Vco design

A voltage controlled oscillator or VCO is an oscillator that can be frequency controlled by an external voltage.

Voltage Controlled Oscillator VCO Design for PLLs

The external voltage is called a tuning voltage and it is typically connected to a voltage variable capacitor varactor within the resonant oscillator circuit. As the voltage changes, the capacitance of the varactor changes which, in turn, dithers the resonant frequency of the oscillator. This may sound simple but there are many constraints to good VCO design including spectral purity, output power, temperature behavior, tuning sensitivity and noise immunity.

All of these constraints depend on the customer's specifications dictated by the application or end-use of the product. For instance, high spectral purity is important in high data rate systems or communication systems where reciprocal mixing may be a problem. Reciprocal mixing occurs when the phase noise skirt of the local oscillator VCO down-converts environmental interferers which may desensitize a receiver.

vco design

Temperature behavior is usually critical for airborne or remote applications such as radar or point-to-point radio base stations. Tuning sensitivity is usually important when the VCO is part of a phase-locked system in a synthesizer or coherent receiver; which depends on the frequency range and the customer's tuning voltage limits.

Noise immunity can be a problem when the VCO is used in an environment where noise or unwanted signals can leak into the VCO either via the power supply or the output load. Output power level and flatness are typically critical when the VCO is used as a local oscillator to drive a mixer circuit. The designer's job is to understand all of the design constraints and optimize the VCO accordingly. All of these constraints will be described in this paper and the reader will be enabled to learn how to incorporate them into a design process.

First, we will discuss basic VCO theory and then we will treat the constraints one-by-one. We use cookies to provide you with a better experience. By continuing to browse the site you are agreeing to our use of cookies in accordance with our Privacy Policy.

Voltage-controlled oscillator

No Comments. Introduction A voltage controlled oscillator or VCO is an oscillator that can be frequency controlled by an external voltage. Report Abusive Comment Thank you for helping us to improve our forums. Is this comment offensive? Please tell us why. Digital Edition. In Print Supplement. Download Now.Voltage-Controlled Oscillator Design. In order to explore the capabilities of the process and explain the difference in predicted and experimental performance, a high-speed, wide-bandwidth voltage-controlled oscillator VCO was developed as a "challenge" chip.

The VCO was developed using digital CML circuits optimized for extreme high-speed operation but with the full-swing characteristics of digital logic.

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It was designed for a bandwidth of 0. Using newer device and interconnection models, the performance of the VCO may now be accurately predicted. Inseveral discrepancies had been uncovered between the circuit performance predicted by the Rockwell-supplied device and interconnection models and actual experimental measurements.

We had designed relatively large circuits that were functional but operated at significantly lower speeds than expected. Furthermore, separate circuits on the same chip would often work but not at the same frequency or power supply level, severely inhibiting testing. The test chip yields were lower than expected but we believed that they would improve with time and the addition of better fabrication tools into the process in particular, a stepper from Canon with higher resolution.

This project was intended to promote the design of very high-speed circuits by combining experimental data from fabricated test structures with CAD tools from Cadence Design Automation and OEA Associates. Four test chips were submitted for fabrication, namely a chip containing simple passive and active test structures, a standard cell test chip, the original RPI testchip, and a high-speed VCO.

The original testchip was included both as a control for comparison to the first RPI fabrication run and in the hopes of obtaining more robust circuits that could provide more test data than before. This chapter describes the VCO design including the architecture, circuits, layout and test results. A voltage-controlled oscillator VCO is simply a circuit that generates an oscillating signal at a frequency proportional to an externally applied voltage.

These types of circuits are useful for tracking and matching signal frequencies as they shift due to thermal variations, power supply fluctuations, and other sources of frequency phase-shifts. VCOs are found particularly often in phase-locked loops PLLs used for clock generation and synchronization.

PLLs combine the variable frequency characteristics of the VCO with a phase detector circuit in order to track a signal as it changes frequency.

Modular Synth VCO Design \u0026 Build

There are basically three types of oscillators that may be voltage-controlled: push-pull oscillators, relaxation oscillators, and ring-oscillators. In the last few years there have been a number of VCOs in the literature based upon a common-base CB amplifier. One common drawback to this type of oscillator is its tendency to change frequency with any variation in the load, referred to as the frequency-pull effect.

An example of a CB oscillator is shown below in Figure 2. Figure 2. Relaxation oscillators or multivibrators are the most commonly used type of oscillator in IC designs. The circuit oscillates by continuously charging and discharging a capacitor between two voltage levels. A multivibrator may be controlled via a voltage-controlled current source that supplies the current for charging and discharging the capacitor an example is shown in Figure 2.

