Integration of Waveguide and Coaxial Components
By Brent Waddoups, Integrated Products Manager and Senior Design Engineer,
Cobham Defense Electronics Systems, Continental Division

Waveguide systems have long been a staple in the microwave industry. The ability to use waveguide and waveguide components to efficiently direct RF signals has been a backbone of systems ranging from mammoth ground stations to tiny space-borne applications to high precision laboratory equipment. Waveguide has seen widespread use in both commercial and military markets. The benefits of waveguide component use in systems include high power transmission capability, extremely low loss, low reflections and structural rigidity.

While waveguide components, in and of themselves, provide exceptional RF performance for many applications, the integration of these components into systems which may contain other forms of technology (coaxial, stripline, etc.) allows for the realization of incredible results. An optimal system performance can be achieved that is much greater than just the sum of the parts. Integration can provide better RF performance, increased system simplicity, smaller size and decreased cost.

There are two typical ways to achieve system integration. We will refer to the first method as the plug-and-play method. As the name suggests, this method involves the specification and purchase of individual components, integration of the components in the as-purchased condition and measurement of the resulting system. The method is a “let’s see what we get” approach and can be very effective for prototyping and feasibility studies at system level. The second method will be referred to as the tuned-system method. In this method, a set of system specifications are generated using a “black box” approach. For optimal system performance, the integrator is responsible for the performance of the system as a whole as well as the components that make up the system. Each method will be examined in detail.

Experimental Verification - Plug and Play Method

There are many ways to configure an RF system, ranging from the very complex to the very simple. For the purpose of this discussion, we will consider a fairly simple system consisting of a power amplifier, low-pass filter, bandpass filter, flexible waveguide and waveguide circulator, as shown in Figure 1. We will investigate the performance of such a system using both the plug-and-play and tuned-system methods of integration. The systems will be evaluated on typical RF performance parameters such, as VSWR, insertion loss, and amplitude variation.

The first step for integration using the plug-and-play method is for the end use customer to create a set of specifications for each component. A simplified example of a parts list for plug-and-play integration is shown in Table 1. The next step in the plug-and-play process is to create purchase part drawings for each part needed and then quote and procure the needed parts. Once the parts have been obtained, each will need to be inspected individually and then integrated into the final assembly by the end use customer. The final assembly is then tested and the results are analyzed.

The example system shown in Figure 1 was built first using plug-and-play integration techniques. The data shown in Figures 2 and 3 illustrate the results obtained by this method. The system performance parameters are captured in Table 3 for later comparison with the tuned-system method.

Experimental Verification - Tuned-System Method
Using the tuned-system method of integration, the end use customer creates a single “black-box” specification and purchased part drawing. This black-box specification contains any and all pertinent information for the proper operation of the system being procured. A subset of the types of parameters that might be specified for the type of system shown in Figure 1 is shown in Table 2. The supplier or integrator to the end use customer is then responsible for either designing or procuring components necessary to create the system. Up to this point the two methods are quite similar. However, if the end use customer utilizes an integrator that manufactures many or all of the components in the system, they will be able to reap the benefits of tuned-system integration. In the example system shown in Figure 1, the system level specification would be sent to an integrator who builds filters, circulators and waveguides in order to obtain maximum tuning potential at the system level.

The integrator will then use a combination of designed/manufactured and procured elements to build the system to the customer’s specification. In the example of the system shown in Figure 1, an integrator could procure the power amplifier and design/manufacture the bandpass filter, low pass filter, circulator and flexible waveguide. Each component would be pre-tuned prior to system assembly to a set of internal specifications. Once the pre-tune is complete, the components will be integrated into the system configuration. At this point in the integration process, the results will track closely with the results obtained from the plug-and-play method. The integrator is then able to analyze the performance of the system as a whole and continue tuning the components in the system to achieve optimal performance. Figures 4 and 5 illustrate the performance of the system in a tuned-system configuration and Table 3 summarizes the results.

This final tuning is the crucial difference between the plug-and-play method and the tuned-system method. Using the plug-and-play method, no avenue exists to mute the effects of cascaded VSWRs, mismatches evident in pass band ripple or group delay variation. It is evident, by observing the data in Table 3, that significant improvements in performance can be achieved when the system is integrated using the tuned-system method.

Conclusion
The development of systems through the integration of RF components allows RF engineers to satisfy the needs of myriad customers from military communications and radar systems to cellular communications systems to space- borne and earthbound systems for satellite usage. The key to obtaining optimal performance in a given RF system is the degree to which the components in the system “play” well together electrically. The ability to integrate and tune an entire system, or subsystems within a larger system, will greatly improve system performance and provide optimal system operation for the end use customer. The system used as an example herein is a simple system, but the concepts used in tuned-system integration can be applied to much more complex systems. Such systems may include diplexed and multiplexed filters, power amplifiers, low-noise amplifiers, switches, circulators, couplers, limiters, etc. As an example, the systems depicted in Figure 6 illustrate fairly complex RF system utilizing several waveguide diplexers, waveguide and coaxial switches, flexible waveguide, waveguide to coaxial adapters, command and control circuit cards and low-noise amplifiers in an optimized, compact EMI compliant package. Whether the system is simple or complex, the use of tuned-system integration techniques will increase performance, decrease size and weight, and provide an optimal RF system solution for the end use customer. The benefits of the tuned-system approach are crucial to the continued success and progress of RF systems as the demand for smaller, cheaper and better performing systems grows.

COBHAM DEFENSE ELECTRONICS SYSTEMS
www.cobhamdes.com
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