For many high-power RF applications, the "Q-factor" of an embedded capacitor is one of the most important characteristics in circuit design. This includes products such as cellular/telecommunication equipment, MRI coils, plasma generators, lasers and other medical, military and industrial electronics.

Usually expressed mathematically, the Q-factor represents the efficiency in terms of the rate of energy loss of a given capacitor. In theory, a "perfect" capacitor would not exhibit any losses and would release the full energy transfer, but in the real world capacitors always exhibit some finite amount of losses. Although there are many high-Q capacitors on the market, performance can vary greatly depending on design and build quality.

The higher this energy loss, the more heat is generated within the capacitor and must be dissipated or cooled. For low power applications, this heat is negligible. However, for higher power applications, this heating can be significant. If the temperature rises significantly, it can damage nearby components and, in extreme cases, parts on the circuit board.

Although many low-power applications do not need to consider the capacitor's Q-factor, energy losses can increase significantly at higher frequencies, causing other performance problems, even in low-power circuits. Reduced receiver sensitivity and link budget can sometimes be associated with higher loss capacitors.

For this reason, high-power RF applications often require high-Q capacitors, which are characterized by ultra-low equivalent series resistance (ESR). In addition to minimizing energy loss, high-Q capacitors reduce thermal noise caused by ESR to help maintain the desired signal-to-noise ratio.

Your performance may vary

Despite its critical role in RF electronics, not all high-Q capacitors are created equal. It turns out that the performance of high-Q capacitors is actually relative and varies widely based on design, manufacturing, quality control and even the type of performance testing. To further muddy the waters, manufacturers use a number of terms to refer to their high-Q capacitors, including "high-Q," "ultra-high-Q," "low loss," and "RF capacitors."

"In many ways, 'high-Q' is a relative term," says Scott Horton of Johanson Technology, a company that makes a variety of multilayer ceramic capacitors (MLCCs). "It seems like every [capacitor] manufacturer has a high-Q product, but the performance of the parts in the circuit can vary widely."

To differentiate the options, most MLCC capacitors publish ESR performance values ​​online. However, Horton said capacitor makers' performance claims should be viewed with skepticism.

ESR testing is performed in a laboratory environment and is typically derived by one of two methods: using a vector network analyzer (VNA) or resonant lines. However, the accuracy of these data is limited by the setup and calibration of these systems. When measuring capacitor Q on a network analyzer, configuration and calibration are critical to ensure meaningful data is collected. Not all measurements of VNAs are equally valid; in fact, poorly calibrated VNAs can produce very inaccurate results.

A more reliable way to test the Q of a capacitor is the proven "resonant line" system; the Boonton 34A resonant line has been the de facto standard in the industry for decades.

Companies such as Johanson Technology publish online ESR performance data from Boonton 34A resonant lines. Since this method depends on the frequency accuracy of the signal generator and a very stable resonant line, measurements can be made with extremely high accuracy that can be repeated over time.

"I can't comment on how some capacitor manufacturers end up coming up with the values ​​they publish, but when I put the capacitors on a mil compliant resonant line and test the parts side by side A/B comparison testing, we see that There are significant differences in the data. I would trust these relative results," Horton said.

Consistent manufacturing, number of layers

Another aspect that affects the ESR of high-Q capacitors is the quality and consistency of the manufacturing process.

By definition, MLCC capacitors consist of a specially formulated stack of ceramic dielectric materials interspersed with a metal electrode system. The layered structure is then fired at high temperature to produce a sintered and volume efficient capacitive device. A conductive terminal barrier system is integrated on the exposed end of the chip to complete the connection.

In an MLCC, the capacitance is mainly determined by three factors: the k of the ceramic material, the thickness of the dielectric layer, the overlap area, and the number of electrodes. Therefore, a capacitor with a given dielectric constant can have more layers and wider electrode spacing, or fewer layers and closer spacing to achieve the same capacitance.

Significantly changing the number of layers in an MLC capacitor can significantly alter the performance characteristics. Therefore, leading capacitor suppliers strictly control the number of layers per component. Unfortunately, this is not set in stone in the industry, with some suppliers offering products with the same part number but variable layers. In short, the same part number can have significantly different designs, which can lead to undesired impedance changes in the capacitor. These differences vary by vendor and can even be seen through a single source.

