Design engineers have developed a switch connector system for use in mobile communications devices.
 Figure 1. The SWD switch receptacle. |
By Jerry Kolbe, Toshitaka Kuriyama and Donald Widener
The dynamics of today's mobile communications market are being driven by the need for voice and data connectivity, anytime, anywhere. Accordingly, high-functionality, portable Internet access devices and multiband/multimode-capable mobile phones are emerging.
These mobile communications devices are high-volume, mass-producible and possess sleek and attractive form factors. By necessity, circuit densities are increasing to enable additional functionality and performance without adding to the size and weight of these mobile devices. These trends are driving design engineers to incorporate new concepts and technologies.
One example of a new concept being developed for use in mobile devices is a switch connector system. The idea was identified out of a need to quickly, easily and reliably test the radio frequency (RF) circuit when an internal antenna is used. The SWD switch connector system consists of a receptacle (see Figure 1) containing a mechanical switch and test probes for manual use or automated production test systems.
Case for the Switch Connector
The need for a switch connector is dictated by the design. Typically, it is used when design engineers want to, or need to, break the continuity of the transceiver or radio for testing purposes, such as when an internal antenna is used.
 Figure 2. The conventional switch connector location. |
To isolate the internal antenna of a mobile phone during testing of the RF section output characteristics, a switch connector is generally used. Without it, electrical signals will continue flowing toward the antenna, making accurate measurements difficult to obtain.
Conventionally, a switch connector is located at the bottom end of the phone, near the charging or vehicle-mounting terminals (see Figure 2). A semirigid cable or printed circuit board (PCB) stripline is typically used to link the switch connector to the antenna, which could create a large loss in signal strength.
 Figure 3. The SWD switch connector location. |
In contrast, the new switch connector is used in-line between the RF section and the antenna (see Figure 3). Because the connection between the RF section and the antenna is shortened, compared to a conventional approach, the loss characteristic is improved.
The need for testing typically occurs during the production phase. The manufacturer usually must check the output of a mobile device to establish its compliance to specification prior to shipment. This can be a PCB-level test or a test performed on the assembled unit, case intact. Field failures require testing to determine root cause(s).
To facilitate testing from outside the assembled unit, it is required that the switch connector be incorporated into the casing. The body of the switch receptacle, with the barrel shape, makes it convenient to integrate into the casing, therefore making it accessible for testing from outside the intact mobile device.
Switch Mechanism
The structure of the receptacle switch mechanism is shown in Figure 4. The terminal labeled "C" is connected to the RF front-end circuit, whereas the terminal labeled "R" is connected to the antenna. In normal operation, the continuity of signal flow from the RF front end to the antenna is maintained as the probe is disengaged and the C and R terminals are mated or in contact. By inserting a specially designed measurement probe (adapter) into the receptacle cavity from above, the internal connection can be turned OFF. The signal is taken out of the C terminal, on the side of the probe, while a circuit, such as an antenna, connected to the R terminal is electrically separated.
In developing the switch connector system, specific development criteria were used, including:
- Operation in frequency bands up to 6 GHz; minimally intrusive with high isolation
- Reliable, durable receptacle
- Suitable size for use in mobile phones or similar devices
- Capable of facilitating automated testing, with a relaxed tolerance for probe insertion mismatches
- Improved manual testing method
Electrical Design
Voice and data connectivity anytime, anywhere is driving development of mobile phones with longer standby and talk times, as well as increased circuit complexity for multimode/multiband use. Enablers of longer and increasingly ubiquitous use include improved battery technology and improvements in transceiver design techniques.
Lower operating voltages and tighter loss budgets in the radio result in improved battery life by reducing energy consumption. Understanding the design trends of mobile devices was critical in establishing the overall goals for electrical performance of the switch connector.
- Operation in RF and microwave frequency bands up to 6 GHz
- Minimally intrusive (low insertion loss) while probe is disengaged
- High RF isolation during engagement
Reliable, Durable Receptacle
During the life of a mobile device, it can be subjected to testing at the point of manufacture, upon delivery to the dealer and for field repair or maintenance. To ensure the robustness of the switch receptacle design, the goal for number of engage/disengage cycles was specified at 500 cycles.
 Figure 4. The SWD switch receptacle mechanism. |
At 500 cycles, there was no visual evidence of mechanical damage or fatigue. Moreover, the parameters for contact resistance and engage/disengage force continued to meet specification. The point of failure was determined to be at least double the specification. To create a reliable switch mechanism, the design engineers made calculations relating to metal fatigue and material mechanics using a 3-D simulator. The simulation contributed to a unique terminal spring design.
The constituents of the switch mechanism include a movable contact, the C terminal, and a fixed contact, the R terminal. In the disengaged state, these terminals are in contact at two points. When engaged with a probe, the receptacle's C terminal displaces from the R terminal to a distance commensurate with the calculated tradeoff between isolation and insertion loss, which is proportional to contact pressure upon return to the ON state (a derivative of long spring life).
