ADS-B Filter Design

Having a wideband LNA is awesome, if you want to amplify wideband signals.  I don’t.

The ADS-B signal is pretty narrow and centered at 1090 MHz.  The RTL-SDR can only do 2 MHz at a time.  If there are nearby signals present, they will alias and interfere with the signal of  interest.  In fact, I can see strong FM broadcast stations in the passband around 1090 MHz, this is a problem.

Filter Design

To deal with strong out of band signals, I designed a bandpass filter centered around 1090 MHz.  I landed on a hairpin filter design to make construction easier and to avoid lossy discrete components.  Adding to the challenge, I decided to use FR-4 PCB material as it is cheap and easy to find.  The design was done using Agilent Genesys and Momentum simulations.  Pretty expensive software, I know.

PCB Fabrication

I milled a prototype using a LPKF S103 PCB mill and measured the result on a VNA.  After optimizing and modeling with all that fancy software, the bandpass was shifted by about 20 MHz.

Hairpin Bandpass Filter

Prototype Hairpin BPF milled on LPKF S103

 I shifted my center frequency in the software and reoptimized the board.  Then remilled a PCB.  I went through this process about 3 times  to get the bandpass centered on 1090 MHz. It would have taken a couple months if I didn’t have the PCB mill to get the filter right.  Hooray for being a student!

Hairpin Filter for ADS-B

Hairpin Filter for ADS-B from OHS Park

Test Results

The response of the OHS Park board is shown below.

Bandpass Filter Response

Bandpass Filter Response

Markers are at key frequencies.  The top end of FM broadcast is down by 62 dB or 1.5 million times lower.  ISM is only 100 times lower, but generally those signals are weaker than FM broadcast.

The insertion loss is 2 dB, which many have argued that is too high to be in front of an LNA.  It only raises to system noise figure to 3.2 dB, instead of the 1.2dB from the LNA alone.  Also, the ADS-B signal is not limited by noise, but by line of sight.  For the cost of the filter, this is acceptable.  It costs around $10 per board from OSH Park, opposed to using some microwave material in the range of $100s per board!

My main goal was to block out FM broadcast.  I also tried to knock out the 915MHz ISM band and cell phone frequencies.  Unfortunately getting rid of radar signals would be next to impossible since they’re so close.  My suggestion would be to add a SAW filter, like the Epcos B1602 after the LNA.  The main purpose for the filter in this post, is to keep the LNA form saturating.

Layout Files

Filter files are here: https://github.com/Reid-n0rc/ADS-B_LNA_Project/tree/master/Filter

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