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Resonant Power Supply Decoupling Aid

IP.com Disclosure Number: IPCOM000045557D
Original Publication Date: 1983-Apr-01
Included in the Prior Art Database: 2005-Feb-07
Document File: 1 page(s) / 12K

Publishing Venue

IBM

Related People

Johnson, M: AUTHOR

Abstract

A disturbing narrow-band interference on a supply line in electronic circuitry may be decoupled through a low value capacitor having a self-resonant frequency which matches the frequency of the narrow-band interference. The self-resonant frequency of a capacitor may be adjusted conveniently through control of the capacitor lead length.

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Resonant Power Supply Decoupling Aid

A disturbing narrow-band interference on a supply line in electronic circuitry may be decoupled through a low value capacitor having a self-resonant frequency which matches the frequency of the narrow-band interference. The self-resonant frequency of a capacitor may be adjusted conveniently through control of the capacitor lead length.

It is common practice in the construction of electronic circuit boards to apply liberal amounts of supply line decoupling in the form of low value capacitors. These are ostensibly of low impedance at the frequencies of interest, and are shorted to ground. Such decoupling can be of paramount importance in certain systems, such as phase-locked loops, sensitive receivers, and high-speed analogue circuitry. I have shown in practice and through systematic measurements that the decoupling obtained by merely choosing a capacitor with the usual impedance design rules is uselessly small (of the order of 1 dB). This is because the self-resonant frequency of normal capacitors is very low, and the capacitor appears as an inductor above this frequency.

However, very close to the self-resonant frequency, supply-line decoupling is very high, though only in a narrow band. The frequency may be varied or tuned through variation of the capacitor lead lengths. For example, a 110 pF silvered-mica capacitor with 0 mm lead length has a resonant frequency of 137 MHz, at which the suppression is 11 dB. The same capacitor with a 30 mm lead length has a resonant frequency of 71 Mz, at which the suppression is 16 dB.

At 200 Mz the...