Voltage Controlled Oscillators (VCOs) are essential elements in a wide range of applications that require frequency variation based controllable signals. Examples of this include phase locked loops (PLLs), frequency modulators, radar, and frequency hopping remote access technology. The basic operation of a VCO is to output a frequency that is dependent on an externally accessible control input. VCOs are available in a wide range of package types and sizes, including Coaxial Connectorized Voltage Controlled Oscillators, SMT Voltage Controlled Oscillators, and monolithic microwave integrated circuit (MMIC) VCOs.
A voltage controlled oscillator, like most oscillators, is based on generating positive feedback to force a sustained oscillation. The difference with a typical oscillator is that a VCO also includes control elements that enable adjustment to the frequency the positive feedback oscillation settles at. In some cases, this is a varactor diode that influences the resonant frequency of a tank circuit used to generate the positive feedback phase shift needed for oscillation. This works, because a varactor diode can work as a reverse-biased diode that exhibits voltage-dependent capacitance characteristics. A voltage-controlled capacitor works as a variable control for the indicator-capacitor (LC) tank circuit used to generate oscillation, as the capacitance value change ultimately changes the resonance behavior of the tank circuit.
VCOs can be made into variable LC oscillator technology, crystal oscillators, clock generators, and more complex frequency synthesizers. Voltage controlled crystal oscillators (VCXOs) are often used in high frequency applications to provide very stable and precise high frequency signals. An additional advancement over a standard VCXO is a temperature compensated VCXO (TCVCXO), which incorporates corrective circuitry to counteract the effects of temperature variation on frequency accuracy.
Key Voltage Controlled Oscillator Specifications and Features
- Frequency Range [Hz]
- Tuning Voltage [V]
- Supply Voltage (DC) [V]
- Supply Current (DC) [A]
- Phase noise (@ 1 kHz, @ 10 kHz, and @ 100 kHz offset) [dBc/Hz]
- Output Power [dBm]
- Tuning Sensitivity (Kvco) [MHz/V]
- Pushing [MHz/V]
- Pulling (pk-pk) [kHz]
- Tuning Port Capacitance [pF]
- Load Impedance [Ohms]
- 2nd Harmonic [dBc]
- Packing type [coaxial connectorized, surface mount technology (SMT)]
There are two main classifications of VCOs, harmonic oscillators and relaxation oscillators. Harmonic oscillators output a waveform that is sinusoidal. Hence, resistor-capacitor (RC), LC, and other resonant tank circuits are all harmonic oscillators. Relaxation oscillators output a waveform that is sawtooth or triangular. Emitter coupled VCOs, grounded capacitor VCOs, and delay-based ring VCOs, are all relaxation oscillator types.