The heart of the filter connector is the capacitor array. The layered planar-array capacitor helps filter connectors go above and beyond in harsh environments.
By Phil Baynes
As military applications push the envelope, RF/microwave filter connectors must increase in complexity. Previous types of capacitors were unable to withstand military specifications for harsh environments. Now, using a dry process to laminate layers of X7R ceramic tape, ceramic multilayer capacitors can achieve capacitance values from 100 pF to 100 nF on the same array.
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In the early 1980s, the filter connector (still in its infancy) used exclusively tubular type capacitors. These capacitors served the needs of the industry well at that time. However, low yields and various quality problems suggested that the tubular capacitor was no longer sufficient for the systems it was designed into. Therefore, in the late 1980s the monolithic planar array was born.
This new technology incorporated the monolithic chip capacitor and adapted it to a multi-line configuration to achieve higher capacitance per line as well as higher withstanding voltages. Tubular capacitors and monolithic planar array technology are vastly different in their design and capabilities (see Fig. 1). The tubular capacitor is, as it suggests, a tube running the length of the contact with electrodes buried inside. The wall thickness of the tube is dictated by the pin-to-pin spacing of the connector, the metal ground plate used to ground the capacitor, and the size of the ferrite in a Pi-section filter.
 FIGURE 1. The parameters of the planar array capacitor stack up nicely against the tubular and chip capacitor technologies. |
For example, in a 150-line Aeronautical Radio Inc. (ARINC) 600 module, the pin-to-pin spacing is 0.100”. With a tube-wall thickness of 0.050”, minus the web dimension of the ground plane, minus the wall thickness of the ferrite, typically the tube wall thickness ends up being around 0.015” thick. This limited thickness has to be designed to withstand the voltage rating of the system, achieve the desired capacitance, and resist system vibration.
The systems of today often require much higher capacitance and voltage ratings. The Eurofighter Typhoon fighter plane has many requirements that exceed 2000 VDC and the vibration requirements are the highest in the industry. The 0.015” tubular capacitor is not designed to handle these high vibration requirements and there is no space to increase either the capacitance or the voltage rating.
System components today are subjected to harsh environmental constraints. The dielectric material in the capacitor typically is X7R type material to achieve the highest capacitance with the least change in capacitance over the temperature range. The tube has the electrodes (which, when stacked together, increase capacitance) running parallel to the contact. This in combination with the pin-to-pin spacing limits the capacitance to about 7000 pF at 200 VDC.
Chip capacitors
The use of chip capacitors is not usually applied to connectors in military applications. The reason is two fold. First, chip capacitors tend to resonate at frequencies around 200 to 300 MHz and during a swept EMC test, tend to fail at those frequencies. They also take up too much space and tend to lower the mean time between failures (MTBF) rating of the connector. The planar-array assembly is much more rugged and not subject to thermal shock and vibration that surface-mounted chip capacitors face. Lastly, the planar array ensures a 360° attachment to ground to maximize insertion loss up to 1 GHz. The chip capacitor will not have as circumferential a ground and some radiated emissions may not be caught by this solution.
Planar array technology
The planar array is much more complex and versatile in its design. The planar uses the same X7R material as the tubular capacitor, however, the electrodes run perpendicular to the contact. This allows higher capacitance and higher voltage ratings, as the pin-to-pin spacing does not affect the design as much. With the electrodes running perpendicular to the contact, we can stack more electrodes, thus increasing capacitance and at the same time, thicken the dielectric between electrodes to increase withstanding voltages.
The planar array also has the advantage of strength. As the layers of ceramics are stacked orthogonally to the contact, we can increase the planar thickness to about 0.100” to withstand high vibration scenarios as in the EFA. This far outweighs the 0.015” found in the tubular capacitor.
The modular design
Designers can integrate both EMI/RFI and EMP protection into a connector with only a small increase in length over the non-filtered version. One unique design approach for diode connectors is to mount the diodes around the outside of the contact arrangement on a multilayer circuit board within the connector.
In this type of design, standard “catalog” diodes are screened using JANTX-quality or equivalent level screening. The factory can provide diode selection and screening level options for transient-voltage suppressor (TVS) devices. All TVS devices in this design are not custom or “sized down” thus lowering the maximum power handling capability of the diode and risking safe system performance.
The diodes are located outside the contact arrangement to allow for size and shape options, preserving the power/pulse width capability. This enables the diode manufacturer to produce the diodes in a familiar manner without special processes.
The structure is modular in that the diodes are attached to multi-layer circuit board with the contact arrangement at its center, permitting detachment. The circuit traces from the contact to the diode are kept as wide as possible and are sandwiched between ground planes. This provides a very low characteristic impedance strip line configuration, thus eliminating any “ringing” of pulse response. Lastly, the diodes are removable, but not accessible to unauthorized personnel.
