Sabritec specializes in custom interconnect solutions for military, space/satellite, medical, telecom, commercial avionics and test & measurement applications.
Filter Connector Technology
Different Technologies to filter connectors
- Planar Arrays
- Tubular Capacitors
- Chip Capacitors
Monolithic Capacitor Benefits:
- Mixed Capacitance, Ground, and Insulated Lines Capacitor
- Voltages Vary up to 1000 VDC Working
- Multilayer Construction
- Voltage Rating: 100 VDC – 1000 VRMS
- Capacitance Range: 100 pF – 100 nF
- Ground Lines & Feed Thru Lines Within Single Capacitor
- Mixed Capacitances 10:1 Ratio Within Same Ceramic (1,000 pF – 10,000 pF Range)
Filter Technology Background
As the connector industry has grown and the products incorporate more technology, so the components have had to increase in complexity. The filter connector is a sterling example of this technology growth.
In the early 1980's 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 an array of quality problems suggested that the tubular capacitor was no longer sufficient for the systems it was designed into. Therefore, in the late 1980's the monolithic planar array was born into existence.
This new technology incorporated the monolithic chip capacitor technology and adapted it to a multi line configuration. This gave both the ability to achieve higher capacitance per line as well as higher withstanding voltages. The 2 technologies are vastly different in their design and capabilities and I will explain those differences in more detail.
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. In a 150 line ARINC 600 module, the pin to pin spacing is .100". Therefore the wall thickness of the tube is .050" minus the web dimension of the ground plane minus the wall thickness of the ferrite. Typically it ends up being around .015" thick. This limited thickness has to be designed to withstand the voltage rating of the system, achieve the desired capacitance and be strong enough for system vibration. As most systems are military the dielectric material in the capacitor must be X7R 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 200VDC.
The systems of today typically require much higher capacitance values and/or require higher voltage ratings. The EFA has several requirements that exceed 200VDC and the vibration requirements are the highest in the industry. The .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.
The planar array, on the other hand, 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. For this reason, we can achieve in the same 150 pin ARINC 600 cavity, a capacitance of 30,000 pF per line per planar at 600VDC. This makes for a total capacitance of 60,000 pF in a Pi section filter as there are 2 planar arrays in a Pi filter.
The planar array also has the advantage of strength. As the layers of ceramics are stacked perpendicular to the contact, we can increase the planar thickness to about .100" to withstand high vibration scenarios as in the EFA. This far outweighs the .015" found in the Tubular capacitor.
The users in the military market all agree that the planar array is the only method for effectively filtering a connector and most users make this statement in their filter connector specifications. The tubular capacitor should never be used in a military airborne application in a filtered connector.
Around 1990 the planar array was tested and accepted as the capacitor design of choice in military applications. It is both rugged and capable of achieving high capacitance values at higher working voltage levels. The design is a multi-layer stacking of anode and cathode electrodes to achieve the desired capacitance. Dues to the diameter of the array in the connector the capacitance could be achieved without too many electrodes being used. This kept the ESR (equivalent series resistance) low to offset any resonance in the array at lower frequencies (around 100-200 MHz).
The chip capacitor has not been accepted for use in military programs for filtering inside of a connector as its ESR is considerably higher and is subject to large resonance between 100 and 200 MHz. This would not show up on insertion loss testing as I.L. is tested at specific points and the part will seem to pass at 100 MHz but fail at around 120-150 MHz (which is not typically measured). Usually this is picked up at the EMC testing stage.
To use a chip capacitor means using a PCB in the connector for ground or by soldering wires that act as springs against the contact. If either method is used the grounding for the filter is poor and also lends itself to poor Insertion Loss measurements. This is acceptable in commercial ground based systems such as personal computers, but in higher vibration scenarios the effect is disastrous! Intermittent opens on filtered lines are quite common.
The planar array technology uses solder to achieve 100% ground around the periphery of the array to shell. This maximizes the insertion loss and further reduces any chance of resonance. The thickness and mounting of the array makes it the most rugged manufacturing technique in use. The 3 largest filter connector manufacturers today use planar arrays as we have all tried chip capacitors and found the problems listed above, which also caused great system problems with the users.
Therefore, Sabritec always recommends the use of planar arrays as the technology to use in filter connectors for all aerospace applications.