Wednesday, July 24, 2024

Gunn Oscillators:A Reintroduction

Parent Category: 2018 HFE

By Yonghui Shu

The semiconductor used in the Gunn oscillator is called the “Gunn diode.” The Gunn diode is a two-terminal “negative” resistance device. The Gunn diode is also referred to as a “Transferred Electron Device” which was invented by J. B. Gunn in 1963. Since their invention, Gunn oscillators have been playing a unique role in replacing the tube to generate low to medium level microwave power in the frequency range of 2 to 140 GHz. This is especially true in millimeter wave frequencies, long before pHEMT and more advanced microwave three-terminal devices reached their maturity and became practical.

Gunn oscillators are an old technology. Newer generation engineers may have limited opportunities to get to know this technology due to the birth of three-terminal based devices such as HBT, MOSFET, and pHEMT. The oscillators based on these devices are playing the dominate role as signal generators in the microwave industry for various system applications, especially in the microwave frequency domain, up to 40 GHz.

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The main advantages of the three-terminal based devices are high power-added-efficiency (PAE), easy-system-integration (ESI) and friendly-surface-mount volume manufacturing. However, they are cumbersome and expensive when upper millimeter wave signal generation is required for labs, system concept approving, and prototyping and production test sets. The three-terminal based signal generation at high millimeter frequencies involves expensive synthesizer or DRO/PLO, frequency multipliers, filters, and amplifiers.

As an alternative, the Gunn oscillator offers simple circuit solutions by utilizing a single Gunn diode and a waveguide cavity. Gunn oscillators are still playing an important role in providing another means to deliver high-performance solutions to microwave and millimeter wave system applications. In addition, Gunn oscillators display less harmonic contents, are smaller, lightweight, and has a substantially lower cost compared to three-terminal based signal generators.

SAGE Millimeter mainly offers two types of Gunn oscillators: mechanically tuned (SOM series) and Varactor tuned (SOV series). Both GaAs and InP compound material Gunn diodes are used in these product families. Despite InP Gunn diodes offering an inherently higher operating frequency, the device manufacturer discontinued the product line a decade ago. The main reason being that there was not enough business to maintain the production line. Based on that fact, SAGE Millimeter’s Gunn oscillators are mainly built with GaAs Gunn diodes. However, SAGE Millimeter also offers limited InP Gunn diode based oscillators by using the “last time buy” devices for high frequency applications from 70 to 140 GHz.

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It is well known that the fundamental operating frequency limit for GaAs diode based oscillators is around 65 GHz and InP is up to 140 GHz, respectively. The standard cavity configurations for the fundamental operations are front-iris-coupled and half-wavelength-backshort defined. The iris coupled oscillator exhibits less load pull effect and good frequency stability. However, the operation frequency is limited to 40 GHz or below due to the mechanical realization limits. The upper millimeter wave frequencies can be reached by combining this type of oscillator with a passive diode multiplier. The SOF series broadband oscillators belong in this category. The half-wavelength-backshort defined oscillators can reach 65 GHz, but this type of oscillator shows poor frequency stability and high load pull effect. Therefore, the isolator is highly recommended for reducing the load pull effects and the heater for improving the frequency stability for this type of oscillator.

For high millimeter wave frequency operations, the second harmonic Gunn oscillators are widely used. This type of oscillator offers incomparable features against its counterpart, the fundamental oscillator. Because the oscillation is established within the cavity, below the cut-off frequency of the waveguide, this type of oscillator offers inherent high isolation between the oscillator and load. Therefore, the isolator is not necessary while performing the system integration. The second harmonic Gunn oscillators are mainly offered in the frequency range of 60 GHz to 140 GHz.

About the Author

Yonghui Shu is the President of SAGE Millimeter.

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