Electronics, Radio & antennas

Cheap single-component RF filters

Low, Band, and High – pass filters are major components when working with RF.
Depending on the application, filters can reduce output noise and unwanted signals or limit a transmit signal to a pre-specified frequency band to prevent interference with other signals.

Here I want to describe how to build simple and cheap single-component (ceramic) filters for your DIY projects.

To build the required filter, we can use capacitors and inductors for lower frequencies. This is classic construction.

Such filter can be built using both through-hole and SMD components.

Both filters are really easy to build but require some calculations. Typically components are widely available. Also, such filters can be fine-tuned. This is a plus and minus at the same time.
Without proper measuring tools, it might not be easy to tune and check such filters. On higher frequencies, all mistakes begin to play a bigger role and might be fatal.

Another well-known type of filter is strip-line filters used on higher frequencies (dozens of gigahertz).

Such a type of filter may look simple but requires more precise calculations (using special software like Microwave Studio) and a more precise production process. Even small extra copper may act as an inductor on high frequencies, which can ruin your schematic.

Thanks to modern technologies, it’s possible to build very compact high-order filters that can work very nicely on frequencies up to 10 GHz (and even higher). You can find such type of filters in compact devices, like mobile phones. Devices are built using LTCC technology.

One of the most popular vendors is Mini-circuits. You can find in their catalog all types of filters for all popular bands. I’m used such devices as band-pass and as low-pass filters for L-band. It’s perfect for DIY projects without special requirements.

This article will use a low-pass filter (0-2400 MHz) as an example.
This type of filter requires only a board and a case with two SMA connectors.

Here is my board. The silkscreen is for 0-2400 filter, but this is only one thing that requires change for different filter types. All devices are in the same package and require almost the same outline.

Here is Eagle (7.2.0) project files: lcfn-2400_eagle

And here are Gerber files if you want to order such board on a factory: lcfn-2400_gerber

This board requires EMI shielding. A milled aluminum case is a perfect solution.
I decided that the best source of such aluminum cases is Bias-T circuits from eBay/Aliexpress:

You can find dozens of such devices for a reasonable price. Keep in mind that this is a really nice milled aluminum case with two SMA connectors. As a bonus, you will get a pass-through capacitor and an actual Bias-T circuit board, which can be used in other projects.

Completed filter:

Here I’m used low-pass filter LFCN-2400

According to the documentation, this filter inserts a noticeable loss starting at 2800 MHz. And after 5000 MHz, performance is dropping.

In my lab, I can measure only up to 4GHz, at this moment. Here are my measurements.
Looks nice, not sure about this spike at 3000 MHz. Probably some interference on my measuring device. You can see that this device really inserts nice loss after 2800 MHz, up to 7dBm. Of course, there are still some fluctuations, but it’s no issue in this case.
Marker 1 is at 3880 MHz, and insertion loss is 66 dBm.

Thanks for reading! 🙂

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