For many high-power RF applications, the “Q factor” of embedded capacitors is one of the most important characteristics in the design of circuits for products from MRI coils to industrial electronics.
Often expressed as a mathematical formula, the Q factor represents the efficiency of a given capacitor in terms of its rate of energy loss.
In theory, a “perfect” capacitor would exhibit no loss and discharge a full energy transfer, however, in the real world they will always exhibit some amount of loss.
The higher this energy loss, the more heat is generated within the capacitor and this needs to be dissipated or cooled.
For low power applications this heat is insignificant. For higher power applications, however, capacitors running hot can damage nearby components and, in extreme cases, de-solder parts from the circuit board.
Energy losses can increase significantly at higher frequencies leading to other performance issues even in low power circuits.Reduced receiver sensitivity and link budget can sometimes be correlated to higher loss capacitors.
For this reason, high power RF applications typically require High-Q capacitors, which are characterised as having ultra-low equivalent series resistance (ESR).In addition to minimising energy loss, High-Q capacitors reduce thermal noise caused by ESR to assist in maintaining the desired signal-to-noise ratios.
| Above: Johanson High-Q capacitors |
Not all High-Q capacitors are created equal.Far from an absolute, it turns out that High-Q capacitors can be quite relative, varying in performance based on their design, manufacturing, quality control and even on the type of performance testing employed.
Further muddying the water, manufacturers use numerous terms to reference their High-Q capacitors, including: “High-Q”, “Ultra-High-Q”, “Low Loss”, and “RF Capacitors”.
“In many ways, ‘High-Q’ is a relative term,” suggests Scott Horton of Johanson Technology, a manufacturer of multi-layer ceramic High Q capacitors.
“It may seem like every [capacitor] manufacturer has a High-Q product, but the performance of the parts in the circuit can be quite different.”
Most MLCC capacitors publish ESR performance values online.However, the performance claims should be viewed with some caution, says Horton.
Conducted in laboratory settings, ESR tests are most often derived by one of two methods: utilising vector network analysers (VNAs) or resonant lines.
| High-Q E series capacitors from Johanson |
The accuracy of this data is limited by set-up and calibration of these systems.
When measuring capacitor Q on a network analyser, the configuration and calibration are critical to ensure meaningful data is collected.Not all measurements on VNAs are equally valid and in fact poorly calibrated VNAs can yield wildly inaccurate results.
A more reliable method of testing the Q of capacitors is well established Resonant Lines systems. The Boonton 34A resonant line has been the de facto standard in the industry for decades.
Many companies look to publish ESR performance data from a Boonton 34A resonant line online.Since this method depends on the frequency accuracy of a signal generator and a very stable resonant line, measurements can be made with extreme precision that is repeatable over time.
“I would tend to believe those relative results,” says Horton.
Consistent manufacturing
Another area that can affect the ESR of a High-Q capacitor is the manufacturing process.
By definition, MLCC capacitors consist of laminated layers of specially formulated, ceramic dielectric materials interspersed with a metal electrode system. The layered formation is then fired at high temperature to produce a sintered and volumetrically efficient capacitance device. A conductive termination barrier system is integrated on the exposed ends of the chip to complete the connection.
In Multi-Layer Ceramic Capacitors (MLCC’s), capacitance is primarily determined by three factors: the k of the ceramic material, the thickness of the dielectric layers, and the overlap area and the number of the electrodes.So, a capacitor with a given dielectric constant can have more layers and wider spacing between electrodes, or fewer layers and closer spacing to achieve the same capacitance.
Significantly changing the layer counts in MLC capacitors can change performance characteristics significantly.As such, the leading capacitor suppliers tightly control the layer counts of each part made.Unfortunately, this is not a “given” in the industry with some suppliers delivering products with the same part number, but a variable number of layers.
In short, the same part number can have significantly different designs which lead to undesirable impedance changes in the capacitor.
“If a manufacturer is not tightly controlling the layer count, they might be providing 10 layer parts in one batch and then later deliver 17 layer parts in a subsequent batch,” explains Horton.These two parts will not perform the same at high frequencies.
Another cause of performance variation occurs when OEMs purchase through resellers who buy from multiple factories.In this scenario, the different factories have different designs which have different high frequency performance.Thus, the items are sourced from different manufacturers which can produce significant variation in high frequency performance – leading to a scenario where parts are not consistent which result in system performance variation.
The Series Resonant Frequency or SRF is a key performance metric affected by varying layer counts. This variation can negatively affect the performance of any LC RF filters where those capacitors are used.Bandpass filters, for example, often use the resonant frequencies of the capacitor to “shape” its performance.
This means that when layer counts vary, filters may not perform as designed and allow radiated emissions to exceed the FCC or ETSI requirement in the finished product.Lot to lot changes in capacitor performance can lead to costly product recalls.
High loss capacitors can impact aspects such as battery life, as well.For systems in which RF amplifiers are utilised, it is inefficient to have power absorbed or dissipated by a capacitor.Engineers must then use amplifiers to make up for losses caused by low Q capacitors which results in faster battery drain in handheld devices.
High-Q capacitors can also improve receiver sensitivity by reducing losses between the antenna and the transceiver.
Variance in design, construction
High-Q capacitors vary from standard capacitors in design.To achieve the lowest losses, companies look to use the lowest loss dielectrics, inks and electrode options.
Low-cost commodity capacitors use 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.
For the highest power RF applications, a number of manufacturers offer pure palladium electrodes.However, silver is a superior conductor when compared to palladium at higher frequencies.
For this reason, Johanson Technology 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, some leading manufacturers are offering 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 1st, 3rd, 5th, etc.).This pushes the first PRF significantly higher in frequency which allows the capacitor to be used at significantly higher frequencies.
When it comes to High-Q capacitors, selecting the ideal MLC capacitor requires more than a voltage, capacitance value and tolerance as has been demonstrated in this article.
“Don’t assume that because the capacitor is labelled ‘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, optimised to minimise energy loss and variation from one batch to another.
“If not, these electronics may not perform as expected in the field.”
