- It is an oscillator whose frequency can be controlled by the external voltage. Now in the conventional oscillator the frequency of the oscillation can be selected by the passive component like the resistor, capacitor and inductor and by changing the values of this passive component it is possible to change the oscillation frequency so in the conventional oscillator if you want to change the frequency then we need either a variable resistor or the variable capacitor and manually we need to tune this component to change the frequency.
- So these voltage controlled oscillator also need this passive components of the frequency generation but with the given set of passive component just by changing the controlled voltage or the tuning voltage it is possible to change the oscillation frequency. So we can say that the voltage controlled oscillator are much easier to tune compared to conventional oscillator and in fact if you want to tune the VCO using the microcontroller then also it is possible. For example if the digital to analog converter is interface with the microcontroller then using the microcontroller the output of the that DAC can be change and by applying that voltage to the VCO, it is possible to change the frequency. So this voltage controlled oscillator can change or tune the frequency in a particular range but commercially available VOC can work from a few Hz to even 10s of GHz. And because of that they are widely used in communication application. So they are used in the modulation and demodulation circuit as well as in the frequency synthesizer and function generator and these VCO are an integral part of the phase lock Loop.
- So there are two type of VCOs. One is the harmonic oscillator and second is the relaxation oscillator. So this harmonic oscillator generate the sinusoidal signal while the relaxation oscillator generate either a square, triangular or the sawtooth wave.
- Now theoretically, any harmonic oscillator can be converted into the VCO. For example instead of tuning the resistor aur the capacitor manually if any of them is replaced by the voltage control element then it can be used as a voltage controlled oscillator.
- In relaxation oscillator, by changing the charging current of the capacitor it is possible to tune the frequency. In this type of oscillator the charging current is proportional to the tuning voltage that means by controlling the tunnig voltage we can change the rate at which this capacitor charges. Now using the Schnitt trigger, the voltage across the capacitor is compared with some reference voltage and when this voltage across this capacitor reaches this reference voltage then the output of the schnitt trigger will change and with the change in the schnitt trigger output the polarity of this reference voltage as well as the direction in which this capacitor charges will also change.
- Now let’s see the sum of the important specification of the voltage control oscillator. So these are the specifications that we need to consider while selecting the vco at the specific application. The first specification is the tuning range and it defines the range over which the frequency of the VCO can be tune. So in the data sheet the manufacturers provide the range of the control voltage and the corresponding tuning range.
- Then the next specification is the tuning sensitivity of the tuning gain. And it defines by what amount the frequency of the VCO can be changed when we change the control voltage by 1V and it is defined in terms of the Hz/V. So, for one VCO if the tuning gain k is equal to 2 MHz/ V, then we can say that, when the voltage will change by the 1 V, then the frequency of the oscillator will change by the 2MHz. Now, this tuning sensitivity may not be constant over the entire tuning range and so in the datasheet manufacturer specify the typical range of the tuning gain instead of the specific value.
- Then the next specification are the supply pushing and the load pulling and these two specification are important for the high-frequency VCOs. So the supply pushing defines the change in the output frequency with the change in the supply voltage. Now ideally, for any voltage controlled oscillator even if there is a change in the supply voltage, then there should be any change in the output frequency. but actually the output vacancy does change with the change in the supply voltage. So the change in the frequency due to this supply voltage is known as supply pushing and it is defined in the unit of Hz/V. So the effect of the supply pushing can be minimised by using the regulated power supplies and further it can be minimised if we use the oscillator circuit which has a high Q-factor.
- Similarly, now let’s see what is load pulling. So ideally for any VCO even if there is any change in the load, then that should not affect the output frequency but actually the output frequency does change with the change in the load and that change in the frequency with the load is known as the load pulling. So in the datasheet, usually it is defined as maximum deviation in the frequency from the nominal frequency.
- Then the next very important specification is the spectral purity. Now whenever this term is referred in terms of the time domain then it is referred as a jitter. And whenever it is referred in the frequency domain than usually is referred as the phase noise.
- So, first of all, let’s talk about the spectral purity in terms of the jitter. Now Ideally at a the particular voltage the VCO should generate only a single frequency. So is VCO signal in the time domain then there should not be any change in the amplitude ad well as the periodicity. But in reality if you see there is some uncertainty in the waveform in the terms of the periodicity and this uncertainty is referred as the jitter. Similarly now let’s see the same term in the frequency domain. So the oscillator should generate only single frequency. So basically this phase noise represents the random fluctuation in the phase of the output waveform and because of this random fluctuation the spectrum form. So noise level or the noise floor should be much lower than the actual signal and it is particularly important when the VCOs used are used for the modulation and demodulation in the communication system. So while the modulating the to signal the phase noise of the one signal should not affect the nearby signal.
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