Don't Bypass the Right Capacitors for Your Design

Don’t Bypass the Right Capacitors for Your Design

Nov. 6, 2018
Bypass capacitors are the focus of this technical paper, which presents actual measurement results of different capacitor bypass networks.

Any engineer knows that capacitors are a fundamental building block in RF/microwave systems. Bypass networks are one example—they require capacitors to bypass ac signals to ground. For such cases, rules-of-thumb or approximate equations can be used to select the best capacitor for the job. However, those approaches may not always be valid. In the technical paper, “Choosing blocking capacitors - it’s more than just values,” Knowles Precision Devices discusses real-world capacitor performance and then presents measurement results of various capacitor bypass networks.

For bypass applications, the paper explains that capacitor values are chosen to provide a low-resistance ground path for unwanted noise signals. It then presents the familiar mathematical expression of capacitor reactance. This equation allows one to determine the theoretical capacitor values needed to provide a low-resistance path to ground for a signal at a given frequency.

In practice, actual capacitors are modeled as a combination of capacitors, inductors, and resistors. A low impedance is realized at a capacitor’s first self-resonant frequency. Above the resonant frequency, the impedance rises as the frequency increases. The paper illustrates this by presenting impedance plots of several capacitors with different values. Furthermore, a common approach to enable broadband RF isolation involves shunting three or four capacitors, each with different values, to ground. Oftentimes, designers employ this method by simply following the recommendations from a manufacturer’s datasheet.

The paper continues by breaking down the parasitic inductance that’s present in a capacitor. It’s noted that increasing a capacitor’s contact pad size can reduce the parasitic inductance. Mention is made of a new manufacturing process that allows for larger pad areas in capacitor footprints without compromising the voltage rating.

Real measurements are shown of the V-Series capacitors, demonstrating how one of them can offer broadband performance that’s typically achieved by bypass networks with multiple capacitors. Additional measurement results compare the performance of a traditional capacitor bypass network with a V-Series capacitor by itself, as well as with a UX-Series capacitor in combination with a V-Series capacitor.

Knowles Precision Devices, 277 Hwy 20, Cazenovia, NY 13035; (315) 655-8710.

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