Agilent 5989-6673EN PWM Waveform Generation Using the U1252A Handheld Digital Multimeter - Application Note free download

File name 5989-6673EN PWM Waveform Generation Using the U1252A Handheld Digital Multimeter - Application Note

Keysight Technologies PWM Waveform Generation Using U1252B DMM Application Note Introduction Square wave is a unique function for many applications such as pulse width modulation (PWM). PWM is widely used in a variety of measurement and digital control applications. It offers a simple method for digital control logic to create an analog equivalent. Most of today's microcontrollers have built-in PWM capability that simplifies the control's implementation. PWM is widely-used in communication systems because the digital signals are more robust and less vulnerable to noise. This application note provides an overview of PWM and shows how the feature-packed Keysight Technolo- gies, Inc. U1252B handheld digital multimeter (DMM), with a built-in programmable square wave generator, can be used to create PWM signals. What is Pulse Width Modulations PWM is a method of digitally encoding analog signal levels. By digitally controlling analog circuits, system cost and power consumption can be drastically reduced. Many microcontrollers and digital signal proces- sors (DSPs) already include the PWM controller chip, thus making implementation easier. Frequency and Duty Cycle Figure 1 shows a circuit with a battery, switch, and LED. This circuit turns on the LED for one second and then turns off the LED for one second using the switch control. The LED is ON for 50% of the period and OFF the other 50%. The period is defined as the total time taken to complete one cycle (from OFF to ON state and back to OFF state). The signal can be further characterized by duty cycle, which is the ratio of the ON time divided by the period. A high duty cycle will generate a bright LED while a small duty cycle will generate a dimmer LED. The example shown in Figure 1 provides a 50% duty cycle. The duty cycle of a square wave is modulated to encode a specific analog signal level using high-resolution counters. The PWM signal remains a digital signal because the DC supply is either ON or OFF. The voltage or current source is supplied to the analog load by repeating a series of ON and OFF pulses. When On, the DC supply is applied to the load and when OFF, the DC supply is switched off. Referring to Figure 2, two waveforms with different frequencies produce the same amount of light. Note that the amount of light is independent from the frequency, but proportional to the duty cycle. The frequency range used to control a circuit is limited by the response time to the circuit. From the example shown in Figure 1, a low frequency can cause the LED to flash noticeably. Whereby, a high frequency can cause an inductive load to saturate. For example, a transformer has a limited frequency range to transfer the energy efficiently. For some designs, harmonics (or beat frequencies) of the PWM frequency can get coupled into the analog circuitry, causing unwanted noise. If the right frequency is selected