Introduction and Specifications of Ceramic Filters

Not a Riddle Wrapped Up in an Enigma Any More!

Electronic surveys discovered that ceramic filters are still receiving a significant amount of interest because they already have become an indispensable component in various electric equipments such as military & space applications and are widely used in the whole sector of commercial equipment.

In the following we will give a short introduction regarding ceramic filters in general and answer some of the most commonly asked questions about these components. Ceramic filters can be either of coaxial form or dielectrically loaded waveguide. Ceramic filters as discussed here are transverse electromagnetic (TEM) coaxial structures using quarter-wave resonators and either capacitive or magnetic coupling. The quarter-wave resonator is formed from a block (usually square) of ceramic material with a hole in the middle parallel to its length. The outside, one end, and the hole are coated with a conductive material (usually silver) to form a shorted quarter wavelength of transmission line. The length vs. frequency of the line is determined by the dielectric constant of the ceramic.

Essentially, ceramic filters are a form of distributed filters. They are much more rugged than LC filters, which are susceptible to vibration and microphonics. Ceramic filters are much less expensive to produce than either LC or cavity-type filters, due to the reduce labor, material, and machining costs. In addition, most modern ceramic materials are extremely temperature stable with temperature coefficients of < 5 ppm. Typically, ceramic filters are much less temperature sensitive than either LC or cavity filters, even after compensation.

Where are ceramic filters most effective?

Coaxial ceramic filters fit within the frequency range of about 500 MHz to 6 GHz. This broad range is accomplished by using ceramic material of various dielectric constants to optimize the size of the resonators. With magnetic coupled filters, bandwidth is limited to about 8 %, but magnetic coupling allows broad and deep stop bands, with 80 dB or more to 6 GHz not uncommon. Capacitively coupled filters can achieve bandwidths of 20 % or more, but the stop band attenuation is limited to about 30 dB or less due to parasitic coupling at the front of the resonator. This problem becomes more pronounced with smaller resonators and higher frequencies.

What determines a filter’s insertion loss?

A filter’s insertion loss is determined by the unloaded Q of the resonator, the bandwidth of the filter, and the number of resonators. Bandwidth and number of resonators are usually determined at the system level and are given for a specific filter. This leaves unloaded Q as the primary loss determination between one filter type or another. On ceramic filters, Q is comprised of a dielectric component (Qd) and a conductive component (Qc). The resultant total Q (Qt) is calculated as 1/Qt = 1/Qd + 1/Qc. Qd is the loss of the ceramic material itself and is typically in the tens of thousands. However, there are some materials that are lossy but provide a required dielectric constant to achieve a certain frequency range. Qc is solely determined by the coating on the outside of the ceramic. This is typically in the hundreds; therefore the process of coating the ceramic will be the primary determining factor of filter loss. At ComNav, we have developed a proprietary coating process that is done in-house.

What determines a filter’s rejection?

The rejection or attenuation a filter can achieve is determined by the number of resonators, how the resonators are connected, and the quality of the ground of the circuit in which the filter is used. As mentioned previously, magnetic coupled filters can achieve extremely high levels of attenuation if grounded properly. To assist in this grounding, the manufacturer ComNav has developed a thru-hole mount package. By grounding along the length of the filter and connecting to the filter on the opposite side of the board, additional shielding and reduced leakage is achieved. This is accomplished in an open lead frame package at frequencies in the 3 GHz and higher range. In addition to grounding, ComNav, all its products available at MSC Vertriebs GmbH, has also developed proprietary pole/zero and cross coupled circuit typologies that optimize the stop band by placing attenuation where it is needed. This reduces the number of resonators required and further reduces the insertion loss.

Can I get 60 dB or greater rejection from a ceramic filter?

Yes. Magnetic coupled filters are not susceptible to the parasitic capacitance between the resonators that affect the performance of capacitively coupled filters. Even in leadless SMT (surface-mount technology) packages with proper grounding, you can consistently achieve 70 to 80 dB rejection specs.

How do ceramic filters act under extreme environmental conditions?

Ceramic filters are basically painted rocks. The size and shape of the rocks determine the filter’s performance. Due to this rugged construction, the filter is not sensitive to shock, vibration, temperature, or humidity. The only environmental effect that can distort a filter is condensing humidity on some types of designs. This is a temporary effect – after the filter dries out, it will come right back into spec with no long-term damage. Once a filter is tuned to spec, short of hitting it with a sledgehammer, it is virtually indestructible.

Are ceramic filters temperature-sensitive?

