Category: pulse Circuits

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

Pulse Circuits – Synchronization In any system, having different waveform generators, all of them are required to be operated in synchronism. Synchronization is the process of making two or more waveform generators arrive at some reference point in the cycle exactly at the same time. Types of Synchronization Synchronization can be of the following two types − One-to-one basis All the generators are operated at a same frequency. All of them arrive at some reference point in the cycle exactly at the same time. Sync with frequency division Generators operate at different frequency which are integral multiples of each other. All of them arrive at some reference point in the cycle exactly at the same time. Relaxation devices Relaxation circuits are the circuits in which the timing interval is established through the gradual charging of a capacitor, the timing interval being terminated by the sudden discharge (relaxation) of a capacitor. Examples − Multivibrators, sweep circuits, blocking oscillators, etc. We have observed in the UJT relaxation oscillator circuit that the capacitor stops charging when the negative resistance device such as UJT turns ON. The capacitor then discharges through it to reach its minimum value. Both these points denote the maximum and minimum voltage points of a sweep waveform. Synchronization in Relaxation Devices If the high voltage or peak voltage or breakdown voltage of the sweep waveform has to be brought down to a lower level, then an external signal can be applied. This signal to be applied is the synchronized signal whose effect lowers the voltage of peak or breakdown voltage, for the duration of the pulse. A synchronizing pulse is generally applied at the emitter or at the base of a negative resistance device. A pulse train having regularly spaced pulses is applied to achieve synchronization. Though the synchronizing signal is applied first few pulses will have no effect on the sweep generator as the amplitude of the sweep signal at the occurrence of the pulse, in addition with the amplitude of the pulse is less than VP. Hence the sweep generator runs unsynchronized. The exact moment at which UJT turns ON is determined by the instant ofoccurrence of a pulse. This is the point where the sync signal achieves synchronization with the sweep signal. This can be observed from the following figure. Where, TP is the time period of the pulse signal TO is the time period of the sweep signal VP is the Peak or breakdown voltage VV is the Valley or Maintaining voltage To achieve synchronization, the pulse timing interval TP should be less than the time period of sweep generator TO, so that it terminates the sweep cycle prematurely. The synchronization cannot be achieved if the pulse timing interval TP is greater than the time period of sweep generator TO and also if the amplitude of the pulses is not large enough to bridge the gap between the quiescent breakdown and the sweep voltage, though TP is less than TO. Frequency Division in Sweep Circuits In the previous topic, we have observed that synchronization gets achieved when the following conditions are satisfied. They are When TP < TO When the amplitude of the pulse is sufficient to terminate each cycle prematurely. Satisfying these two conditions, though synchronization is achieved, we may often come across a certain interesting pattern in the sweep with regard to sync timing. The following figure illustrates this point. We can observe that the amplitude V’S of the sweep after synchronization is less than the unsynchronized amplitude VS. Also the time period TO of the sweep is adjusted according to the time period of the pulse but leaving a cycle in-between. Which means, one sweep cycle is made equal to two pulse cycles. The synchronization is achieved for every alternate cycle, which states $$T_o > 2T_P$$ The sweep timing TO be restricted to TS and its amplitude is reduced to V’S. As every second pulse is made in synchronism with the sweep cycle, this signal can be understood as a circuit that exhibits frequency division by a factor of 2. Hence the frequency division circuit is obtained by synchronization. Learning working make money

UJT as Relaxation Oscillator An oscillator is a device that produces a waveform by its own, without any input. Though some dc voltage is applied for the device to work, it will not produce any waveform as input. A relaxation oscillator is a device that produces a non-sinusoidal waveform on its own. This waveform depends generally upon the charging and discharging time constants of a capacitor in the circuit. Construction and Working The emitter of UJT is connected with a resistor and capacitor as shown. The RC time constant determines the timings of the output waveform of the relaxation oscillator. Both the bases are connected with a resistor each. The dc voltage supply VBB is given. The following figure shows how to use a UJT as a relaxation oscillator. Initially, the voltage across the capacitor is zero. $$V_c = 0$$ The UJT is in OFF condition. The resistor R provides a path for the capacitor C to charge through the voltage applied. The capacitor charges according to the voltage $$V = V_0(1 – e^{-t/RC})$$ The capacitor usually starts charging and continues to charge until the maximum voltage VBB. But in this circuit, when the voltage across capacitor reaches a value, which enables the UJT to turn ON (the peak voltage) then the capacitor stops to charge and starts discharging through UJT. Now, this discharging continues until the minimum voltage which turns the UJT OFF (the valley voltage). This process continues and the voltage across the capacitor, when indicated on a graph, the following waveform is observed. So, the charge and discharge of capacitor produces the sweep waveform as shown above. The charging time produces increasing sweep and the discharging time produces decreasing sweep. The repetition of this cycle, forms a continuous sweep output waveform. As the output is a non-sinusoidal waveform, this circuit is said to be working as a relaxation oscillator. Applications of Relaxation Oscillator Relaxation oscillators are widely used in function generators, electronic beepers, SMPS, inverters, blinkers, and voltage controlled oscillators. Learning working make money