Variable-frequency oscillator

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,

The advantage of using a VFO as a heterodyning 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.

An analog VFO is an electronic oscillator where the value of at least one of the passive components is adjustable under user control so as to alter its output frequency.

The variable capacitor is a mechanical device in which the separation of a series of interleaved metal plates is physically altered to vary its capacitance.

Adjustment of this capacitor is sometimes facilitated by a mechanical step-down gearbox to achieve fine tuning.

The varactor bias voltage may be generated in a number of ways and there may need to be no significant moving parts in the final design.

[4] Varactors have a number of disadvantages including temperature drift and aging, electronic noise, low Q factor and non-linearity.

Modern radio receivers and transmitters usually use some form of digital frequency synthesis to generate their VFO signal.

The advantages include smaller designs, lack of moving parts, the higher stability of set frequency reference oscillators, and the ease with which preset frequencies can be stored and manipulated in the digital computer that is usually embedded in the design in any case.

Pre-programmed frequency agility also forms the basis of some military radio encryption and stealth techniques.

Extreme frequency agility lies at the heart of spread spectrum techniques that have gained mainstream acceptance in computer wireless networking such as Wi-Fi.

Filtering may be required to ensure the transmitted signal meets regulations for bandwidth and spurious emissions.

When examined with very sensitive equipment, the pure sine-wave peak in a VFO's frequency graph will most likely turn out not to be sitting on a flat noise-floor.

Slight random 'jitters' in the signal's timing will mean that the peak is sitting on 'skirts' of phase noise at frequencies either side of the desired one.

Because of the stability of the reference used, digital oscillators themselves tend to be more stable and more repeatable in the long term.

In the shorter term the imperfections introduced by digital frequency division and multiplication (jitter), and the susceptibility of the common quartz standard to acoustic shocks, temperature variation, aging, and even radiation, limit the applicability of a naïve digital oscillator.

This is why higher end VFO's like RF transmitters locked to atomic time, tend to combine multiple different references, and in complex ways.

Such an arrangement can then give all of the longer term stability and repeatability of an exact reference, the benefits of exact digital frequency selection, and the short term stability, imparted even onto an arbitrary frequency analogue waveform—the best of all worlds.