Building a Colpitts Oscillator Test circuit

Learn to build a Colpitts oscillator test circuit to test your crystals. Easy-to-follow steps and measure results are included.

Oscillators are essential components in many electronic devices, providing clock signals for microprocessors and serving as the heartbeat of various communication systems. The Colpitts oscillator, a popular design due to its simplicity and effectiveness, can be modified to test various crystal oscillator values without requiring inductors. In this blog post, we will guide you through the steps to build a Colpitts oscillator test circuit using only capacitors and a transistor and explain how to use it to test different crystal oscillator values.

What is a Colpitts Oscillator?

A Colpitts oscillator is an LC oscillator where the inductance (L) and capacitance (C) are arranged in such a way that the feedback needed for oscillation is provided by a capacitive voltage divider. Check this article to know more about Colpitts oscillators. The traditional Colpitts oscillator uses both inductors and capacitors, but in this tutorial, we’ll adapt the circuit to avoid inductors by leveraging crystals. You may also want to check this post or this one.

Warning

Messing around with electronics could be harmful if you don’t know what are you doing. You could burn yourself with the soldering iron and be exposed to potentially toxic fumes. Proceed at your own risk! Please, read the full disclaimer here.

Required Components

  • 2 x NPN Transistor (e.g., 2N2222, BC547)
  • 2 x 100pF ceramic capacitors (you can use 10pF to 100pF)
  • 1 x 1nF ceramic capacitor
  • 1 x 2.2uF electrolytic capacitor
  • 1 x 3k3 resistor
  • 2 x 1k resistor
  • 2 x 1n4148 diode
  • 1 x LED (I’m using green)
  • 1 x 4 slots female header
  • 4 x 2 pins male header
  • Crystal oscillator (the one you want to test)
  • Power supply (any voltage from 5V to 15V)
  • Perfboard or Breadboard and connecting wires

Circuit Diagram and Work principle

Kicad schematics

I like to use Kicad to develop my boards, even when I do hand-soldering.

The circuit uses the Colpitts oscillator configuration comprising capacitors C1, C2, the Crystal under test, Q1 and the feedback from R2. C3 filter the AC signal which will be pumped by D1 and D2 to the peak detector created by D2 and C4 which will turn on the LED by turning on Q2.

Circuit Design

KiCad place and route components.

A bit of a challenging design because I tried hard to put all the components on a small piece of perf board I had lain around.

Building and Testing the Circuit

Nice job!

I used a silicon tube to isolate the wires.

Job done!

I completed the soldering job in 40 minutes. The most difficult part was holding in place some components.

Nice tiny board.

Once your circuit is built, you can test different crystal oscillators by simply swapping them in and out of the circuit.

Power and oscilloscope attached.

Use a power supply to power the circuit. I used 5V to test most of my crystals but higher frequencies require more voltage to turn on the LED.

Oscilloscope readings.

Use an oscilloscope to observe the output signal. Connect the probe to the output net. You should see a waveform if the circuit is oscillating correctly.

The measured frequency should match the crystal’s nominal frequency you are testing.

This particular crystal was a 10Mhz one as you can read 9.998Mhz at the oscilloscope screen.

I could test crystal values from 32khz to 16Mhz with ease. 20Mhz crystals and above could not be tested as my oscilloscope does not have the power needed.

Conclusion

Building the Colpitts oscillator test circuit is a straightforward task that allows you to test various crystal oscillator values efficiently. By understanding the basics of transistor biasing and capacitive voltage dividers, you can create an oscillation circuit that relies on crystals for frequency determination. This setup is not only educational but also practical for anyone working with crystal oscillators in their projects.

See ya!