The working or operational voltage is the maximum voltage that can be continuously sustained. The dielectric used to manufacture the capacitor sets this value, which is directly proportional to the distance between ground planes and electrodes, whether tubular capacitor or a planar array.
Insulation resistance is generally measured at the capacitor or connectors working voltage. This ensures that resistance between contacts and from a contact to ground is sufficient, so as not to cause electrical shorts. Typical values are approximately 5000 MΩ. Lower values are required for high capacitance values.
Capacitance is a product of the overlap between ground planes and electrodes, and the dielectric constant of the ceramic, k. Capacitance plays a key role in filter performance. Capacitors impedance lowers as frequency increases. The greater the frequency, the greater is the effect of filtering or attenuation.
 FIGURE 3. The noise floor (red), typically 75 to 85 dB, must not be exceeded by the connector. |
The noise floor for this design shows attenuation still increasing at 80 dB (see Fig. 3). The noise floor is the value at which the connector will not exceed, typically 75 to 85 dB. This is limited by capacitor performance, source and load impedance, and ground resistance.
Capacitor design
One of the main components of a filter connector is the capacitor. The capacitor consists of multiple layers of ceramic insulators and precious metal conductors. The ceramic part has unique ability to store charge. The amount of charge that a capacitor can store depends on its capacitance and the voltage applied. The capacitance depends upon two factors; the first is the composition of the insulator (better known as the dielectric). Air, for example, has a dielectric constant of about 1.0; mica has a dielectric constant of 6.0. In other words, mica has the ability to hold 6 times more charge than air. The dielectric materials C0G and X7R have dielectric constants of 95 and 3000, respectively (see Fig. 4).
 FIGURE 4. With a dielectric constant of 95, the ceramic material called C0G is a low-dielectric with stable capacitance over the temperature range of -55° C to 125° C. |
The second determining factor is the geometry of the capacitor. For a simple single-layer capacitor, the capacitance increases with area. The capacitance can also increase with decreasing thickness.
The capacitor design is based on a customer’s requirements. There are four major guidelines when designing a particular capacitor array. First, the design must be large enough to compensate for shrinkage. Second, multi-capacitance arrays require multi-screen designs. Third, high capacitance designs cannot exceed a certain number of layers. And fourth, high-voltage designs must meet minimum fired thicknesses.
The capacitance is influenced by the number of active printed layers, the overlap area, and the layer thickness. There must be a balance between all three parameters to ensure and reliable and economical component. With each printed layer, precious metal is used, which is expensive. The amount of overlapping area between the ground plane and positive pattern must be small enough to minimize alignment variations, which can lead to failure, yet large enough to minimize the number of printed layers required to obtain a particular capacitance target. Large overlapping areas can increase the distribution of capacitance between the hole population within a part. This wide distribution may exceed customer’s specifications.
Finally, the layer thickness must be large enough to safely exceed the customer’s voltage requirements. If the layer thickness design is too large than more printed layers are needed, increasing the overall thickness, making the capacitor too thick to fit into the connector design. If the capacitor is too thin, it may be prone to cracking during processing. There will always be at least two screens used for any one ceramic design, the ground plane and positive pad. The ground plane provides the ground connection to the connector shell. The positive pad provides connection to the contact pins.
The increased sensitivity of electronic systems and mandated performance requirements such as RCTA DO-160 make transient protection paramount in system design today. Transient suppression built into the connector provides the most space-efficient and effective method of electromagnetic protection (EMP), lightning protection, nuclear EMP, and protection from voltage transients. The excess energy is shunted to ground at the connector interface before it can enter the system.
High signal transmission speeds coupled with low-level operating voltages have given rise a need for high-speed EMP protection circuitry. A complete series of EMP products is ideally suited for this need. Densely packaged and protected within the connector shell, low-voltage TVS bi-polar diodes can be connected in series to a parallel network of back-back rectifiers.
 FIGURE 5. Diodes are mounted inside the connector around the periphery of the insert arrangement. |
The TVS diodes are mounted inside the connector around the periphery of the insert arrangement (see Fig. 5). Standard “catalog” diodes are used as opposed to custom or downsized diodes in order to increase reliability and minimize cost. Prescreening of diodes with component level testing and burn-in eliminate infant mortality.
PHIL BAYNES is the eastern regional sales manager for Sabritec EMI/RFI solutions, 17550 Gillette Avenue, Irvine, CA 92614. Tel: (949) 250-1244; email: pbaynes@sabritec.com.