The temperature performance of ceramic filters is determined mostly by the ceramic material itself, and to a lesser extent the coupling, loading, and other external components. Over the years, we have worked with our ceramic suppliers and have developed materials with the lowest possible temperature coefficient that is a compromise across our various construction techniques. The worst material we use has a temperature coefficient of 5 ppm. At 5 GHz, this translates into a temperature drift of 1.5 MHz across our standard temperature range of -40° C to +85° C. Most of the materials we use have temperature coefficients of 1 ppm or less. This reduced temperature drift means that we can squeeze additional performance out of the filter. Since we do not have to compensate for significant temperature drift, we can design closer to the spec.

What kinds of filters can be made from ceramics?

Most ceramic filters are narrowband band pass filters. But band stop filters also lend themselves nicely to ceramic construction. Since ceramic resonators are basically narrowband devices, they do not lend themselves well to broadband low pass or high pass filters. These devices are much better suited to LC or printed structures.

I’ve noticed the silver peeling off of filters I’ve used in the past. What causes this? Is it some thing I am doing wrong?

The problem you are asking about is silver adhesion. Adhesion is a big problem with ceramic filters. The coating process is a very delicate operation. The slightest process variation, and the first thing to go is adhesion. This also affects the unloaded Q of the resonator. Most ceramic resonators use a thick-film silver sintering process to coat the resonators. We were lucky in that we had three years of pure R&D time to develop and refine our process and discover what can go wrong. Whether we use outside vendors or our own in-house process, we know immediately when something goes wrong and we fix it. As a result, over the last couple of years we have refined our process and assisted our outside vendors to improve their process to the point where we can no longer measure adhesion (either the ceramic or the pull tester breaks).

To answer your second question, no you are not doing anything wrong, and most likely your filter supplier didn’t either, as the problem started before they even received the resonators. A properly coated resonator is impervious to soldering temperatures, fluxes, and normal bench handling. However, an improperly coated resonator can peel, have silver leach off during soldering, or degrade over time if humidity can get under the silver and pull the silver away. At ComNav, we qualify every batch of resonators before they go into production with sampled Q and pull tests.

What are typical failure modes of ceramic filters?

Over the years, we have come upon three distinct failure modes that can be induced by customers – one temporary and two fatal. I will not go into bad coating problems, since I consider that to be our problem, not yours. The temporary mode was mentioned previously, which is condensing moisture. Some construction techniques use a pin and bushing assembly to perform the impedance matching in and out of the filter. Electrically this makes up a loading capacitor. When moisture in the form of droplets gets in the loading capacitor, it will change the capacitance and distort the filter’s tuning. However, once it dries out it will be back to its original tuning. One fatal mode is silver bubbling, which occurs when the filter mounted on a PC board is cleaned with an ultrasonic cleaner. The silver coating is bonded to the ceramic with a glass frit. The molecular vibration of the ultrasonic cleaner more than 5 minutes is strong enough to break the glass frit under the silver. Eventually the silver will bubble, a small hole will appear, and water vapor gets in. Over time, this water vapor expands and contracts, pushing the silver away from the ceramic. The worst thing about this problem is that it is a time bomb. It can take years to develop and, when it does, can destroy the filter and vastly degrade the performance of the system with a problem almost impossible to troubleshoot. Our recommendation: never, under any circumstances, use ultrasonic cleaners on boards with ceramic filters or any thick film component, including resistors. The third failure mode is obvious but I’ll mention it anyway, that of chipping. Ceramic, along with its thick-film coating, are very brittle. If dropped on a hard surface, it can chip just like a ceramic ashtray or figurine. This will distort the tuning of the filter.

I am currently using a cavity filter, which is big and expensive. Can it be replaced by a ceramic filter?

In some cases, yes. Ceramic filters can achieve unloaded Qs of 1000 or more in 12 mm profile resonators. Insertion loss and stop band performance are relatively close between cavity and ceramic typologies. There have been numerous instances where our customers have replaced machined cavity filters with ceramics. In some cases, not only are there size and price advantages, but also a performance advantage. By using cross coupling and pole/zero circuits, we have been able to achieve similar or even better performance with fewer resonators. There will always be a trade-off, because nothing is for free. Usually it boils down to the question of whether that 0.5 dB less of insertion loss is really worth an extra $200 to $400 per filter, which is usually the price difference between a cavity and ceramic filter.

Can diplexers/duplexers be made with ceramic filters?