"If the MLCC manufacturer didn't strictly control the number of layers, they might deliver a 10-layer part in one batch and then deliver a 17-layer part in a subsequent batch," explains Horton. The two parts behave differently at high frequencies.

Another reason for performance variation occurs when OEMs source through dealers who buy from multiple factories. In this scenario, different factories have different designs with different high frequency performance. Therefore, these products are from different manufacturers, which can make significant differences in high frequency performance. This can also lead to very real situations of parts inconsistency, leading to changes in system performance.

The series resonant frequency (SRF) is a key performance metric affected by different layer counts; this variation can negatively impact the performance of any LC RF filter using these capacitors. Bandpass filters, for example, often use the resonant frequency of a capacitor to "shape" their performance.

In other words, when the number of layers is varied, the filter may not perform as designed and allow radiated emissions to exceed FCC or ETSI requirements in the finished product. Lot-to-lot variation in capacitor performance can lead to costly product recalls.

"If the series resonant frequency changes, your filter may no longer meet FCC emissions requirements," Horton said. “So, by tightly controlling the number of layers, manufacturers help ensure that LC filter performance is consistent from batch to batch, day to day, month to month, year to year.”

High loss capacitors can also affect aspects such as battery life. For systems using RF amplifiers, it is inefficient to absorb or dissipate power by capacitors. Engineers must then use amplifiers to compensate for the losses caused by the low-Q capacitors, which cause the handheld's battery to drain faster. High-Q capacitors can also improve receiver sensitivity by reducing losses between the antenna and transceiver.

Differences in capacitor design and construction

High-Q capacitors are designed differently than standard capacitors. To achieve the lowest loss, leading companies use the lowest loss dielectric, ink and electrode options.

For example, most low-cost commodity capacitors use nickel electrodes; however, nickel is a poor conductor known for its high losses at RF and microwave frequencies. Silver and copper electrodes offer superior performance over nickel and can be used in most high-Q applications. This type of electrode has the added advantage that it does not generate a magnetic field like nickel does. This factor is important for applications involving strong magnetic fields, such as MRI receiver coils.

For the highest power RF applications, many leading manufacturers offer pure palladium electrodes. However, at higher frequencies, silver is an excellent conductor compared to palladium. For this reason, Johanson Technology has added silver electrodes to its high power standard 1111, 2525 and 3838 size capacitors in its ultra-high Q (lowest ESR loss) product E-Series multilayer RF capacitors.

Vertically Oriented Capacitors

Even small details like the orientation of the capacitors in the tape and reel can have a direct impact on the performance of the circuit. Traditionally, high-Q capacitors have mostly been in horizontal electrode configurations when installed in tape and reel. Some manufacturers offer MLCC capacitors in horizontal and vertical electrode orientations.

However, mounting capacitors in a vertical configuration is also an industry "trick" for effectively extending the frequency range available to capacitors.

In addition to the SRF (based on a given physical size/structure and a given capacitance value), capacitors also exhibit a parallel resonant frequency (PRF). As a rule of thumb, the PRF is approximately twice the SRF. At the PRF, the transmission impedance is relatively high, and capacitor losses are very high around this frequency.

Odd-numbered PRFs (ie, 1st, 3rd, 5th, etc.) can be eliminated by mounting the capacitors in a vertical position. This arrangement significantly increases the frequency of the first PRF, allowing capacitors to be used at significantly higher frequencies.

High Q Relativity

If there's one lesson to be learned from the discussion of high-Q capacitors, it's that choosing the ideal MLC capacitor requires more than just voltage, capacitance value, and tolerance. This can also explain why one supplier's capacitance value may not correspond directly to another supplier in a critical matching circuit. The design and quality/consistency of manufacturing is just as important as the type of testing that validates performance.

"Don't assume that because a capacitor is labeled 'high-Q' it will provide the desired performance," Horton concluded. “These capacitors play a key role in RF transmission and reception in military, medical and industrial electronics, so they must perform as expected and be optimized to minimize energy loss and lot-to-lot variability. Otherwise, these electronic Equipment may not function as intended in the field."

Reviewing Editor: Guo Ting

Leave a Reply

Your email address will not be published.