Measure Transistor Coupled Oscillator from Infrared Remote

In this picture, the Oscium iMSO-104 handheld oscilloscope and iPad 2 are displaying the waveforms of an oscillator circuit that’s part of an infrared transmitter. The three traces plotted on the oscilloscope screen represent the time varying voltages at critical points in the oscillator circuit that generate the transmitter’s baseband signal.

The transmitter in the picture is part of a transmitter/receiver pair of circuit boards that form an invisible barrier system that “squawks” loudly when the path between the two boards is blocked. The components on the transmitter board can be grouped into three functional sections: dc power source select, the baseband oscillator, and the infrared emitting diodes.

The IR transmitter board can be battery powered or connected to a constant DC power source. Provisions for battery powered operation consist of a carrier for a 9 V battery and an On/Off switch to disable the transmitter when not in use.

The transmitter’s oscillator produces a square wave with period and frequency set by passive components. In another blog post, the pros and cons for this type of oscillator are discussed, including the effects of component tolerance variation on the square wave’s frequency. The output of the oscillator circuit drives the base of a bipolar junction transistor in the infrared diode section.

The IR transmitter output stage consists of two infrared light emitting diodes modulated at the frequency set by the oscillator in the previous stage.

Oscilloscope enables several measurements simultaneously

There are three traces plotted on the iPad’s display in this picture. The analog channel (green) is probing the voltage at the last stage in the transmitter. Some of iMSO’s built in measurements are active and visible in the iPad display: Maximum Voltage, Peak-to-Peak Voltage, Frequency, Duty Cycle, and Pulse Width. These parameters are important to designers and manufacturers because they are used to calculate power dissipation and efficiency.