Mini-Circuits LFCN and HFCN Filters: How to Choose the Right Low-Pass or High-Pass Filter

Two families, one idea

Mini-Circuits' LFCN and HFCN families are tiny ceramic filters that do a single, well-defined job:

• LFCN — low-pass filters: pass low frequencies, reject harmonics and high-frequency noise.
• HFCN — high-pass filters: pass high frequencies, reject low-frequency interference and DC.

Decoding the part number

The number in the part name is the cutoff frequency in MHz. So LFCN-80+ is a low-pass filter with a cutoff near 80 MHz, while LFCN-6000+ cuts off near 6 GHz. The + suffix denotes the RoHS-compliant version. HFCN-3100+ is then a high-pass filter cutting off near 3.1 GHz.

When to use a low-pass filter

• After a power amplifier, to suppress harmonics before the antenna.
• Before a receiver, to keep out-of-band energy away from sensitive stages.
• Anywhere a transmitter must meet spectral-mask or EMC requirements.

When to use a high-pass filter

• To block low-frequency interference and DC in broadband receive chains.
• To separate bands in a diplexer alongside a low-pass section.
• To protect a front end from strong nearby signals below the band of interest.

Selecting the cutoff

Place the cutoff frequency just outside your band of interest so insertion loss stays low in-band while rejection builds quickly out-of-band. These ceramic filters roll off fast but are not infinitely steep — check the datasheet's rejection curve, and if you need sharper skirts, cascade two filters or move to a higher-order design.

Sourcing Mini-Circuits filters

Because they are used in almost every RF board, LFCN and HFCN filters are high-turnover items and frequently show up on the spot market in mixed date codes. Confirm the cutoff and the RoHS suffix, and buy from a supplier who can show you the actual goods.

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