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When Bandwidth is Stretched Thin, Voltage-Controlled Oscillators are a Key Solut...

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When Bandwidth is Stretched Thin, Voltage-Controlled Oscillators are a Key Solution

December 09, 2020 by Antonio Anzaldua Jr.

With bandwidth demand at an all-time high, designers may find themselves using voltage-controlled oscillators more frequently in wireless communication applications.

The demand for high bandwidth is at an all-time high with 5G on the horizon and more people working from home than ever. The modern IC has, luckily, afforded complex digital modulation schemes to make the most of these high-bandwidth communication channels.

Still, one component that is also helping designers address the issues associated with bandwidth demand is voltage-controlled oscillators, which appear increasingly in many analog and RF signal processing systems.  

The Fundamentals of Voltage-Controlled Oscillators

Design engineers can utilize voltage-controlled oscillators (VCOs) to improve RF and wireless communications applications without worrying about excess noise, high power consumption, and system footprint. 

The fundamentals of an oscillator can be studied by looking at the most commonly-used oscillator in the electrical world, the LC circuit. The basic functionality of this oscillator is to alternate energy back and forth.

An LC oscillator produces non-ideal signals
An LC oscillator produces non-ideal signals due to the decay of the capacitors' and inductors’ strength over time. Image used courtesy of Worcester Polytechnic Institute

Energy flows through a capacitor where it is stored in the form of an electric field between the plates before discharging to an inductor. The inductor stores energy as a magnetic field, which is then able to deliver charge back to the initial capacitor plate, restarting the process—this time, with the current flowing in the opposite direction.

One drawback of this type of oscillator is its limited design flexibility: there is no way for designers to control the output signal without adding accompanying circuits. 

Since oscillators are known to be autonomous non-linear circuits, it makes sense that creating a controllable variation would become an instant game-changer. The need for controllable solutions led to the creation of voltage-controlled oscillators (VCOs). VCOs were designed to output periodic signals that could be controlled via an input voltage. 

Types of VCOs

Oscillators fall under two groups: tuned and untuned. Tuned oscillators use frequency-selective circuits within the path of feedback and produce sinusoidal signals. Untuned oscillators, on the other hand, avoid loop-assisted circuits and produce nonlinear signals (triangular, sawtooth, square).

The LC, RC, and SC circuits are oscillators that fall under the tuned category, also known as harmonic oscillators in classical mechanics. 

Untuned oscillators, such as the relaxation oscillator, exhibit repetitive behaviors that are sensitive to noise in instances of high switching and charging currents. Tuned oscillators can be difficult to integrate into a system due to their large size and the lack of high-quality, passive inductors.

It's also difficult to manipulate frequencies in both harmonic- and relaxation-based oscillators to alter the output. This is where a VCO is a useful solution for designers.

VCOs Utilized in Wireless Communications

Integrating VCOs for wireless communications on transceivers will require external resonators in order to boost wireless channels. 

VCOs work well in phase-locked loops (PLL), which is a control system that generates a signal with a fixed reference signal. Many designers reach for ring oscillators to use in PLLS for digital clock generation and data recovery because of their compact size. Ring oscillators, however, are not stable and will dissipate energy fast. These oscillators will require monitoring at the input stage, which means added components and costs. 

Diagram of a PLL, including a VCO
Diagram of a PLL, including a VCO. Image used courtesy of Peter Kinget, Bell Labs

Keep in mind that because VCOs require high power consumption, a large tuning range, and external discrete components, they will lead to a larger design footprint.

Analog Devices Introduces Quadband VCOs

Illustrating some of the core tenets of VCOs, Analog Devices (ADI) has recently launched a series of quadband VCOs said to offer wideband capabilities, noise mitigation, and low power consumption for RF and microwave applications.

Functional diagram of the HMC8074
Functional diagram of the HMC8074. Image used courtesy of ADI

ADI’s four new narrowband VCOs include the HMC8364, 8362, and 8074. These devices have a leg up on high-density instances because they are composed of gallium arsenide. Each solution is equipped with resonators, negative resistance devices, and voltage-controlled varactor diodes. 

It seems ADI sought to address the key challenges of current oscillators with the new series of VCOs. For instance, in regard to size, the footprint is 6 mm x 6 mm and requires no external matching components. ADI also says the devices consume little power and deny unwanted noise from entering the system with built-in resonators.


What is your experience working with voltage-controlled oscillators? What are the challenges and merits of this device? Share your thoughts in the comments below. 


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