Planar Filters

A planar filter is a type of electronic filter built using flat, planar transmission lines (like microstrip or stripline) on a dielectric substrate. These filters are compact and suitable for integration in printed circuit boards (PCBs).

Common types include:

• Microstrip filters

• Stripline filters

• Coplanar waveguide filters

• Substrate-integrated waveguide (SIW) filters

Each has different trade-offs in terms of size, performance, and manufacturing complexity.

They are widely used in:

• Wireless communication systems

• Satellite and radar systems

• Mobile phones

• RF/microwave circuits

• Signal processing

• Compact size

• Lightweight

• Low cost for mass production

• Easy integration into PCBs

• Good repeatability and reliability

They are typically fabricated using standard PCB manufacturing techniques, such as photolithography and etching, on dielectric substrates like FR4, Rogers, or ceramic materials.

Planar filters can be designed for a wide range of frequencies, from a few hundred MHz up to tens of GHz, depending on the technology and materials used.

• Losses due to dielectric and conductor materials

• Unwanted coupling or spurious responses

• Tight tolerance requirements for high-frequency applications

• Thermal stability and performance variation with temperature

Design methods include:

• Lumped-element modeling

• Distributed element modeling

• Full-wave electromagnetic (EM) simulation

• Optimization algorithms for tuning performance

Tools like HFSS, CST, and ADS are commonly used.

Key parameters include:

• Center frequency

• Bandwidth

• Insertion loss

• Return loss

• Out-of-band rejection

• Group delay

Yes. By integrating varactors, MEMS, or PIN diodes, planar filters can be made tunable or reconfigurable, allowing frequency agility for modern communication systems.

Choosing the right planar filter for your system depends on several technical and practical factors. Here’s a step-by-step guide to help you select the appropriate filter:

1. Define System Requirements

Start by identifying your system’s core needs:

• Center frequency (f₀) – What is the target frequency?

• Bandwidth (BW) – How wide is the frequency range that should pass?

• Filter type – Do you need a low-pass, high-pass, band-pass, or band-stop filter?

• Rejection requirements – How much attenuation is needed outside the passband?

2. Choose Filter Technology Based on Frequency

Different planar filter types suit different frequency ranges:

• Microstrip – Good for moderate to high frequencies (up to ~30 GHz)

• Stripline – Offers better shielding, suitable for higher performance

• SIW (Substrate Integrated Waveguide) – Excellent for high-Q filtering at mm-wave frequencies (>10 GHz)

• Coplanar waveguide – Useful when tight coupling is required on a single layer

3. Consider Physical Constraints

• Size – Limited board space? Microstrip and lumped-element filters are more compact.

• Weight – Planar filters are lightweight, but SIW is bulkier than microstrip.

• Substrate type – FR4 is cheap but lossy at high frequencies. Use Rogers or ceramic substrates for better performance at GHz+.

4. Analyze Performance Parameters

Use simulations or datasheets to evaluate:

• Insertion Loss (S21) – How much signal is lost in the passband?

• Return Loss (S11) – How much signal reflects back due to impedance mismatch?

• Group Delay – Critical for systems where phase linearity matters (e.g., radar, communication)

• Stopband attenuation – Are out-of-band signals sufficiently blocked?

5. Evaluate Manufacturing Capabilities

• Is your PCB fabricator capable of producing fine features or high-frequency substrates?

• Some filter designs may require tight tolerances or multilayer boards.

6. Consider Reconfigurability

Do you need a tunable or adaptive filter?

• Choose varactor-loaded or MEMS-integrated planar filters for real-time reconfigurability in software-defined radios or multi-band systems.

7. Budget and Volume

• Low-cost systems may use standard microstrip filters on FR4.

• High-performance systems (e.g., aerospace) might require expensive materials and tight design control.

To place an order for planar filters please contact us and we will help you!