Introduction of Voltage Controlled Oscillator
An oscillator is a circuit that generates a continuous, repetitive, alternating waveform in the absence of any input. Oscillators, in essence, convert unidirectional current flow from a DC source into an alternating waveform of the desired frequency, as determined by its circuit components.
A voltage controlled oscillator (VCO) is an output signal oscillator whose output can be adjusted throughout a specific frequency range that is controlled by the DC voltage input. It is an oscillator whose output frequency is directly proportional to the voltage applied (FM control).
The sweeping time is a key parameter of the VCO: it is the shortest amount of time required to turn or sweep from minimum frequency to highest frequency or vice.
The VCO can be modified by amplitude using an external analog signal (AM). An external power amplifier may be required to achieve the desired RF power level.
Types of Voltage Controlled Oscillator
The VCOs can be categorized based on the output waveform:
- Harmonic Oscillators
- Relaxation Oscillators
Harmonic oscillators provide a sinusoidal output waveform. This also applies to the oscillator that controls the linear voltage. Examples are the LC and Crystal oscillators. In this case, the capacitance of the diode fluctuates with the voltage around it. This, in turn, changes the capacitance of the LC circuit. As a result, the output frequency will change. The benefits include frequency stability in terms of power supply, noise, and temperature, as well as precision in frequency control. The sole disadvantage is that this type of oscillator cannot be easily implemented on monolithic ICs.
A screwed tooth is the waveform output of harmonic oscillators. Because of the reduced number of components, this kind may provide a wide range of frequency. It is most commonly used in monolithic integrated circuits. The relaxation oscillators can have the following topologies:
- Delay-based ring VCOs
- Grounded capacitor VCOs
- Emitter-coupled VCOs
The gain stages in delay-based ring VCOs are coupled in a ring form. As the name says, the frequency is linked to the delay at every single location. VCOs of the second and third types behave almost identically. The time spent in each stage is directly related to the charging and discharging times of the capacitor.
Voltage Controlled Oscillator Requirements
When constructing a voltage-regulated oscillator, or VCO, various parameters must be considered before the design process begins. These parameters define the primary performance requirements for the VCO.
VCO tuning range
It is obvious that the oscillator that is powered by voltage must be able to tune over the range that the loop is supposed to work over. This requirement is not always simple to meet and, in extreme cases, may necessitate switching the VCO or resonant circuit.
VCO tuning gain
The gain of the voltage-controlled oscillator is critical. It is calculated in terms of volts per Hz (or V/MHz, etc.). It is the tuning shift for a given change in voltage, as shown by the units. The voltage-controlled oscillator gain has an effect on all of the overall loop design factors and measurements.
At lower frequencies, the VCO response curves are relatively straight. They typically flatten out at higher voltages, where the capacitance changes from the variable diodes decrease.
VCO V/f slope
It is a critical condition for any voltage-driven oscillator used in a phase-locked loop that the voltage to frequency curve is monotonic, i.e. it always shifts in the same context, usually increasing voltage frequency. If ti changes, as it can in some cases due to spurious resonances, etc., the loop can become unstable. This must therefore be avoided if the phase-locked loop is to function properly. This curve features a tiny dip, which will result in an unstable phase-locked loop.
Efficiency of phase noise: In some PLL applications, the phase noise performance of the voltage regulated oscillator is critical, especially when used in frequency synthesizers. The phase noise output of the voltage-regulated oscillator is the dominant factor in phase noise outside of the PLL loop bandwidth. While the PLL reduces close-in noise, it has no effect on VCO phase noise outside of the loop bandwidth.
These are some of the key specifications that must be understood from the start of the VCO design process. Careful tuning of the tuned circuit Q, particularly the use of variable diodes with the highest Q possible, selection of the active system, and optimization of the oscillator feedback.
Voltage Controlled Oscillator Feedback
A VCO, like any other oscillator, can be thought of as an amplifier and a feedback loop. The gain of the amplifier can be denoted as A, and the feedback as B. For the circuit to oscillate, the phase shift around the loop must be 360° and the gain must be unity. Signals are transmitted back around the loop in this manner in order to become addictive, and as a result, any tiny disturbance in the loop is fed back and accumulates. The signal is based on one frequency since the feedback network is frequency-dependent, the feedback network is resonant, and a single frequency signal is created.
Many oscillators and consequently VCOs use a standard emitter circuit. This causes a 180° phase shift, leaving the feedback network to provide another 180°. A basic base circuit with no phase shift between the emitter and collector signals (assuming a bipolar transistor is used) can be used by other oscillator or VCO circuits, and the phase shift network must offer either 0 ° or 360 °.
To ensure that the oscillation occurs on a given frequency, the device requires a resonant circuit for the oscillator to oscillate on that frequency. Depending on the circuit, the resonant circuit may be one of several LC resonant circuit configurations, or a quartz crystal, etc., in either series or parallel resonance.
Voltage Controlled Oscillator Varactor Issues
When using varactor diodes inside a voltage-driven oscillator, care must be taken in the design of the circuit to ensure that the drive frequency in the tuned circuit is not too high. If this is the case, the varactor diodes can be forced into forwarding conduction, lowering the Q and increasing the number of spurious signals.
There are two main types of varactor diodes that can be used within a VCO—the name refers to the diode junction, which influences their output.
As the name implies, abrupt diodes have a somewhat sharp transition between the diode’s regions. They can provide a greater Q than their hyper-abrupt siblings, although abrupt varactor diodes cannot provide such a wide tuning range or linear transfer characteristic. This leads in improved voltage-controlled oscillator phase noise output. Another thing to keep in mind is that abrupt varactor diodes may require a high tuning voltage in order to have the required tuning range, as certain diodes may require a tuning voltage for the VCO to differ up to 50 volts or slightly more. This can present issues with providing a sufficiently high voltage supply to the drive circuits.
The capacitance curve has a relatively linear voltage for hyper-abrupt diodes. As a result, they provide a very linear tuning characteristic that may be required in some applications. They can also tune over a wide range, and can normally tune over an octave range with a tuning voltage shift of less than 20 volts. They do not, however, set a particularly high Q standard. Because this reduces the overall Q of the tuned circuit, the output of the phase noise is as good as that obtained with an abrupt varactor diode.
The voltage-controlled oscillator design is far from simple, despite its apparent simplicity. A design would also involve careful optimization of the input levels in conjunction with the system and layout. The VCO’s design must carefully balance sometimes contradictory requirements, such as a wide tuning range and a low noise phase.
Once the design has been completely configured and the design has been completed, the efficiency standards that can be achieved are surprisingly good.