Duty Cycle Measurement

Experiment: Pulse Width and Duty Cycle Measurement

1. Aim

To measure the frequency and duty cycle of a square wave signal using the digital frequency counter input (IN2) and to observe the effect of pulse-width modulation.

2. Apparatus / Components Required

• SEELab3 or ExpEYES-17 unit
• Connecting wires
• PC or Smartphone with SEELab3 software

3. Theory & Principle

A square wave is characterized by its Frequency (how many cycles per second) and its Duty Cycle (the percentage of time the signal remains in the “High” state).

The Duty Cycle ($D$) is calculated as: \(D = \frac{T_{on}}{T_{total}} \times 100\%\)

Where:

• $T_{on}$ is the time the signal is at high level (Pulse Width).
• $T_{total}$ is the total time for one full cycle ($T_{on} + T_{off}$).

The IN2 terminal on the SEELab3 acts as a high-speed digital timer. It measures the time interval between a rising edge and a falling edge ($T_{on}$) and between two consecutive rising edges ($T_{total}$) to calculate the duty cycle accurately.

4. Circuit Diagram / Setup

1. Signal Source: Connect SQ1 to IN2.
2. Monitoring: Connect SQ1 to A1 so you can visually confirm the waveform on the oscilloscope.
3. Note: The digital inputs (IN2, SEN) expect a “Logic Level” signal. The Low level should be $0V$, and the High level should be between $3V$ and $5V$.

5. Procedure

1. Open the SEELab3 software and select the “Duty Cycle” or “Frequency Counter” tool.
2. Set the Source: In the Waveform Generator panel, set SQ1 to a frequency (e.g., $1000\text{ Hz}$) and an initial duty cycle of 50%.
3. Measure: Tap or click on the IN2 icon in the software interface. The device will perform the timing measurement and display the Frequency and Duty Cycle.
4. Vary the Width: Change the duty cycle of SQ1 to 20% and then 80%. Re-measure using the IN2 icon and observe how the $T_{on}$ width changes on the A1 trace.
5. Timeout Check: Disconnect the wire from IN2 and try to measure again. The software will report a “Timeout” because it cannot find the signal edges.

6. Observation Table

7. Results and Discussion

• The frequency and duty cycle of the signal from SQ1 were accurately measured by the IN2 terminal.
• It was observed that the $1000\text{ Hz}$ frequency remained constant while the pulse width varied according to the duty cycle setting.
• The “Timeout” error confirms that the digital timer requires a continuous series of pulses (edges) to perform a calculation.

8. Precautions

1. Voltage Levels: Do not apply more than $5V$ to the IN2 or SEN terminals, as these are digital logic inputs.
2. Grounding: Ensure a clean common ground to avoid “triggering” on electrical noise, which could lead to incorrect frequency readings.
3. Frequency Limits: For very high frequencies, the precision of the duty cycle measurement may decrease due to the internal clock resolution of the microcontroller.

9. Troubleshooting