Here a crystal oscillator design using 2N2222 transistor is illustrated. Such crystal oscillator can be used in making RF transmitter and also can be used at the RF receiver. When used in this way the crystal oscillator is also called local oscillator. Here the circuit consist of crystal oscillator with buffer amplifier circuit. The buffer amplifier is also build with 2N2222 transistor. The buffer amplifier is not used to provide amplification but provide a impedance matching or transformation between the oscillator and most probably mixer. The output impedance from the buffer amplifier is high so as not to load the next circuit stage which is the RF mixer or AM modulator.
The following typical RF transmitter block diagram shown below shows where the crystal oscillator/carrier oscillator is used.
In the above circuit diagram, a crystal which ranges from 1MHz to 15MHz is used. This crystal along with the Q1 transistor forms the crystal oscillator. This oscillator is based on Colpitts oscillator with the crystal is inductor and the capacitor C1,C2 and C3 are the capacitors. The resistors R1, R2 and R3 are used to DC bias the Q1 transistor. The Q2 transistor forms the buffer amplifier. It is DC biased by resistors R4, R5 and R6. The capacitor C5 is coupling capacitor and R7 along with 500Ohm potentiometer is used to control the output signal. The capacitor C6 is again a coupling capacitor which is connected to a load resistor RL of 50Ohm. The whole circuit is powered using 9V power supply and the capacitor C4 is used to smooth any possible power supply noise.
The local crystal oscillator output signal can then be connected to a simple diode based am modulator circuit to generate AM modulated signal. Then the modulated signal can be filtered to filter out unwanted signal in the modulated signal. After that modulated and filtered output can be amplified using 2N3904 or 2n2222 rf amplifier circuit to amplify the modulated signal. Finally the amplified AM signal is coupled to an antenna. A complete AM crystal oscillator transmitter can thus be build with couple of transistors, resistors, capacitors and crystal.
The tutorial How to make simple Crystal Oscillator shows actual crystal oscillator circuit build on breadboard. There are other methods to make electronics oscillator. The following tutorials shows some ways to make oscillator.
- Colpitts oscillator with LM358 and TL072 Op-Amps
- 2N2222 Phase Shift Oscillator
- Twin-T oscillator with LM741 operational amplifier
- BC547 Transistor Phase Shift Oscillator
- Hartley Oscillator Design with Enhancement MOSFET
Useful Recommendations
- For BJT amplifier design such as 2N2222 RF amplifier circuit mentioned above you can use the online bjt amplifier calculator. To learn how to bias a BJT transistor see BJT transistor biasing tutorial.
- For different oscillator design use the oscillator calculator to calculate component value for particular frequency.
