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Selecting Miniature Passive Components for WiMAX and WLAN Applications

Wed, 08/08/2007 - 7:50am
When exploring passive component options, asking the right questions upfront can ensure the correct component is selected for your design.

By Michael Dakhiya, Chris Reynolds, Kevin Christian, Avital Yaish, AVX Corporation

1. Basic design steps for selecting the correct standard passive component
In a wireless world, consumers have come to expect smaller, faster, and more sophisticated electronic/electrical products from equipment manufacturers. The miniaturization of devices equates to smaller, more feature rich designs. As the trend accelerates, many design engineers are finding difficulty in identifying and selecting
The AVX CP0402P is an integrated thin film, high directivity coupler providesa directivity of 20 to 27 dB and an ultra tight ±0.5dB coupling factor tolerance. Based on the proprietary RFAP thin film technology,the device improves PA gain control in high frequency applications such as cellular and WiMAX wireless system transmitters.
the correct passive components to meet their design specifications. For example, Low-Pass Filters, 3dB Hybrid Couplers, and other RF components can be realized as discrete components down to 0402 size and also as integrated elements in passive arrays. Many designs are taking advantage of downsized discrete parts, however, limited space in modern RF front-end blocks is dictating the use of more miniature components with a high level of integration.

Product size constraints directly influence the circuitry design and in turn can determine component’s case sizes and termination layout. Knowing the packaging measurements that the circuitry must fit into allows a design engineer to appropriately select components based on case size and termination requirements.

The IEEE 802.16 and 802.11 standards specify broadband and high data rate wireless connectivity. The higher the data rate of the transmitted signal, the higher the signal-to-noise ratio required by the system. When selecting standard passive components for these applications, such as low pass filters (LPFs) or 3 dB 90deg> couplers, the first step is to determine the frequency range: cut-off and harmonics to be rejected for LPF; pass band for 3 dB.

Understanding the components (S parameters), the behavior of a device under power at frequency range, typically characterized by magnitude, decibel and phase, is a vital element of the passive component selection process. Gathering this information from the manufacturer also allows the design engineer to convert to other parameters such as hybrid or admittance parameters as well as stability factor and other gain parameters.

In many cases, it is also necessary to evaluate the component at application conditions and for components such as couplers and low pass filters, AVX can provide the designer with dedicated measurement fixtures which allow various port configuration options.

Once this information is gathered, the designer can request specific samples (or designer kits) for the standard passive components. One advantage of the discrete (thin film) RF product family is their stability and reproducibility of parametric performance. Once the design is proven out, the circuit characteristics will remain the same as the design is moved to full production.

Another factor relating to the accuracy of the discrete parts is that small geometric changes of the part in relation to the board layout (such as whether a chip inductor is flipped over or not) can produce changes in the second order coupling with the ground plane; this effect can be used for fine tuning the circuit and, when in full production, parts can be supplied at different orientations in tape to match the original design orientation.

For Passive Micro Circuit design (PMC), which integrates multiple passive technologies in three dimensions, modeling of the parasitics to provide the final target functionality is required and HFCC simulation is used to accomplish this.

In all cases, direct contact with AVX engineering staff is recommended for design process support.

2. Challenges/Problems Customers Present to AVX
If an off-the-shelf solution will not meet the design challenges faced by the engineer, AVX offers customized engineering solutions that, in many cases, solve space constraints, signal integrity, repeatability, reproducibility, and tolerance control problems. The flexibility of RFAP technology allows fast development of various custom components. AVX engineers start by talking with a
Passive Component Manufacturers
Hittite
Micro Lambda Wireless
Vectron International
Miteq, Inc.
Teledyne Cougar
Fox Electronics
NDK America
Narda Microwave East – L3Comm
Krytar
Triquint Semiconductor
Reactel
American Technical Ceramics
Skyworks Solutions
Anaren Microwave
Dielectric Labs
Microsemiconductor
Stratedge
Vishay
TDK Components
Omniyig
customer with a design problem and analyzing the feasibility of the design from the first principle. The preliminary specification provides specifics of the desired RF performance, physical size and terminations layout. AVX then simulates the complex design using HFSS (high frequency structure simulation) modeling processes, providing the customer with S-parameters files and updated specifications.

3. Questions Customers Typically Ask AVX
Cost, final package delivery options and lead times are the most prominent questions customers ask. Customers also typically ask what our technology can do for their designs and if we can help solve the design problems they have encountered.

4. Asking the Right Questions to Ensure Proper Selection
When exploring passive component options, whether it is a custom or off-the-shelf solution, it is vital to collect the correct information and ask the right questions to ensure the correct component selected for the specific design. Although cost and lead times are important elements, a components electrical performance and construction characteristics, such as:

  • Power handling capacity
  • Insertion loss
  • Operating temperature
  • Tolerance
  • Frequency bandwidth
  • Size
  • Termination options
  • Ruggedness in harsh environments

    5. Questions AVX Asks the Customer
    In order to better serve the customer, we will go through a questionnaire that contains 20 or more questions so that we can suggest an appropriate part. We want to know information such as, what the component is going to be used for, what is the application, is it a harsh or mission critical application. AVX also wants to know what, if any, are the design requirements, including real estate, tolerance, termination preferences.

    6. Technical Support
    AVX provides technical support during the design process. S-parameters for standard and custom components will be made available. Customers' boards, measurement techniques, and components mounting process are addressed by AVX engineering staff. Component performance on the customer's board is simulated and fine adjustments are made in order to optimize the component performance in each particular case.

    Measurement procedures are available for each standard and custom component. Application notes for the mounting process include recommendations for soldering materials, soldering profiles, and preferred board layout.

    7. Summary
    When selecting a passive component, whether it is an off-the-shelf or a custom solution for a mission critical application, knowing the design parameters enables the component manufacturer to more affectively aid the design engineer in selecting the correct part for the specific application requirements.

    AVX Corporation is located at 801 17th Ave. South, Myrtle Beach, SC; (843) 448-9411.


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