In the latest PCB version of the SCARP receiver board (Arduino Project for Software Controlled Aviation Receiver), I couldn't get the bandpass filter and integrated Low Noise Amplifier to behave properly. I wanted to change out the LNA in my design to a better IC, and why not try to improve the bandpass filter at the same time?
It's not easy to integrate major changes into an existing PCBa, yet I'm giving it my best effort. I appended a different LNA to the design, and the photo shows the test setup with an external bandpass filter. The next board layout will recombine stuff back into one board.
(The older version of the PCBa works extremely well, and my most recent video demonstrating it at an airport is here:)
Have a look at the photo below. My Vector Network Analyzer (VNA), is sweeping the circuit from 50 to 250 megahertz. (Aircraft band is 118 - 138). You can see that the response in the blue trace of the screen on the VNA.
The math is simple: the VNA is putting out a sweeping signal at a power level of 0 dbm. This is attenuated (reduced) by 30 dbm arbitrarily, so that the signal is now closer to that of a very strong RF signal. After filtering through an external bandpass filter, the signal then passes into my circuit board, and is boosted by about 25 dbm, so that the signal displayed on the blue trace is shown at around minus 5 dbm, peak.
In my PCB, this then continues to be processed. My white paper covers this and more. Sign up below, and I will (soon, I promise) send you the paper. The first draft has been written.
