Voltage Controlled Oscillators (VCO) Selection Guide
Application Notes for Microwave Sources
VCO GENERAL
An oscillator will employ an inductor and capacitor to determine the oscillation frequency. Varying the capacitance will change the oscillator's frequency. It's possible to use a variable capacitor, but these tend to be expensive. A cheaper alternative is a voltage-controlled oscillator (VCO). A VCO works on the principal that a reverse-biased diode acts like a capacitor, ideally not letting any current through. A VCO typically employs a varactor diode, whose capacitance varies with the applied voltage.
You can categorize VCOs by the type of waveform they produce:
- harmonic oscillators, which produce a sinusoidal waveform, and
- relaxation oscillators, which produce a triangular waveform.
markets three types of VCOs:
- Octave Band VCOs: These VCOs employ a high-Q silicon hyperabrupt varactor to cover an entire octave, or range of frequencies up to double the initial frequency. Half octave VCOs are also available. Silicon varactors are better than GaAs varactors for high-speed applications, because of faster settling time [= time for circuit's signal to go from the initial to its final value] and fewer long-term charging effects.
- Miniaturized VCOs: These VCOs are less than one-tenth the weight (typically only 4.34 grams) of Octave Band VCOs. The family of VCOs covers a range of frequencies limited at present to 2-6 GHz, each type of VCO covering no more than a half-octave. They are useful in airborne electronic warfare (EW) applications and ground-based simulators.
- Custom Military and Commercial VCOs: These oscillators employ special circuitry to achieve linearity, so that the gain is the same over the range of frequencies, despite the non-linear nature of VCOs.
- Linear X band (8-12 GHz): Used in radar, and satellite and terrestrial communications
- Linear Ku (12-18 GHz): Used for jamming applications
Uses of Voltage Controlled Oscillators
VCOs are used in:
- Electronic jamming equipment. For example, sending out radio waves along the same frequencies that a radio or cellular phone uses, causes enough interference with the communication between the radio or cell phone and the transmission tower to render the phone or radio unusable.
- Function generators, which generate either repetitive or single-shot (trigger) waveforms
- Phase-locked loops, which generate an output signal whose phase is matched with that of an input reference signal
- Frequency synthesizers used in communication equipment.
