pulse generator

source: foysal sir slides

1. Fundamentals of Pulse Generators

(PYQ 2024: What is meant by a pulse generator?)

• Definition: A pulse generator is an electronic test instrument used in laboratories specifically for generating rectangular pulses.
• Purpose:
  • Used to stimulate and test digital logic circuits.
  • Used alongside an oscilloscope (CRO) as a measuring device to display waveforms at specific points in a system under test.
  • Provides both qualitative and quantitative information about the device being tested.
• Distinct Feature: Unlike function generators that can generate multiple waveforms (sine, triangular, etc.), pulse generators are dedicated exclusively to rectangular pulses.

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2. Pulse Generator vs. Square Wave Generator

(PYQ 2024: Differentiate between a pulse generator and a square wave generator)

Feature	Pulse Generator	Square Wave Generator
Waveform Type	Generates rectangular pulses of varying widths.	Generates strictly symmetrical square waves.
Duty Cycle	Variable duty cycle (typically adjustable from 25% to 75%).	Fixed duty cycle of exactly 50% ($0.5$).
Average Value	Depends on the variable duty cycle setting.	Always exactly $0.5\times \text{Peak Value}$.
Application	Ideal for testing specific logic states and transient responses where asymmetric timing is needed.	Ideal for general clock signals and basic symmetric switching.

Note: Duty Cycle is defined as the ratio of pulse width to pulse period $DutyCycle=\frac{\text{Average Value}}{\text{Peak Value}}=\frac{t}{T}$

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3. Characteristics of a Pulse Waveform

(PYQ 2024: Define the following terms: (i) Rise time, (ii) Overshoot, (iii) Pulse height, and (iv) Pulse droop)

Pulse Characteristic	Definition
Rise Time (PYQ 2024)	The time required for the pulse amplitude to rise from 10% to 90% of its steady-state value.
Overshoot (PYQ 2024)	The maximum height or spike of the signal distortion measured right after the leading edge, exceeding the normal pulse amplitude.
Pulse Height (Amplitude) (PYQ 2024)	The voltage level measured from the baseline (0V reference) to the steady-state value of the pulse.
Pulse Droop (Sag) (PYQ 2024)	The gradual fall or decline in the pulse amplitude over time during the “high” state.
Baseline	The DC level or reference line (usually 0 volts) at which the pulse starts and finishes.
Ringing	The combination of positive and negative peak distortions (excluding the initial overshoot) that occur as the pulse settles.
Pulse Width / Duration	The time interval measured between the 50% amplitude points on the leading (rising) and trailing (falling) edges.
Settling Time	The period needed for the pulse ringing to stabilize within a specified percentage of the pulse amplitude.

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4. Internal Architecture and Operation

(Key Theory & Block Diagram Concepts)

A standard pulse generator consists of several internal control loops that shape the pulse:

• Components: Upper and lower current sources, Schmitt trigger, ramp capacitor, multiplier, and output amplifiers.
• Operating Mechanism:
  1. The Upper Current Source provides a constant current that charges the Ramp Capacitor.
  2. As the capacitor charges, the ramp voltage increases linearly.
  3. Once the voltage reaches a predetermined upper limit, the Schmitt Trigger changes state.
  4. This state change reverses the current (via the Lower Current Source), causing the capacitor to discharge linearly.
  5. When the negative ramp reaches a predetermined lower limit, the Schmitt trigger switches back, and the cycle repeats.
• Panel Controls:
  • Symmetry Control: Adjusts the ratio between the charging and discharging current sources, which dictates the duty cycle of the output.
  • Multiplier: Selects the size of the ramp capacitor to shift frequency ranges.

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5. Practical Pulse Generation using a 555 Timer

(PYQ 2023: Describe the procedure of pulse generation using a 555 timer with necessary diagrams)

To practically generate a rectangular pulse, a 555 Timer is connected in an Astable Multivibrator configuration.

Circuit Setup:

• The circuit utilizes an external capacitor ($C$) and two external resistors ($R_A$ and $R_B$) to dictate the timing intervals.
• Pins 2 (Trigger) and 6 (Threshold) are tied together to the capacitor, allowing the circuit to re-trigger itself automatically.

Operating Procedure:

1. Charging (High State): When the timer output is high, the internal discharge transistor is off. The external capacitor $C$ charges toward the supply voltage ($V_{CC}$) through both $R_A$ and $R_B$.
2. Upper Threshold: Once the capacitor voltage reaches $\frac{2}{3}{V}_{CC}$, the internal comparator resets the flip-flop. The output goes LOW, and the internal discharge transistor turns on.
3. Discharging (Low State): The capacitor $C$ now discharges through resistor $R_B$ down to ground.
4. Lower Threshold: Once the capacitor voltage drops to $\frac{1}{3}{V}_{CC}$, the second internal comparator triggers the flip-flop. The output goes HIGH, the discharge transistor turns off, and the charging phase repeats, creating a continuous train of pulses.

Mathematical Expressions:

• Time High ($T_{high}$): $T_{high}\cong 0.7(R_A+R_B)C$
• Time Low ($T_{low}$): $T_{low}\cong 0.7(R_B)C$
• Total Period ($T$): $T=T_{high}+T_{low}\cong 0.7(R_A+2R_B)C$
• Frequency ($f$): $f=\frac{1.44}{(R_A+2R_B)C}$