Planar Filters
Manufactured using a thin-film process, Microstrip (planar) filters can offer a high quality factor (Q) and a reduced packaging envelope when compared to discrete lumped element designs, and are more practical at higher frequencies. The thin-film design can hold tighter design tolerances due to the distributed transmission lines forming resonant structures. Planar filters are a robust solution, attractive for applications ranging from established platforms, such as military warfare, to emerging technologies, like 5G. Below are some general-purpose resources for additional background, applications, and benefits of Microstrip filters:
+ Filter Terminology and Specifications
+ Shape Factor and Selectivity
+ Opportunity for Reduced Size at High Frequencies
+ Cost Advantages of Surface-Mounted mmWave Microstrip Filters
+ Microstrip Filter Topologies
Given this wide range of applicability, it is important for the design engineer to tailor the specification requirements of the filter to the intended application to optimize component performance, size, and cost. The key specification categories are filter type, mechanical, electrical, and attenuation. The specification criteria for each category is described below.
Specification Criteria
Type of filter required
Filter Response Type– defines the types of frequencies removed or allowed by the filter
| Low Pass filters allows frequencies below a given frequency to pass (to be transmitted or received) while rejecting frequencies above the given frequency. |  |
| High Pass filters let frequencies above a given frequency to pass through the filter, while rejecting frequencies above the given frequency (opposite of low pass). |  |
| Band Pass filters pass frequencies between two frequencies while rejecting all others. |  |
| Band Stop (or Band Reject) filters prevent all frequencies between two frequencies from passing while allowing all others to pass (opposite of band pass). |  |
Mechanical Specification
Mounting Options
- Wire-Bonded – integrated connections that offer a flexible, reliable connection that can reject frequencies up to 100 GHz
- Surface Mount(SMT) – filter is mounted to the surface of the circuit board
- This mounting method is more packaging-dense, but typically offers reduced rejection (<60 dB) over wire-bonded mounts in volume production.
- Max length and max width – govern packaging footprint available for filter
- Height required to cover or shield filter – defined as the distance from PCB to shield/cover
- Cover provider – define who will provide filter cover
Electrical Specification
Frequency Range, Bandwidth and Loss
- Center Frequency(MHz) – geometric or arithmetic mean of the upper and lower cutoff frequencies
- Passband(MHz) – range of frequencies in which signals pass through a filter
- 3dB Bandwidth (MHz) – defines the filter’s bandwidth (difference between upper and lower 3dB points); the point at which a signal is attenuated by 3dB
- Relative/Fractional Bandwidth (%) – the ratio of a filter’s bandwidth to its center frequency; it can be expressed as a fraction or a percentage
- Max. Allowable Insertion Loss (dB, related to s21) – losses associated with connection inefficiency, typically caused by either connection integrity or material purity. This is defined as the ratio of the signal level in-test with a filter present to that of a test without a filter in place.
- Insertion loss is specified in reference to either the passband or center frequencies
- Return Loss Requirement (dB, related to S11, the Scattering Parameter that represents the Reflection Coefficient (𝛤) at the input) – measure of the amount of signal returned/reflected by the filter
Attenuation Specification
Attenuation (dB) – degree by which a signal sees a loss in amplitude after passing through a filter. Examples of specification values include:
- System requirements
- Mirror frequencies to suppress
- Out-of-band signals of interest
Validation requirements
- Test fixture requirements, if any
- Connectors planned for use with test fixture
- Application-specific test requirements, such as;
- Temperature range
- Test documentation requirements
- Mounting requirements
- Packaging envelope/footprint
Accurate and complete specification of a filter is critical to ensure that a design engineer can meet the required performance, size, and cost of a given application. If you don’t have the full definition of the filter type required for an application, contact a knowledgeable supply partner to review your specification. Taking that step will provide an added level of technical confidence that the product you select will be the optimal choice for its intended use.
References