Category: pulse Circuits

Pulse Circuits – Sampling Gates Up to now, we have come across different Pulse circuits. At times, we get the need to restrict the application of such pulse inputs to certain time periods. The circuit that helps us in this aspect is the Sampling gate circuit. These are also called as linear gates or transmission gates or selection circuits. These sampling gates help in selecting the transmission signal in a certain time interval, for which the output signal is same as input signal or zero otherwise. That time period is selected using a control signal or selection signal. Sampling Gates For a Sampling gate, the output signal must be same as the input or proportional to the input signal in a selected time interval and should be zero otherwise. That selected time period is called as Transmission Period and the other time period is called as Non-transmission Period. This is selected using a control signal indicated by VC. The following figure explain this point. When the control signal VC is at V1, the sampling gate is closed and when VC is at V2, it is open. The pulse width Tg indicates the time period for which the gate pulse is applied. Types of Sampling Gates The types of Sampling gates include − Unidirectional sampling sgates − These type of sampling gates can pass either positive or negative going pulses through them. They are constructed using diodes. Bidirectional sampling gate − These type of sampling gates can pass both positive and negative going pulses through them. They are constructed using either diodes or BJTs. Types of Switches Used The sampling gates can be constructed using series or shunt switches. The time period for which the switch has to be open or close is determined by the gating pulse signal. These switches are replaced by active elements like diodes and transistors. The following figure shows the block diagrams of sampling gates using series and shunt switches. Sampling Gate using a Series Switch In this type of switch, if the switch S is closed, the output will be exactly equal or proportional to the input. That time period will be the Transmission Period. If the switch S is open, the output will be zero or ground signal. That time period will be the Non-transmission Period. Sampling Gate using a Shunt Switch In this type of switch, if the switch S is closed, the output will be zero or ground signal. That time period will be the Non-transmission Period. If the switch S is open, the output will be exactly equal or proportional to the input. That time period will be the Transmission Period. The sampling gates are entirely different from logic gates of digital circuits. They are also represented by pulses or voltage levels. But they are digital gates and their output is not the exact replica of the input. Whereas the sampling gate circuits are the analog gates whose output is exact replica of the input. In the coming chapters, we will discuss the types of sampling gates. Learning working make money

Pulse Circuits – Switch A Switch is a device that makes or breaks a circuit or a contact. As well, it can convert an analog data into digital data. The main requirements of a switch to be efficient are to be quick and to switch without sparking. The essential parts are a switch and its associated circuitry. There are three types of Switches. They are − Mechanical switches Electromechanical switches or Relays Electronic switches Mechanical Switches The Mechanical Switches are the older type switches, which we previously used. But they had been replaced by Electro-mechanical switches and later on by electronic switches also in a few applications, so as to get over the disadvantages of the former. The drawbacks of Mechanical Switches are as follows − They have high inertia which limits the speed of operation. They produce sparks while breaking the contact. Switch contacts are made heavy to carry larger currents. The mechanical switches look as in the figure below. These mechanical switches were replaced by electro-mechanical switches or relays that have good speed of operation and reduce sparking. Relays Electromechanical switches are also called as Relays. These switches are partially mechanical and partially electronic or electrical. These are greater in size than electronic switches and lesser in size than mechanical switches. Construction of a Relay A Relay is made such that the making of contact supplies power to the load. In the external circuit, we have load power supply for the load and coil power supply for controlling the relay operation. Internally, a lever is connected to the iron yoke with a hard spring to hold the lever up. A Solenoid is connected to the yoke with an operating coil wounded around it. This coil is connected with the coil power supply as mentioned. The figure below explains the construction and working of a Relay. Working of a Relay When the Switch is closed, an electrical path is established which energizes the solenoid. The lever is connected by a heavy spring which pulls up the lever and holds. The solenoid when gets energized, pulls the lever towards it, against the pulling force of the spring. When the lever gets pulled, the moving contact meets the fixed contact in order to connect the circuit. Thus the circuit connection is ON or established and the lamp glows indicating this. When the switch is made OFF, the solenoid doesn’t get any current and gets de-energized. This leaves the lever without any attraction towards the solenoid. The spring pulls the lever up, which breaks the contact. Thus the circuit connection gets switched OFF. The figure below shows how a practical relay looks like. Let us now have a look at the advantages and disadvantages of an Electro-magnetic switch. Advantages A relay consumes less energy, even to handle a large power at the load. The operator can be at larger distance, even to handle high voltages. No Sparking while turning ON or OFF. Disadvantages Slow in operation Parts are prone to wear and tear Types of Latches in Relays There are many kinds of relays depending upon their mode of operation such as Electromagnetic relay, solid-state relay, thermal relay, hybrid relay, reed relay etc. The relay makes the connection with the help of a latch, as shown in the following figure. There are four types of latch connections in relays. They are − Single Pole Single Throw (SPST) − This latch has a single pole and is thrown onto a single throw to make a connection. Single Pole Double Throw (SPDT) − This latch has a single pole and double throw to make a connection. It has a choice to make connection with two different circuits for which two throws were connected. Double Pole Single Throw (DPST) − This latch has a double pole and single throw to make a connection. Any of the two circuits can choose to make the connection with the circuit available at the single throw. Double Pole Double Throw (DPDT) − This latch has a double pole and is thrown onto double throw to make two connections at the same time. The following figure shows