Although multivibrators are relatively simple and require few devices, the oscillation frequency is partially dependent upon the capacitor value and is rather sensitive to thermal effects due to their dependence upon the device V BE. These circuits are capable of high speeds a peak value of 7. Ring oscillator VCOs are fundamentally different from multivibrators. The basic circuit block is a variable delay cell that is voltage-controlled. By connecting several delay elements as a ring oscillator, the circuit will oscillate with a period equal to the voltage-controlled cell delay multiplied by twice the number of delay stages the signal must propagate through the inverting path twice to return to its original value.

One example of a ring-based VCO is shown below in Figure 2. Unlike multivibrators, voltage-controlled ring oscillators are not highly sensitive to temperature or capacitor values.

vco design

In addition, the ring oscillator circuits may be created with multiple quadrature outputs that may be used to double the frequency. Quadrature signals are useful in many PLL applications and are possible using multivibrator circuits.

Multiple quadrature outputs, however, are unique to voltage-controlled ring oscillators and makes possible multiplication by factors of 4 and higher. Multiplying the signals we get.The voltage controlled oscillator performance governs many aspects of the performance of the whole phase locked loop or frequency synthesizer. Accordingly careful design is necessary.

The design of a high performance voltage controlled oscillator, VCO, is not is not a trivial task. Consideration of the circuit, components used and the layout all play a role in determining the performance. This requires sound theoretical design, followed by careful choice of all the components and then a good PCB layout. Even with circuit simulation, it may take a couple of iterations of the VCO layout. When designing a voltage controlled oscillator, VCO, there are several parameters that must be considered before the design starts.

These define the key performance parameters needed for the VCO. These are some of the main requirements that need to be known from the outset of the design of the VCO. Careful optimisation of the Q of the tuned circuit, especially using varactor diodes with as high a Q as possible, choice of active device, optimisation of the feedback within the oscillator.

Like any oscillator, a VCO may be considered as an amplifier and a feedback loop. The gain of the amplifier may be denoted as A and the feedback as B. In this way signals are fed back round the loop so that they are additive and as a result, any small disturbance in the loop is fed back and builds up.

In view of the fact that the feedback network is frequency dependent, the build-up of signal will occur on one frequency, the resonant frequency of the feedback network, and a single frequency signal is produced.

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Many oscillators and hence VCOs use a common emitter circuit. For the oscillator to oscillate on a given frequency, the system includes a resonant circuit to ensure that the oscillation occurs on a given frequency. The resonant circuit can be one of a number of configurations from an LC resonant circuit in either series or parallel resonance dependent upon the circuit, or a quartz crystal, etc. Of the two, the Colpitts circuit is the most widely used, but both are very similar in their configuration.

These circuits operate as oscillators because it is found that a active device such as a bipolar transistor with capacitors placed between the base and emitter C1 and the emitter and ground C2 fulfils the criteria required for providing sufficient feedback in the correct phase to produce an oscillator. For oscillation to take place the ratio C1 : C2 must be greater than one. The resonant circuit is made by including a inductive element between the base and ground.

In the Colpitts circuit this consists of just an inductor, whereas in the Clapp circuit an inductor and capacitor in series are used. The capacitance for the overall resonant circuit is formed by the series combination of the two capacitors C1 and C2 in series. In the case of the Clapp oscillator, the capacitor in series with the inductor is also included in series with C1 and C2.

In order to make the oscillator tune it is necessary to vary the resonant point of the circuit. This is best achieved by adding a capacitor across the indictor in the case of the Colpitts oscillator. Alternatively for the Clapp oscillator, it can be the capacitor in series with the inductor.

For high frequency applications a circuit where the inductive reactance is placed between the base and ground is often preferred as it is less prone to spurious oscillations and other anomalies.

The bipolar transistor has a low input impedance and is current driven, while the FET has a high input impedance and is voltage driven. The high input impedance of the FET is able to better maintain the Q of the tuned circuit and this should give a better level of performance in terms of the phase noise performance where the maintenance of the Q of the tuned circuit is a key factor in the reduction of phase noise.

Another major factor is the flicker noise generated by the devices.

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Oscillators are highly non-linear circuits and as a result the flicker noise is modulated onto VCO as sidebands and this manifests itself as phase noise.

