Handheld Oscilloscope Investigates Critical Nodes Oscillator Circuit

The circuit in the schematic below is an infrared transmitter. The infrared beam is transmitted in a nearly line-of-sight pattern to another device with an infrared receiver. The plotted waveforms are the output voltages of the two intermediate stages (purple and yellow) and the transmitter’s output stage (green).

To measure the voltage pulsed across the transmitting IR LEDs, use iMSO-104’s analog input as a traditional oscilloscope. Clip the ground clamp to a suitable anchor point on the circuit, and attach the scope probe lead to the collector leg of transistor T3, as indicated with a green dot in the schematic. The voltage oscillations of T1 and T2, purple and yellow respectively, drive the base voltage of T3 and create the pulsed output.

From inspection of the plot of the waveforms, the user can learn the following about the behavior of this circuit:

  • AMPLITUDE: The output stage voltage swings nearly 5 Vpk-pk
  • FREQUENCY: The oscillation frequency is 7.69 kHz and the waveform is in the shape of a squarewave

From inspection of the schematic, the user can lean about the components that create the oscillation frequency: 

  • Components R1,and C1 have an RC time constant for charging the base voltage at T2
  • Components R2 and C2 have an RC time constant for charging the base voltage at T1
  • Capacitor C1 only charges when T1 is turned on and conducting, and T2 is turned off with its base voltage “pulled high” (meaning that the base voltage is essentially the same as the supply rail voltage)

Changing Oscillation Frequency

The oscillation frequency is set by the values of four components: R1 and R2, and C1 and C2. There are two ways that these component values affect the frequency.

  • RC Time Constant: Choose values for these components that provide the new desired frequency, where T = R*C with R in Ohms and C in Farads
  • Use tight tolerances, say 1% instead of 20%, to be sure the result is close to the design

Oscium's handheld oscilloscope is now available with universal platform support! So, if you're interested in using the scope on iOS, Android, PC or Mac, Oscium supports you. Please go to iMSO-204x for more information.