A 2-Stage Colpitts Variable Frequency Oscillator

An electronic oscillator plays an essential role in the construction of radio circuits. An oscillator is an electronic circuit that produce an electronic wave or signal which can be a sine wave or a square wave. An LC oscillator circuit produces a repetitively polarizing wave signal of constant amplitude and stable frequency called as the sine wave. The sine waves are produced by the oscillator when a required positive feedback is fed to the oscillating LC components resonating at the required frequency to sustain it. The sine wave signal is utilized in a radio circuit for mixing with the incoming RF signal and to detect and demodulate the modulated RF signal. The oscillator signal is utilized in a circuit for performing several functions. Oscillators can also be considered as the heart of a radio circuit as a perfect sine-wave signal is needed for the circuit to function.

7 Mhz VFO Circuit Diagram

This is the construction of a Colpitts LC oscillator, which is a transistorized oscillator circuit. This oscillator is also called as the variable frequency oscillator (VFO) as it can be varied in frequency by varying the inductor or capacitor values. It is the most commonly used and a popular oscillator circuit. Waveform generated is a repetitive sine-wave AC signal at the output terminals and the output is usually of constant amplitude. The frequency of the oscillator is determined by the LC network at the input of the transistor. Here the oscillator is tuned to around 7 Mhz range. The oscillator consists of a transistor amplifier stage with its output coupled to the input tank circuit containing a parallel LC tuned to the required frequency. The feed back is fed through a capacitor divider network at the correct amplitude and in correct phase. The LC or the inductance and capacitance acts as a parallel resonance network that oscillates at the frequency of the LC tank circuit.

7 Mhz Variable Frequency Oscillator

An FET is utilized as the oscillator stage and a BJT transistor is used as the buffer stage. The voltage that is supplied to the oscillator is regulated to 9 volts by utilizing a 9 volt IC regulator. It is further reduced in voltage using a drop down resistor and filtered using 0.01 ceramic disc capacitors. The oscillator voltage is further regulated by a 6 volt zener diode in order to reduce any drift in the oscillator signal. The capacitors and inductance used in the LC network should be of good quality for obtaining a smooth and stable output signal. The capacitors used are of styroflex or equivalent variety and the inductor is 20 guage wire wound several times on a small PVC former tuned to around 7 Mhz. The output of the oscillator stage is coupled to the input of the buffer stage through a small value capacitor of 12 pf. The buffer stage base voltage is provided through a high value single resistor. The output of the buffer stage is obtained from the collector lead using a 30 pf capacitor. The signal can be further amplified utilizing an amplifier stage for different applications.

7 Mhz Variable Frequency Oscillator

Actually an oscillator derives its power from the DC power of the power supply and converts into an AC form. Oscillators can be made to oscillate on a wide range of frequencies starting from the audio frequency range into the RF range of frequencies that covers UHF, extremely high frequencies (EHF) and above. The sine wave generated by the oscillator is used in radio reception by mixing with the RF signal from the antenna. The sine-wave can also be in a transmit mixer to generate a DSB or SSB signal. The sine-wave or the carrier signal can be amplified and coupled to an antenna for continuous wave (CW) transmission.