The ‘Relativity’ of High-Q Capacitors

Estimated reading time: 8 minutes

High-Q capacitors vary from standard capacitors in design. To achieve the lowest losses, companies such as Johanson utilize the lowest-loss dielectrics, inks and electrode options.

For example, most low-cost commodity capacitors utilize nickel electrodes. However, nickel is a poor conductor known for high loss at RF and microwave frequencies.  

Silver and copper electrodes are superior and perform better than nickel and are used for most High Q applications. This type of electrode has the added advantage that it does not create a magnetic field like nickel.  This is important factor for applications such as MRI receiver coils where strong magnetic fields are involved. 

For the highest power RF applications, a number of leading manufacturers offer pure palladium electrodes. However, silver is a superior conductor when compared to palladium at higher frequencies. For this reason, Johanson incorporates silver electrodes in its ultra-high-Q (lowest ESR loss) offering, the E-Series multilayer RF capacitors in their high power standard 1111, 2525 and 3838 size capacitors.

Capacitors in Vertical Orientation

Even minor details like the orientation of the capacitor in the tape reels can have a direct impact on the performance of a circuit.

Traditionally, high-Q capacitors are available primarily in a horizontal electrode configuration when mounted in tape and reels.  Now, manufacturers such as Johanson offer the MLCC capacitors in both horizontal and vertical electrode orientation configurations. 

However, mounting capacitors in a vertical configuration is also an industry “trick” that effectively extends the usable frequency range of capacitors. 

In addition to the SRF (which is based on the given physical size/construction and a given capacitance value), capacitors also exhibit parallel resonant frequencies (PRF). As a rule of thumb, PRF is approximately double the SRF.

At the PRF, the transmission impedance goes relatively high, and the capacitor is very high loss around this frequency.

By mounting the capacitor in a vertical position instead, the odd PRFs are eliminated (e.g., the 1^st, 3^rd, 5^th, etc.). This pushes the first PRF significantly higher in frequency which allows the capacitor to be used at significantly higher frequencies. 

High-Q Relativity

If there is a lesson from this discussion of High Q capacitors, it is that selecting the ideal MLC capacitor requires more than a voltage, capacitance value and tolerance. This may also explain why a capacitance value from one supplier may not be a one to one correspondence with another supplier in critical matching circuits. The design and quality/consistency of manufacturing plays just as big a role, as does the type of testing to verify performance.

“Don’t assume that because the capacitor is labeled ‘high-Q’ it is going to deliver the required performance,” concludes Horton. “These capacitors play a critical role in RF transmission and reception of military, medical and industrial electronics, so they must perform as expected, optimized to minimize energy loss and variation from one batch to another. If not, these electronics may not perform as expected in the field.”

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