VCO ICs generate output signals of variable frequency. The frequency of the output voltage can be varied using the input DC voltage. The output voltages can be waveshape sinusoidal, triangular, or saw-tooth depending on the type of IC.
There are two types of voltage-controlled oscillator. One type generates sinusoidal waveforms and is called a sinusoidal oscillator or a harmonic oscillator. The non-sinusoidal oscillator, or relaxation oscillator, generates non-sinusoidal signals.
The design parameters of VCO ICs are output level, sweep range, linearity, output harmonic level, characteristic gain, tuning sensitivity, load pulling, frequency pushing, and VCO phase noise.
Test the performance of a stereo amplifier using VCO generated signal
If you want to test the performance of a stereo amplifier, you need signals in the audio frequency range (20 Hz to 20 kHz). Audio signal generators are used for this purpose. Now consider a communication system in general—broadcasting transmitters modulate communication signals with high-frequency carrier waves. These applications make use of voltage-controlled oscillators (VCO) ICs for generating signals of varying frequency. Wherever signal generators are required, system designers include VCO ICs, as they offer excellent features including stable operation in high temperatures, low power dissipation, and easy control.
VCO ICs in Communication Systems
First of all, we must erase the idea that alternators can be used for signal generation in communication systems. Periodic signals presented in communication systems are in the high-frequency range. In radio broadcasting, the frequency ranges from 500kHz to 25 MHz. The television broadcasting carrier frequency varies from 45 MHz to 250 MHz. Generally, we are in need of high-frequency signals that become difficult to generate using alternators. The impracticalities of using alternators are the greater number of poles and the high speed of rotation of the armature. These limitations can be easily overcome by using VCOs, which produce stable and accurate periodic oscillating signals of required frequency within a small footprint.
The periodic oscillating signals needed in the transmission and reception of wireless communication signals are supplied by VCO ICs. VCO ICs generate output signals of variable frequency. The frequency of the output voltage can be varied using the input DC voltage.
The frequency of the output voltage is a function of input DC voltage
The output voltages can be waveshape sinusoidal, triangular, or saw-tooth depending on the type of IC. The frequency of these waveforms can be regulated or varied by varying the magnitude of the input DC voltage.
The frequency of the output voltage increases with input DC voltage
The frequency of the output signal when the input DC voltage is at the nominal value is called the center frequency (fmid). The variation in the frequency is illustrated in the figure above.
Types of VCOs
There are two types of VCOs. One type generates sinusoidal waveforms and is called a sinusoidal oscillator or a harmonic oscillator. The non-sinusoidal oscillator, or relaxation oscillator, generates non-sinusoidal signals.
For generating sinusoidal signals, tuned circuits oscillators (LC or tank circuits), RC oscillators, and crystal oscillators are used. In an LC oscillator, the resonant frequency of the LC network is the frequency of the signal generated. The charging and discharging of the capacitor regulates the frequency of the output signal in RC oscillators.
In certain VCO applications, varactor diodes are used as capacitors. The reverse-biased varactor diodes are analogous to capacitors and the capacitance varies with the applied DC voltage.
Harmonic oscillators are also called a linear voltage controlled oscillator. In such oscillators, the output frequency increases with the increase in input DC voltage. The advantages of linear VCOs are that the frequency of the generated signal is not sensitive to noise or temperature. Additionally, linear VCOs have frequency stability wrt to input DC supply and accurate frequency control.
The implementation of harmonic oscillators is challenging, but the stability is much better compared to relaxation oscillators. The main drawback of this type of VCO is the difficulty to implement it as a monolithic IC package.
Compared to the implementation of harmonic oscillators, relaxation oscillators are easy to implement. They use active devices such as op-amps and Unijunction Transistors (UJT), along with capacitors. They are widely used VCOs, and are found in various applications such as frequency modulation, tone generation, frequency synthesizing, electronic jamming, and function generation.
The relaxation oscillator waveforms can be square, saw-tooth, or triangular waveforms. Relaxation VCOs are classified as delay-based ring VCOs, Ground capacitor VCOs, or emitter-coupled VCOs.
In delay-based ring VCOs, the gain stages are cascaded in a ring form. The frequency of the signal generator holds a relationship with the delay in each stage. In the ground capacitor and emitter-coupled oscillators, the charging and discharging time of the capacitors controls the time period of the oscillator stages. Generally, the relaxation oscillators are fabricated as monolithic ICs.
Design Parameters of VCO ICs
The various design parameters of VCO ICs are:
Output Level-In wireless applications, VCO ICs drive a variety of circuits including mixers and synthesizers. Considering this, the VCO IC is equipped with buffer amplifiers for load isolation and drive capabilities.
Sweep Range-The possible frequency range generated by VCO ICs is described by the sweep range. A broad sweep range which satisfies the requirement of the application should be employed in each circuit.
Linearity-The linearity represents the frequency change with the change in control input DC voltage. The linear characteristic of VCO ICs makes frequency control of the generated signal easier.
Output Harmonic Level-Nonlinear active devices are the main sources of harmonics in VCOs. The gain of the VCO ICs should be designed, keeping the harmonic levels low.
Characteristic Gain-The ratio of VCO output frequency to the VCO input DC voltage is called characteristic gain.
Tuning Sensitivity-The range of frequency available in VCO ICs by tuning the input DC voltage or capacitor is called tuning sensitivity, expressed in Hz/V. High values in hundreds of kHz/V are expected for tuning sensitivity in VCO ICs.
Load Pulling-The sensitivity of free-running VCOs to load changes at the output terminal is called load pulling. The load pulling should be low enough that the VCO operation isn’t affected by the load variations.
Frequency Pushing-The sensitivity of free-running VCO output frequency to input DC voltage variations is called frequency-pushing. Generally, VCO ICs have excellent pushing performance, which is 5-10% of the tuning sensitivity.
VCO Phase Noise-The relationship between noise-sideband levels to carrier power level in free-running VCOs is called VCO phase noise. Low phase noise in VCO ICs limits its tuning range.
VCO ICs are an inevitable part of frequency modulators, frequency synthesizers, and phase-locked loops (PLL). You cannot ignore the sensitivity parameters and frequency characteristics of VCO ICs when using them in wireless communication systems. If VCO ICs are selected appropriately, keeping in mind all the application requirements, then signal generation tasks can be completed effectively and efficiently.