Variable Frequency Oscillator or VFO and its Functions

A variable-frequency-oscillator is an electronic oscillator whose frequency can be varied or tuned over a range of frequencies and then amplifies the signal that is produced. It is an essential part of a radio receiver or a transmitter. It is a very important component in household equipment such as radio, television, mobile phones, satellite receivers, walkie-talkies, communication devices, radio operated toys etc.

Variable Frequency Oscillator Testing

The VFO produces a waveform in the radio frequency spectrum or even into the audio frequency range depending on the frequency of use. The waveform most commonly used is the sine wave, which is mostly used in communication. There are also other types of waves such as square waves, triangular waves, saw-tooth waves etc.

 

 

The main signal generating stage in a VFO is the oscillator. The variable frequency control and the band selector varies the frequency of oscillator in a VFO. Then there are other stages of the VFO, which buffers, isolates and amplifies the required signal. There are different types of radio frequency oscillators, some of which I have described in my previous videos.

Types of VFO:
There are two main types of VFO that are commonly in use and they are:
1. Analog VFO. The analog VFO uses the LC network to provide feedback in the oscillator. A tuning capacitor or a varactor diode is used to change the frequency of oscillation in an analog VFO.
2. Digital VFO. À digital frequency synthesizer is used to generate the signal in a digital VFO.

The three types of digital VFOs are,

A. Direct digital synthesis (DDS).

B. Direct frequency synthesis (DFS).

C. Phase locked loop (PLL) VFOs.

 
Electrical Characteristics of an LC Oscillator:
The stability of an LC oscillator is of utmost importance in both reception and transmission of radio signals. So, proper design of an LC oscillator helps to provide frequency stability over a wide range of frequencies that it operates. Only an adequate amount of feedback is given that is needed to quickly start the oscillation, provide a pure sine wave pattern and to minimize pulling by external load changes.

 

VFO Circuit Using the Colpitts Oscillator

It is essential to bias the oscillator only to a required power level that is needed to drive the buffer stage, which is about 10 mW of power. Higher oscillator driving power may increase the chances of heat generation and therefore frequency drifting. The selection of components in an oscillator is critical, so those components that have the least temperature change and maintain higher frequency stability are used. BJT transistors with high fT such as 250 MHz must be used and a transconductance of 2000 and higher must be used in an FET. Short leads and mechanical stability are also important parameters in VFO design. Long leads may introduce inductance into the circuit and it also lowers the Q. Mechanical stability helps to hold the VFO components in place and reduce frequency variation. There is also a possibility of oscillator drift that occurs with fluctuation of input voltage. Therefore, a stable power supply is essential for a VFO. VFOs must be sufficiently shielded to prevent external physical factors such as external capacitance, temperature, humidity or external signals to influence its operation. The output of a VFO must be taken out through a shielded wire or a 50 ohm coaxial cable.

Parts of a VFO:
There are four main functional stages in a variable frequency oscillator and they are
1. Oscillator stage: The oscillator is the signal generating stage of the VFO. There are different types of oscillators used in VFO such as the LC oscillators, varactor tuned oscillators, crystal oscillators, phase locked loop oscillators, direct digital synthesis (DDS) oscillators etc. The oscillator stage produces sinusoidal oscillations or sine waves at a specific frequency. The frequency of oscillations are controlled by a variable capacitor, potentiometer or a rotary encoder depending on the type of oscillator.

 

2. Buffer stage. This is a stage that comes between the oscillator stage and the amplifier stage. It acts as a buffer between these two stages, which prevents pulling of the oscillator by subsequent amplifier stages. It also helps in the impedance matching between the input and the output.

 

 

3. Amplifier stage. This is the third stage of the VFO. It selectively amplifies the required signal from the oscillator output. It amplifies the weak signal from the oscillator and feeds it to the next stage. It can also selectively amplify a required harmonic frequency with the use of a tuning network and provides the amplified signal at the output. There may be one or more amplifier stages for amplifying the signal to the optimum level at the output.

 

4. Power supply regulator: The operating voltage for the VFO should be well filtered and regulated. The power supply to the oscillator and the buffer stages are regulated by 6-8 volt zener diodes. The VFO circuit is powered from a well regulated supply using the 7809 IC voltage regulator.

Applications of VFO:

1. VFOs are extensively used in communication receivers and transmitters.
2. It is used in household equipment and devices such as broadcast radio receivers, television sets, mobile phones, satellite receivers, walkie-talkies, communication devices, radio operated toys, WiFi routers, connectivity gadgets etc.
3. It is used for instrumentation purposes such as signal generators,  oscilloscopes, reflectometers, radars etc.