Hurtling toward the airport in late-afternoon traffic, huddled in the back seat of a taxi, Ernie's fingers flew across the keys of his laptop as he scrambled to put the finishing touches on his latest digital-system design. He finalized the netlist and typed "send" just as the taxi turned off the freeway and into the airport. Perfect timing! His job was done. Thirty minutes later he stepped onto the Jetway a free man, bound for Aspen, CO.
Ernie had meticulously researched every aspect of his design, especially the power system. Every bypass capacitor was the proper size, and every dielectric layer was the proper thickness. Ernie relaxed in the first-class cabin, sipping champagne—a model of supreme confidence.
Or maybe not. Maybe Ernie didn't have time to research the power system. Maybe he just copied the same old approach he had used last year. Maybe the design took longer than he had planned, and, in the crunch for time at the end, he had to give up his vacation. Does this situation sound familiar?
However you design your board, take the time to check the health of its power system when the board comes back from fabrication. Use an oscilloscope to directly measure the noise present on VCC (with respect to ground). Because the natural impedance between power and ground is low (less than 1 Ω), you need not use a fancy high-impedance probe to measure power-supply noise. You can use a plain coaxial cable directly soldered to the VCC and ground nodes of the pc board. Connect the other end of the coax to your scope and engage the scope's internal 50-Ω terminator.
To connect the coaxial cable to your board, remove one bypass capacitor and solder the coax signal and ground connections diectly to the capacitor's VCC and ground mounting pads (Figure 1). This arrangement keeps the exposed coaxial signal conductor short, so it doesn't pick up much extraneous noise. Make the measurement at several locations. At high frequencies, the power system is a distributed circuit, so noise may vary across the board.
Your noise will be a small ac signal superimposed on a relatively large dc bias equal to your power-supply voltage. If you can't adjust the vertical position on your scope far enough to clearly see the noise, you may improve your resolution by using your scope's ac-coupling mode. The mode has a hitch, however: Although many scopes permit ac coupling, and many scopes provide an internal, built-in 50-Ω termination, you often cannot simultaneously engage these features. If this situation exists with your scope, you may want to provide an external ac-coupling circuit. This circuit consists of a dc-blocking capacitor in series with the coaxial connection to the board.
You can use a bypass capacitor already present on the board to perform the dc-blocking function. Lift off one of the capacitors, lay down some insulating tape on its ground pad, and resolder the VCC end of the capacitor. The previously grounded end of the capacitor now sits on the insulator and is not connected to anything. Connect your coaxial signal connection to the previously grounded end of the capacitor and connect your coaxial ground connection to any nearby ground via. A 0.1-µF capacitance feeding into the 50-Ω load of your oscilloscope produces a highpass filter with a time constant of 5 µsec, which comfortably passes all frequencies greater than 100 kHz.
Alternatively, you may build an external dc-blocking black-box attachment for your scope. The black box should contain one series-connected, 0.1-µF capacitor. It should have SMA coaxial- cable connectors attached to both ends of the capacitor with 50Ω traces. A well-constructed black box should easily produce good performance through about 1 GHz.
Checking your power system's health always returns useful information. If you see too much noise, you know you have some serious work cut out for you. If you see very little noise, you may have the opportunity to save some money, space, and weight on your pc board by stripping out some of the bypass capacitors or reducing their sizes. Either way, measuring the noise on the power system gives you useful information that will help improve your design.