01 Crystal Oscillator

I. Introduction

I saw a book on RF circuit design today. A reference circuit for a crystal Colpitts oscillator is given. The reason why I am interested is that when building this kind of oscillation circuit, the circuit often failed to oscillate. - Let's test the working of this circuit with a breadboard.
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▲ Figure 1.1.1 Colpitts crystal oscillator circuit

▲ 图1.1.1 Colpitts 晶体振荡电路

2. Test results

1. Build a test circuit

Build this test circuit on a breadboard with the components you have at hand. The circuit is relatively simple, and the components used are relatively few. Some of the capacitance parameters are slightly different from the reference circuit given in the book. This is the actual circuit schematic. Among them, capacitor C2, since there is no 82pf capacitor at hand, uses 100pf instead. The bias resistors R1 and R2 are 220k ohms and 910 ohms respectively. The working power supply uses 12V.

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2. Measure the oscillation waveform

After the circuit is powered on, the working current is about 6mA. The circuit starts to oscillate. The two signals measured are the waveform on transistor emitter resistor R2 and the waveform on crystal Y1. It can be seen that the amplitude of the waveform on R2 is relatively small, and there is distortion. Use the FA-2 frequency meter to measure the R2 waveform frequency, about 12MHz. This is very close to the nominal value of the crystal in the circuit. This experiment verified that the circuit can work stably.
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▲ Figure 1.2.1 Circuit oscillation waveform

▲ 图1.2.1 电路振荡波形

▲ Figure 1.2.2 Oscillating signal frequency

▲ 图1.2.2 振荡信号频率

In the previous test circuit, it was often encountered that the circuit did not vibrate. The main reason is that the parameter setting is unreasonable. For capacitor C3, if its capacitance value is reduced, the circuit will also oscillate, and the voltage amplitude on the crystal will increase.
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After testing, if C3 is removed, the circuit can still oscillate. At this time, the oscillating voltage signal on the crystal is very high, and the waveform is greatly distorted.

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※ Summary ※

This article tests a Colpitts crystal oscillator circuit, built the circuit on a breadboard and tested its oscillation waveform.
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● Links to related diagrams:

Figure 1.1.1 Colpitts crystal oscillator circuit
Figure 1.2.1 Circuit oscillation waveform
Figure 1.2.2 Oscillating signal frequency

Crystal oscillator circuit test