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IMPEDANCE INVERSION TECHNIQUE PREVENTING OSCILLATOR SATURATION

IP.com Disclosure Number: IPCOM000005631D
Original Publication Date: 1987-Oct-01
Included in the Prior Art Database: 2001-Oct-22
Document File: 1 page(s) / 64K

Publishing Venue

Motorola

Related People

Lim Tiam Hock: AUTHOR

Abstract

When a conventional, common base Colpitts oscillator (Figure IA) is subjected to low supply voltage, the sustaining amplifier can be sent into saturation due to excessive feedback from the oscillator tank. Such satura- tion seriously loads down the resonanting tank and degrades the tank Q factor. Moreover, the input resistance of the amplifier(transistor) may be so low that it loads down the resonating tank. The oscillator sideband noise ratio is a function of the tank power and the tank Q factor. Degradation in either of these factors likewise degrades the sideband noise ratio. This invention provides a solution to these deficiencies by employing an impedance inversion technique. With such improvements, the oscillator is able to perform at a higher sideband noise ratio.

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Technical Developments Volume 7 October 1987

IMPEDANCE INVERSION TECHNIQUE PREVENTING OSCILLATOR SATURATION

by Lim Tiam Hock

   When a conventional, common base Colpitts oscillator (Figure IA) is subjected to low supply voltage, the sustaining amplifier can be sent into saturation due to excessive feedback from the oscillator tank. Such satura- tion seriously loads down the resonanting tank and degrades the tank Q factor. Moreover, the input resistance of the amplifier(transistor) may be so low that it loads down the resonating tank. The oscillator sideband noise ratio is a function of the tank power and the tank Q factor. Degradation in either of these factors likewise degrades the sideband noise ratio. This invention provides a solution to these deficiencies by employing an impedance inversion technique. With such improvements, the oscillator is able to perform at a higher sideband noise ratio.

   With a proper feedback ratio between Cl and C2, amplifier saturation can be avoided. However, the power developed across the tank is now lowered, causing the oscillator to exhibit a lower sideband noise ratio. Also, a closer look at the simplified equivalent of the oscillator (Figure 1B) shows that the input resistance of the amplifier seriously loads down the tank Q factor. This is due to the low input resistance of the common base transistor. To increase the apparent resistance looking through C2, while also increasing the tank power, a capacitor C3 is added in...