.11...1.18. Voltage diagrams explain the processes of formation of the output pulse. The circuits shown in the same figure are similar in logic and have the same voltage pattern at the test points.
Rice. 1.11. Single vibrator with one timing circuit
Rice. 1.12. Single vibrator based on RS flip-flop
Rice. 1.13. Single vibrator on the front of the input signal
Rice. 1.14. single vibrator
Rice. 1.15. Pulse shapers after the end of the trigger signal
Fig 1.16 Pulse formers
Fig 1.17 Pulse formers
Rice. 1.18 Single vibrators with two timing circuits Unlike the simplest version (Fig. 1.9), the circuits shown in Fig. 1.11 ... 1.14 are not sensitive to the duration of the input pulse, which is why they are most widely used in equipment. Schemes, fig. 1.9, 1.15 ... 1.17, the restart property is inherent, i.e. if another trigger appears during the formation of the output pulse, then the countdown of the duration of the generated pulse will start anew from the end of the last trigger. The diodes used in the circuits accelerate the process of recharging the capacitance, which reduces the possibility of impulse noise at the LE output. In order for the output resistance of the LE not to affect the accuracy of the calculation, and also not to overload the output, the resistor R1 must have a nominal value of at least 10 ... 20 kOhm. In order to neglect the mounting capacitance in the calculations, the minimum capacitance C1 can be 200 ... 600 pF. To obtain high temperature stability of the time interval, the value of R1 should be < 200 kΩ, and the capacitor should not exceed 1 μF. The use of electrolytic capacitors increases the instability of the time interval. To reduce the influence of the spread of Unop values on the duration of the generated pulse, you can use circuits with two timing circuits (Fig. 1. 18). If the time constants of both timing circuits are the same, then with a maximum spread of Unop values from 0Upit to 33Upit, the change in the duration of the generated pulse does not exceed 0,69%. Execution of single vibrators on the RS-trigger, fig. 9. 1 and 19. 1, makes it possible to have two separate trigger inputs (on the leading edge of the pulse), as well as immediately receive a direct pulse and an inverted pulse at the outputs. Another advantage of RS-triggered single vibrators is the ability to trigger from a slowly varying input voltage.
Rice. 1.19. Waiting multivibrators: a) on a D-flip-flop; b) on a JK-flip-flop, c) with increased stability when the power changes
Figure 1.20. Standby multivibrators with increased output pulse slope a) on a D-trigger; b) on a JK flip-flop The duration of the triggering pulses applied to the input S must be less than the generated one (the mode when a log. "1" is simultaneously present at the inputs S and R is prohibited). At input C, the duration of the trigger pulse can be any. Diode VD1 accelerates the discharge of the capacitor through the trigger output and allows you to increase the frequency of triggering pulses (its use reduces the recovery time of the circuit). The duration of the generated pulses is approximately ti=0,69R1C1. The minimum value of the resistance R1 is limited by the maximum allowable output current of the trigger. It can be changed within 20 kΩ ... 10 MΩ, while the pulse duration will change 500 times. Simultaneous change in the values of R1 and C1 allows you to adjust the pulse duration within four orders of magnitude. The scheme in fig. 1.19v provides more stable pulses when the supply voltage changes (a similar circuit can also be assembled on JK flip-flops). To increase the steepness of the decays of the output pulses, the schemes shown in Fig. 1.20, but in them the capacitors C1 must be non-polar. In this case, the duration of the generated pulse at the same values of the RC circuit as in the circuits in Fig. 1.18, it turns out about 2 times more.
Figure 1.21. Standby multivibrator with increased stability Better stability when changing the supply voltage compared to those shown in fig. 1.19 provides options for a single-vibrator circuit on two triggers, Figure 1. 21. In addition, in this case, connecting the load does not affect the duration of the generated pulses. The circuit consists of two single vibrators having a common trigger input, but generating pulses of different durations at independent outputs. The pulses at output 5 will be almost independent of the supply voltage.
Rice. 1. 22 Schemes of delayed pulse shapers. A waiting universal single vibrator can be made on a microcircuit specially designed for this purpose (Fig. 1. 22a). In one 564AG1 (1561AG1) package there are two single vibrators, which, depending on the combination of control signals at the input, have the property of normal triggering on the leading edge (input S1) or trailing edge (S2), and can also be restarted if necessary. Input R is a priority in relation to other inputs and sets the value of the signal Q=0 (if input R is not used, it is connected to +Upit). The duration of the generated signal (ti, Q=1) is set by the corresponding external RC circuit: ti=0,5RC for С>0,01 uF. The diagram given in the handbook [L8] allows to determine more precisely.
