To meet the environmental isolation requirement, an air-to-air heat exchanger was designed to allow the unit to shed heat, while preventing interchange of internal and external air. In order to minimize total system volume, reduce weight, and increase structural stiffness of the chassis, the heat exchangers and exhaust air ductwork are used to form the side walls of the chassis. The third and probably most important challenge was cooling the payload (see Figure 3). Four fans are used to pull external air through the external side of the heat exchangers, while internal air is re-circulated through the inner air passages of the heat exchangers. Additional cooling is provided for the electronics package by applying four smaller fans to pull air across the electronics package heat sink. The heat exchanger is a dual-passage counter-flow design where the internal air flows in the opposite direction of the external air.

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System Monitor
In RF Mil-Spec systems, system monitoring falls into five major categories: temperature monitoring, fan monitoring, voltage monitoring, remote access, and other options. Temperature monitoring is becoming more critical as the value of the payload continues to rise. To address this, strategically located thermistors feed temperature values to a system monitor. The monitor can then evaluate the temperatures and increase/decrease the fan speed as needed. If a specified temperature is reached, a warning can be sent, and more importantly, if a temperature level is reached, the system monitor can inhibit the power to the backplane, shutting down and protecting the boards. It is also important to monitor other functions such as the health of the fans or system voltages. If one of these should fail, the system could be jeopardized.

In addition to the basic monitoring functions, new system requirements are generated every day. As the cost of processing boards rises, users are more interested in alerts that may not require system shutdown, which is moving customers towards boards and systems that can self-monitor the performance of the electronics package in addition to environmental factors.

This article was written by Ryan Pellecchia, Senior Technical Application Engineer, at Hybricon Corp. in Ayer, MA. For more information, contact Mr. Pellecchia at This e-mail address is being protected from spam bots, you need JavaScript enabled to view it , or visit http://info.hotims.com/10968-401.