A second example of an application for multiple decoupling branches is provided by the need to suppress several frequencies simultaneously. In this case, several decoupling branches, each one tuned to a different target frequency, might be used. Though the following analysis is for dual decoupling branches, the extension to more than two branches can be made relatively easily.
Consider a dual branch decoupler in which the each of the branches is tuned to different frequencies, f 1 and f 2 . The equation is:
where
and
for f 2 > f 1 .
For the real-life application of these techniques, consider the reduction of radiated emissions from the I/O harness of a pre-production electronic module. Shielding the harness was not an option. Furthermore, since the design was all but "frozen," extensive rework of the pc board was out of the question. Appreciable reduction of radiated emissions, with minimal intervention, was required.
Regulator Noise
The offending frequencies were the 750-kHz fundamental and 1.5-MHz second harmonic, produced by a commercially available switching regulator. The power-input pin of the regulator was connected to the filtered output of a pre-regulator by 1.5 in. of a 100-mil wide, 2-mil thick isolated trace. Using Rostek's formula 4 , the inductance of this trace calculates to be 29.4 nH. The regulator was returned directly to a very low-impedance ground plane.
In an initial attempt at decoupling, the board already had a high-quality dielectric, 1-µF ceramic surface-mount capacitor "downstream" of the regulator at a site designated C236. The capacitor at this site was connected to the power-input pin of the regulator by a two-section, series-connected trace. One section of the trace measured 0.94 in. long, 100 mils wide, and 2 mils thick. The other section measured 0.150 in. by 100 mils by 2 mils. The capacitor was returned to the ground plane by a 0.32-in. by 50-mil by 2-mil trace. All of these traces were located at a height of 20 mils over the ground plane. The inductance of the complete trace calculates to be 17.6 nH.
A 1-µF capacitor of this type has a minimum impedance, which is a zero, at about 5 MHz. This yields an intrinsic inductance (equation 6) of about 1 nH. Thus, the zero of the decoupling loop, for a 1-µF capacitor at C236, calculates to 1.17 MHz, with a pole (equation 7) at 728 kHz.
Preliminary benchtop tests were conducted using an H-field "wand" and a spectrum analyzer. The tests measured the relative strengths of the frequencies radiating from the harness. Baseline measurements were made with no decoupling. For the second case, measurements were made with the 1-µF surface-mount capacitor at the C236 site. The decoupling significantly reduced the second harmonic, as expected, but had little effect on the fundamental. This latter result is due in part to the pole at 728 kHz being located so close to the fundamental.
The surface- mount capacitor was replaced by a 1- µF radial capacitor for the third case. This was tuned to have a zero at 700 kHz, obtained by providing about 52 nH of lead inductance. With this capacitor, the decoupling loop has a zero at 600 kHz and a pole at 500 kHz, yielding significant reduction at the fundamental frequency, and some reduction at the second harmonic.
Capacitor Returned
For the fourth case , the surface mount capacitor was returned to the C236 site, and the radial capacitor was connected directly on the power input and ground pins of the regulator. This resulted in zeros at 698 kHz and 1.17 MHz, unity at 847 kHz (equation 11), and poles at 531 kHz and 916 kHz. The reduction at the fundamental is not as great as in the third case, probably because of the proximity of the unity to the higher-frequency pole. However, the reduction at the second harmonic is as substantial as that obtained in the third case, in spite of the 916-kHz pole.
Subsequent radiated emissions measurements—in an anechoic chamber—supported the "wand" results. A variation of the fourth case was used for the production module.
The use of tuned decouplers can be very helpful, particularly in those situations in which the main contributors to interference are sharply defined, well-separated frequencies. As shown, however, the technique introduces poles as well as zeros, and should be applied with these considerations in mind. For maximum effectiveness, the poles should be located in the frequency-range regions in which there is no interference structure. They should also be located at a sufficient distance from the zeros, so that the presence of the poles does not degrade the decoupling effect.
References:
- Shi, H., Yuan, F., Drewniak, J. L., Hubing, T.H. and VanDoren, T.P., "Simulation and Measurement for Decoupling on Multilayer PCB DC Power Buses," IEEE Electromagnetic Compatibility Symposium Digest , Santa Clara, CA, 1996. pp. 430-435.
- Hubing, T., Van Doren, T., Sha, F., Drewniak, J., and Wilhelm, M., "An Experimental Investigation of 4-layer Printed Circuit Board Decoupling," IEEE Electromagnetic Compatibility Symposium Digest , Atlanta, GA, 1995, pp. 308-312.
- Daijavad, S. and Heeb, H., "On the Effectiveness of Decoupling Capacitors in Reducing EM Radiation from PCBs," IEEE Electromagnetic Symposium Digest , Dallas, TX, 1993, pp. 330-333.
- Rostek, P. M., "Avoid Wiring-Inductance Problems," Electronic Design , Dec. 6, 1974, pp. 62-65.
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