Rice. 1. 23 Standby triggered multivibrator with restart capability.
Rice. 1. 24 Standby multivibrator with restart capability. If it is required to have a restart of the single vibrator on the trigger, in the event of the arrival of the next input pulse during the formation of the interval, then the circuit in Fig. 1.23 allows you to increase the duration of the output pulse due to the start of counting from the end of the trigger signal. A similar scheme is shown in fig. 1. 24. When the log is valid at the input. "0", the capacitor is charged to the value of the supply voltage (log. "1"). When a trigger pulse is received with a duration sufficient to discharge the capacitor, the trigger will flip and generate a pulse. The duration of this pulse, after the end of the input signal, is determined by the necessary time to charge the capacitor to the log level. "1".
Rice. 1.25 Waiting multivibrator with increased steepness of the front of the output pulses. The circuit (Fig. 1.25), in contrast to the above, allows you to get steeper fronts of the signal at the trigger outputs. The second advantage of this circuit is that at the end of the generated pulse, the capacitor quickly discharges through the diode from the level Upor instead of recharging to the power level (E ) Because of this, the next trigger pulse can be significantly shorter, while maintaining a zero recovery time The second method obtaining a pulse of the desired duration is associated with the use of counters - digital single vibrators They are used when the time interval must be very large or high requirements are placed on the stability of the formed interval. In this case, the minimum duration obtained is limited only by the speed of the elements used, and the maximum duration can be any (unlike from circuits using RC circuits).
Rice. 1. 26 Digital single shot on a programmable counter. The principle of operation of a digital single vibrator is based on the trigger being turned on by an input signal and turned off after a time interval determined by the counter conversion factor. The use of counters with a switchable division ratio in a single vibrator, fig. 1.26, allows you to get an impulse of any duration. The 564IE 15 chip consists of five subtractive counters whose counter modules are programmed by parallel loading of data in binary code. It takes three cycles to load numbers into the counters, so you can set the division factor N>3 [L2].
The table shows the maximum possible division ratios depending on the value of M. For values of M=0, counting is prohibited. The signal at input S controls the mode of periodic (0) and single (1) counting. The binary code for different values of the module M is taken from Table 1.3 (# - account prohibition, x - any state, log. "0" or "1"). The overall division factor of the microcircuit is determined by the formula: N=M(1000P1+100P2+10P3+P4)+P5 . When a digital single vibrator with a quartz self-oscillator of clock frequency is used, a higher stability of the output pulse duration is provided, which allows them to be used in measuring instruments.
Rice. 1.27. Digital Single Shot with Enhanced Time Interval Stability
Rice. 1.28. Digital Single Vibrator On fig. 1.27 shows an example of the simplest circuit for obtaining an impulse using a counter. The operation of single vibrators is explained by the diagrams shown in the figures. A common drawback of the circuits shown in Figures 1.27 and 1.28 is the random error associated with the arbitrariness of the phase of the master oscillator at the time of launch. The error can be up to the period of the clock frequency and decreases with increasing oscillator frequency and counter conversion factor. This shortcoming can be eliminated by the circuit in Fig. 1.28 (the generator turns on when a trigger pulse appears). In the initial state, the output of the counter D2 / 3 (4) has a log voltage. "1", which disables the operation of the oscillator on D1.1, D1.2. The trigger pulse resets the counter D2, and its output D2 / 3 will be a log. "0" until the moment he counts until the appearance on D2 / 3 log. "1". Since the formation of the output pulse always starts from the same state of the master oscillator, a random error in the pulse duration is excluded, but this circuit has another drawback: when the power is turned on, it generates a pulse of indefinite duration at the output (within a given interval). The scheme has the property of restarting if during the formation of the output pulse there is another trigger (the countdown of the duration of the generated pulse starts anew).
Figure 1.29. Single vibrator with synchronization of the duration of the output pulse with the frequency of the clock generator The circuit shown in Figure 1.29 at the moment the trigger pulse arrives at the input provides a signal at the output, the duration of which is equal to the period of the clock frequency (T=1/ft). With quartz stabilization of the generator frequency (ft), the circuit can be used as a highly stable single vibrator. Publication: irls.narod.ru
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, where E is the supply voltage of the circuit; Uthr - used threshold level (Fig. 1.10) to switch the element.
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