the diagrammatic view of all the four types of latch connections. Electronic Switch The next kind of switch to be discussed is the Electronic Switch. As mentioned earlier, transistor is the mostly used electronic switch for its high operating speed and absence of sparking. The following image shows a practical electronic circuit built to make transistor work as a switch. A Transistor works as a switch in ON condition, when it is operated in saturation region. It works as a switch in OFF condition, when it is operated in cut off region. It works as an amplifier in linear region, which lies between transistor and cut off. To have an idea regarding these regions of operation, refer to the transistors chapter from BASIC ELECTRONICS tutorial. When the external conditions are so robust and high temperatures prevail, then a simple and normal transistor would not do. A special device named as Silicon Control Rectifier, simply SCR is used for such purposes. This will be discussed in detail, in the POWER ELECTRONICS tutorial. Advantages of an Electronic Switch There are many advantages of an Electronic switch such as Smaller in size Lighter in weight Sparkles operation No moving parts Less prone to wear and tear Noise less operation Faster operation Cheaper than other switches Less maintenance Trouble–free service because of solid-state A transistor is a simple electronic switch that has high operating speed. It is a solid state device and the contacts are all simple and hence the sparking is avoided while in operation. We will discuss the stages of switching operation in a transistor in the next chapter. 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Time Base Generators Overview After having discussed the fundamentals of pulse circuits, let us now go through different circuits that generate and deal with Saw tooth waves. A Saw tooth wave increases linearly with time and has a sudden decrease. This is also called as a Time base signal. Actually, this is the ideal output of a time base generator. What is a Time Base Generator? An Electronic generator that generates the high frequency saw tooth waves can be termed as a Time Base Generator. It can also be understood as an electronic circuit which generates an output voltage or current waveform, a portion of which varies linearly with time. The horizontal velocity of a time base generator must be constant. To display the variations of a signal with respect to time on an oscilloscope, a voltage that varies linearly with time, has to be applied to the deflection plates. This makes the signal to sweep the beam horizontally across the screen. Hence the voltage is called as Sweep Voltage. The Time Base Generators are called as Sweep Circuits. Features of a Time Base Signal To generate a time base waveform in a CRO or a picture tube, the deflecting voltage increases linearly with time. Generally, a time base generator is used where the beam deflects over the screen linearly and returns to its starting point. This occurs during the process of Scanning. A cathode ray tube and also a picture tube works on the same principle. The beam deflects over the screen from one side to the other (generally from left to right) and gets back to the same point. This phenomenon is termed as Trace and Retrace. The deflection of beam over the screen from left to right is called as Trace, while the return of the beam from right to left is called as Retrace or Fly back. Usually this retrace is not visible. This process is done with the help of a saw tooth wave generator which sets the time period of the deflection with the help of RC components used. Let us try to understand the parts of a saw-tooth wave. In the above signal, the time during which the output increases linearly is called as Sweep Time (TS) and the time taken for the signal to get back to its initial value is called as Restoration Time or Fly back Time or Retrace Time (Tr). Both of these time periods together form the Time period of one cycle of the Time base signal. Actually, this Sweep voltage waveform we get is the practical output of a sweep circuit whereas the ideal output has to be the saw tooth waveform shown in the above figure. Types of Time base Generators There are two types of Time base Generators. They are − Voltage Time Base Generators − A time base generator that provides an output voltage waveform that varies linearly with time is called as a Voltage Time base Generator. Current Time Base Generator − A time base generator that provides an output current waveform that varies linearly with time is called as a Current Time base Generator. Applications Time Base Generators are used in CROs, televisions, RADAR displays, precise time measurement systems, and time modulation. Errors of Sweep Signals After generating the sweep signals, it is time to transmit them. The transmitted signal may be subjected to deviation from linearity. To understand and correct the errors occurred, we must have some knowledge on the common errors that occur. The deviation from linearity is expressed in three different ways. They are − The Slope or Sweep Speed Error The Displacement Error The Transmission Error Let us discuss these in detail. The Slope or Sweep Speed Error (es) A Sweep voltage must increase linearly with time. The rate of change of sweep voltage with time must be constant. This deviation from linearity is defined as Slope Speed Error or Sweep Speed Error. Slope or Sweep speed eror es = $frac{difference : in: slope: at : the: beginning: and: end: of: sweep}{initial : value :of : slope}$ $$= frac{left (frac{mathrm{d} V_0}{mathrm{d} t} right )_{t = 0} – left( frac{mathrm{d} V_0}{mathrm{d} t} right)_{t = T_s}}{left( frac{mathrm{d} V_0}{mathrm{d} t}right )_{t = 0}}$$ The Displacement Error (ed) An important criterion of linearity is the maximum difference between the actual sweep voltage and the linear sweep which passes through the beginning and end points of the actual sweep. This can be understood from the following figure. The displacement error ed is defined as ed = $frac{(actual: speed)thicksim (linear: sweep : that: passes: beginning : and : ending: of: actual: sweep)}{amplitude: of: sweep: at: the : end: of: sweep: time}$ $$= : frac{(V_s – V′_s)_{max}}{V_s}$$ Where Vs is the actual sweep and V’s is the linear sweep. The Transmission Error (et) When a sweep signal passes through a high pass circuit, the output gets deviated from the input as shown below. This deviation is expressed as transmission error. Transmission Error = $frac{(input): thicksim :(output)}{input: at : the: end: of: the: sweep}$ $$e_t = frac{V′_s − V}{V′_s}$$ Where V’s is the input and Vs is the output at the end of the sweep i.e. at t = Ts. If the deviation from linearity is very small and the sweep voltage may be approximated by the sum of linear and quadratic terms in t, then the above three errors are related as $$e_d = frac{e_s}{8} = frac{e_t}{4}$$ $$e_s = 2e_t = 8e_d$$ The sweep speed error is more dominant than the displacement error. Learning working make money