In choosing the material for the C terminal, high durability and good elasticity were deemed prerequisites. This led to the selection of stainless steel. The role of the R terminal in the switch mechanism dictated that it be made of a malleable and durable alloy with good electrical properties, which is why phosphor bronze was chosen.
Suitable Size
The goal was to make the switch receptacle small and short enough to accommodate dense PCB mounting. The target size was 3.0 X 3.0 X 1.75 mm, which was achieved. This translated into a 20 percent decrease in the mounting dimension occupied by the switch connector (9.0 mm2), compared to the first-generation switch connector it is based on (11.2 mm2). Furthermore, the height of the switch connector (1.75 mm) was reduced by 8 percent over that of the first-generation switch connector (1.9 mm).
Key to accomplishing this was a design change of the terminal structure. This change facilitated reducing the size of the molded body, height and outline, as well as changing the design of the receptacle/probe interface from a flat surface to a barrel shape. The receptacle requires less PCB area for mounting compared to the first-generation switch connector, and the receptacle shape lends itself to integration into the case of a mobile device.
Tolerance for Probe Insertion
With the market for mobile devices expanding rapidly, production systems are evolving with the capability to handle higher volume, increased complexity and higher operating frequency PCBs. Along with adding capital to increase production capacity, reduced cycle time and increased yield and efficiency can be realized through implementation of automatic testing. To accommodate automatic measurements in high-volume production, a probe for high-speed automatic measurement was designed.
This straight probe incorporates a spring. By managing the strokes of the automatic measuring machine, users can determine the appropriate pressure for switching the switch connectors OFF and ON. Upward facing contact points of the switch receptacle facilitate automatic measurement.
Engaging the C terminal requires a force of 3.4 N minimum (4.5 N maximum) when applied in a downward vertical direction (relative to the axis of the fixed measurement probe). To accommodate for position mismatches between the fixed measurement probe and the receptacle, the central contact section of the probe is tapered.
In addition, an inverted cone is molded into the cavity of the receptacle to facilitate guiding the probe into the body and in contact with the C terminal. The result is that linear displacements of 0.7 mm from the vertical axis of the receptacle to the vertical axis of the probe can be accommodated, as well as small, angular misalignments.
Manual Testing Method
The probe designed for automatic measurement does not have a locking mechanism, making it impractical for experimentation. To facilitate taking measurements manually, for experimental fabrication purposes or for low-volume production, a manual probe is recommended. This probe has a locking mechanism to keep the switch receptacle engaged in the OFF state.
In addition to measurements while the PCB is exposed, the manual probe can be used for testing mobile devices that have the receptacle integrated into the casing, thus accessible externally without disassembling the case. The locking mechanism of the manual probe is integrated into its body. Its locking function is engaged by applying a maximum force of 30 N when interfaced with the upward facing contact points (barrel) of the receptacle. The design of the receptacle facilitates manual probe engagement within a few degrees of an imaginary vertical axis, perpendicular to the plane of the body. Disengaging the manual probe requires application of a force (5 N minimum, 40 N maximum) along the vertical axis.
Conclusion
In today's rapidly expanding mobile communications market, high-functionality, portable Internet access devices and multimode/multiband-capable mobile phones with complex designs are emerging to meet the need for voice and data connectivity, anytime, anywhere. To ensure these mobile devices meet the consumer demand for sleek and attractive form factors, without a significant increase in size or weight, design engineers are incorporating new technologies and concepts to facilitate increased circuit density. Moreover, these mobile devices are being designed to be high-volume and mass-producible.
A switch connector system is an example of a new concept being promoted for use in mobile device designs. Use of this switch connector system facilitates reliable, high-volume, automated testing of an RF circuit in radio designs using an internal antenna.
Adapted from a presentation given at the 33rd Annual IICIT Connector and Interconnection Technology Symposium, Oct. 23-25, 2000, Lake Buena Vista, Fla.
JERRY KOLBE is Market Segment Group Manager, and DONALD WIDENER is Product Manager, Murata Electronics North America Inc., 2200 Lake Park Drive, Smyrma, GA 30080-7604; (770) 436-1300; Fax: (770) 436-3030; Web site: www.murata.com. TOSHITAKA KURIYAMA is Design Engineer for the SWD Connector, Kanazawa Mfg. Co., a division of Murata Mfg. Co., Kanazawa, Japan.
SPEC SHEET
End Applications:
Testing of RF circuits in radio designs with internal antennas
Related Products:
Switch connectors, PCBs
Main Point:
In today's mobile communications market, portable phones and other devices with complex designs are emerging to meet the need for voice and data connectivity, anytime, anywhere. Design engineers are incorporating new technologies to facilitate increased circuit density. A switch connector system consisting of a receptacle that contains a mechanical switch and test probes for manual use or automated production test systems is one example of a new concept.
