2N3904 Crystal Oscillator for SSB-AM Local Oscillator

 Here it is shown how to make a crystal based local oscillator for SSB-AM with single crystal, single 2N3904 transistor and few capacitors and resistors. The local oscillator can be used for SSB-AM transmitter or receiver. Here the local oscillator is tuned to 5.1MHz which is the frequency of oscillation of the crystal. But any other frequency dependent crystal can be used.

The following shows a typical transmitter block diagram with local oscillator(carrier oscillator).

AM Transmitter block diagram

There are various ways to make the carrier oscillator such as LC oscillator like Colpitts oscillator, Hartley oscillator, Wein bridge oscillator, RC oscillator, VCO(Voltage Controlled Oscillator) etc. Another way to make this carrier oscillator or local oscillator is using Crystal Oscillator which is the topic of this tutorial. Crystal oscillator are much more stable and usually used in RF transmitter and receivers.

The crystal oscillator based local oscillator shown here is for SSB AM circuit. SSB-AM stands for Single Sideband Amplitude Modulation which is a technique for sending or receiving AM(Amplitude Modulation) signal. In SSB AM there is only one sideband which is transmitted instead of two sidebands because single sideband contains the same information of the two sidebands. The advantage of SSB is that AM signal with less power can be transmitted, thus power efficiency of SSB is higher than DSB.

The following is the circuit diagram of the crystal oscillator.

crystal oscillator circuit diagram

The above crystal oscillator circuit is based on Colpitts oscillator configuration. The capacitor values are in pF. The crystal which has value of 5.1MHz along with frequency adjustable capacitor and the 56pF capacitor forms the feedback circuit for the oscillator. The 2N3904 transistor is the amplifier part of the oscillator. The resistor forms the biasing network for the oscillator. The output is taken from the emitter via the coupling capacitor of 56pF. The output of the oscillator can be connected to buffer amplifier or directly to a mixer as shown in the above RF block diagram.

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