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Phase Locked Loop Schmitt Trigger

3.5 Schmitt Trigger

The Schmitt Trigger is an essential part of the Phase-Locked Loop. The basic function of the Schmitt Trigger is a fast acting switch. It turns on and stays on when the input voltage reaches a certain level. The switch will stay on until the input voltage goes down to the turnoff value. The input voltage controls how fast the switch will turn off and on. By changing the frequency of the input to the Schmitt Trigger, the frequency of the output of the Schmitt Trigger changes.


Figure 3.5.1 PLL Schmitt Trigger

Figure 3.5.2 shows a simplified schematic of a schmitt trigger. The basic configuration is two common emitter amplifiers in series. The two transistors operate in either the "cutoff" or "saturation" region. The output voltage is either at a maximum or a minimum level. The switch is on when running in Saturation and off when running in cutoff. The "trigger" voltage is determined by the resistor values and the values of the D.C. power supplies. A 0.65 volt voltage drop was assumed. Circuit analysis was used to determine the bias currents and voltages. Figure 3.5.2 shows the hysterisis curve of the schmitt trigger. When the input voltage reaches about 1.3 volts, the schmitt trigger "triggers". In other words the output voltage goes from approximately -0.5 volts to just under 6 volts almost instantaneously. The output will stay at this level until the input goes down to about -1 volt. At this point the output will go from about 6 volts back down to -0.5 volts. The schmitt trigger acts exactly like a fast acting switch.

Figure 3.5.3 shows the simulation of the output voltage versus input voltage. The simulation agrees with the theory of operation. The switch turns on according to the input.


Figure 3.5.2 Schmitt Trigger Theory of Operation



Figure 3.5.3 PSPICE Hysteresis Curve

Figure 3.5.3 shows the input and output voltages as a function of time. Note that the output is either all the way high or all the way low. As the input voltage crosses the trigger voltage, the output rapidly changes from high to low.


Figure 3.5.4 Schmitt Trigger Switching Characteristics

Figure 3.5.4 shows the schematic of the schmitt trigger. The circuit consists of various buffer stages, two inverting stages, and one non-inverting stage. Figure 3.5.5 shows the full schematic of the schmitt trigger. Transistors Q22, Q24,Q26,Q28,Q29, and Q30 are saturation-limiting transistors. If the transistors are driven deep into saturation then the schmitt trigger will not be able switch rapidly. By adding these transistors the voltages across Q37 and Q31 will not go lower than about 0.65 volts. Therefore, enabling the schmitt trigger to quickly change state.


Figure 3.5.5 Schmitt Trigger Simplified Schematic





Figure 3.5.6 Schmitt Trigger Enhanced Simplified Schematic








Electrical Engineering lab key words: PLL, Schmitt trigger, Phase-Locked Loop, switch, voltage level, VCO, feedback signal, cutoff, saturation, transistors, circuit comparator, threshold, hysteresis, open loop application, dual reference, diode, switching, inverting, non-inverting, decoder, logic, pulse, rise time, fall time, trimming, IC, Vcc, Pspice, hysteresis curve, switching characteristics, limiting, inverting, stages, simplified circuit, circuit model, BJT, bi-polar junction.

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