Purpose
In a simple superhet radio receiver, the incoming radio frequency signal (at frequency ) from the antenna is mixed with the VFO output signal tuned to, producing an intermediate frequency (IF) signal that can be processed downstream to extract the modulated information. The IF signal frequency is chosen to be either the sum of the two frequencies at the mixer inputs (up-conversion), or more commonly, the difference frequency (down-conversion), depending on the receiver design.
In addition to the desired IF signal and its unwanted image (the mixing product of opposite sign above), the mixer output will also contain the two original frequencies, and and various harmonic combinations of the input signals. These undesired signals are rejected by the IF filter. If a double balanced mixer is employed, the input signals appearing at the mixer outputs are greatly attenuated, reducing the required complexity of the IF filter.
The advantage of using a VFO as a hetrodyning oscillator is that only a small portion of the radio receiver (the sections before the mixer such as the preamplifier) need to have a wide bandwidth. The rest of the receiver can be finely tuned to the IF frequency.
In a direct-conversion receiver, the VFO is tuned to the same frequency as the incoming radio frequency and Hz. Demodulation takes place at baseband using low-pass filters and amplifiers.
In a radio frequency (RF) transmitter, VFOs are often used to tune the frequency of the output signal, often indirectly through a hetrodyning process similar to that described above. Other uses include chirp generators for radar systems where the VFO is swept rapidly through a range of frequencies, timing signal generation for oscilloscopes and time domain reflectometers, and variable frequency audio generators used in musical instruments and audio test equipment.
There are two main types of VFO in use: analog and digital.
Read more about this topic: Variable-frequency Oscillator
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