In general bipolar transistors offer a lower level of flicker noise and as a result VCOs based around them offer a superior phase noise performance.Basic oscillator design specifications often require a given output power into a specified load at the design frequency.

The drive level and bias current set the fundamental output current and the oscillation frequency is set by the resonator components. Transistor selection of the transistor should consider noise, frequency, and power requirements.

Based on the particular device, the design may account for parasitics of the device affecting resonator components as well as nonlinear performance specifications. All the VCO schematics presented below were practical build using the Infineon SiGe transistor BFP, and any of them can be re-tuned for different frequency ranges changing varicaps and LC tank values. VCO Specifications. VCO Design Recommendations. Nonlinear Effects in VCOs. Oscillator circuit nonlinearities cause low-frequency noise components to be up-converted and to appear as noise sidebands on the VCO output.

Although this statement is intuitively obvious, quantifying this mechanism is much more complex. Second-order nonlinear distortion determines the degree of noise contamination of the oscillator output for instance. Therefore, second-order distortion in the oscillator should be minimized. The degree to which any oscillator accomplishes this goal can be judged based on the second harmonic output level of the oscillator.

A good oscillator should exhibit 2 nd harmonic levels on the order of dBc.

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Another useful indicator of good oscillator design is the change in oscillation frequency versus DC bias reduction. A slow reduction of the supply voltage from nominal to the point at which oscillation just ceases, should result in a very small frequency change for example, should be on the order of about of 20 kHz for a well-designed 2 GHz oscillator. The oscillator excess open-loop gain which is necessary for initial oscillator build-up should be minimized in order to prevent amplitude fluctuations from being converted into significant frequency fluctuations.

The 2 nd harmonic currents in the oscillator sustaining stage can appear in phase quadrature with the fundamental current, thereby worsening the conversion of AM noise to PM noise.

Ideally, the 2 nd and 3 rd harmonic frequencies should be placed well above the f T cut-off frequency of the oscillator sustaining stage transistor, thereby minimizing this effect. A change in the RF voltage amplitude across the tuning varactor normally affects the observed tuning capacitance value in the resonator, thereby providing one substantial AM-to-FM conversion mechanism in the oscillator. Other VCO impairments including injection locking, load pulling, and power supply frequency pushing can cause serious oscillator performance degradation, particularly in phase-locked systems.

If the induced impairments fall within the closed-loop bandwidth of the system, potentially chaotic spectral behavior can result.

vco design

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VCO Design Tutorial: Part 1

Be Inspired.An Oscillator in electronics generally refers to a circuit which is capable of producing waveforms.

This waveform can either be of Sine, triangle or even a saw tooth type. Some of the most common oscillator circuits are LC circuit, Tank circuit etc. A Voltage Controlled Oscillator is an oscillator which produces oscillating signals waveforms with variable frequency.

The frequency of this waveform is varied by varying the magnitude of the Input voltage. For now you can imagine a Voltage Controlled Oscillator VCO to be a black box which takes in Voltage of variable magnitude and produces an output signal of variable frequency, and the frequency of the output signal is directly proportional to the magnitude of the input voltage. We will learn more about this black box and how to use one in our designs in this tutorial.

There are many types of VCO circuits ; a very basic one can be built by just utilising a capacitor, inductor and resistor to make a tank circuit. Also Op-Amps, Multivibrator, transistorstimers can also be utilised to build oscillating circuits. In a RC oscillator the frequency of the output wave depends on the value of the capacitor used in the circuit, since the frequency is given by the formulae. Hence in this case the frequency of oscillation is inversely proportional to the value of capacitance used in the circuit.

So now to control the output frequency and to make it work as a VCO we have to vary the capacitance of the Capacitor based on the value of the Input voltage. This can be achieved with the help of varactor diodes. These diodes change the value of capacitance across them based on the voltage applied. A sample output graph of a VCO is shown below. Let us assume the control voltage to be Vc and the output frequency as fo.

As the input voltage control voltage is increased the output frequency increases and the vice versa is also possible. There are many types of VCO circuits used in different applications, but they can be broadly classified into two types based on their output voltage.

vco design

Harmonic Oscillators: If the Output waveform of the oscillator is sinusoidal then it is called as harmonic Oscillators. These types of oscillators are harder to implement but they better stability than the Relaxation Oscillator. Harmonic oscillators are also called as linear voltage controlled oscillator. Relaxation Oscillator: If the output waveform of the oscillator is in sawtooth or triangular form then the oscillator is called as Relaxation Oscillator.

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