Ceramic filters lend themselves very well to diplexing. The high impedance inputs can be prototyped to provide both contiguous and non-contiguous crossovers with a full match on the opposite, or output, end of the filters. Band pass / band pass diplexers are most common, but band pass/notch diplexers also work well. Notch/notch diplexers, classically called duplexers, are a little more difficult and in some frequency ranges are not practical.

But we have built them for several of the standard communications bands. How much power can a ceramic filter handle?

Since ceramic filters for the most part are relatively low impedance, arcing from high voltages is generally not a problem. The main limiting factor will be how hot the part can get before damage occurs. With this in mind, the filter’s insertion loss will be a prime determinant of power handling and will determine the temperature rise caused by the dissipated power. There can, in fact, be instances were a 4 mm leadless SMT filter can handle more power than a thru-hole-mount 12 mm filter. It will all depend on loss, bandwidth, heat sinking, and duty cycle and will be different for each device. The general rule of thumb I use is that a 4 mm filter can handle 2 watts, a 6 mm filter can handle 5 Watts, and a 12 mm filter can handle 20 Watts. But each type will probably handle more depending on the circumstances. Generally a thru-hole package grounded to a good heat sink can double the power-handling ability given in rule of thumbs.

How high of a reflow temperature can ceramic filters withstand?

The main limiting factor in reflow is not the ceramic, but the carrier board and soldering of internal components. Our internal construction uses SN-95 or SN-10 solder depending on the filter type. The carrier or insulating board will initially discolor, then delaminate, and then boil, when exposed to excessive temperature. As limits, we recommend not exceeding the following time/temps: 215° C/60 seconds, 230° C / 30 seconds, 245° C / 15 seconds, 260° C / 10 seconds (see Figure 1).

Figure 1: Reflow process

Why do I not get the same response as the test data that came with the filters?

This is a common problem that occurs with pretty much all SMT filters – ours and everyone else's. We had to standardize our I/O (input/output) pads at some dimension in order to somewhat standardize our product and keep our costs down. Our standard pad width is 0.058 Inches, which is 50 Ohms for 30 mil thick FR4. The problem is that just about every one of our customers uses a different board material with different 50 Ohms line dimensions. If there is an overhang of the filter pad relative to your trace, this will result in a parasitic shunt capacitor at the input and output of the filter. This parasitic shunt cap will cause a mismatch and distort the filter’s performance. Our recommendation in this situation is to attach a 5 to 12 nh coil in a shunt configuration off of the input and output of the filter. By adding a shunt coil, it is possible to resonate this cap and effectively remove its effect. We suggest getting an engineering kit of CoilCraft microsprings (http://www.coilcraft.com/smspring.cfm) and try different values until the best value is found. This technique usually solves the problem and our customers have done this and have had great success.

Glossary of Terms

To alleviate the inevitable confusion that occurs when engineers from different backgrounds and countries communicate, this glossary of terms is provided to define what we mean when discussing certain specifications. Amplitude Flatness: Defined in dB, this term means the total amplitude variation across a given frequency passband. This value includes roll-off associated with finite unloaded Q.

• Amplitude Ripple: Defined in dB, this value refers only to the signal variation caused by the VSWR and does not include the roll-off associated with finite unloaded Q.
• Absolute Attenuation: Defined as dBa, this term describes the absolute attenuation at a given frequency.
• Bandwidth: Unless specifically stated otherwise, our filters are specified according to the 3 dB or half-power bandwidth.
• Group Delay Variation: Defined as a unit of time, typically nanosecond.
• Linear: This is defined by the difference in the end points of the group delay curve.
• Parabolic: The total variation of the group delay from a flat line, usually expressed as a +/- value.
• Insertion Loss: Defined in dB, this term describes the attenuation or loss at the reference point of the filter, usually the center frequency.
• Phase Linearity: Defined in degrees, this term describes the total phase variation from a straight line across a given frequency band, usually expressed as a +/- value.
• Pass band Loss: Defined in dB, this term describes the absolute loss across a band of frequencies the filter is supposed to pass.
• Rejection: Defined as dBc, this term describes the attenuation at a given frequency relative to the lowest loss point on the filter, usually the filter’s center frequency.
• VSWR: Defined as a ratio, this term describes the reflection coefficient of the filter and can be associated with a frequency pass band; but, it is usually described as typical which is some percentage of the filter’s 3 dB pass band, which depends mostly on the number of resonators.

by Martin J. Geesaman, President ComNav Engineering
Rolf Aschhoff, Line Manager Frequency Devices MSC Vertriebs GmbH