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Old Man Mike

Reducing 1280 Mhz interference to GPS & FASST

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After observing some interference to GPS and FASST from my 1280 Mhz Video TX, I decided to try designing a microstrip PCB to eliminate the interference. I soon found that low pass filters were not effective since 1280 Mhz and 1575 Mhz are so close together. Instead, I ended up using notch filters. Here is the initial analysis for the filter (red is attenuation and blue is Ref Coeff which is like SWR) as designed in the Agilent Genesys program:


Rusty used his CNC milling machine to fabricate the boards. We had to iterate the design a few times to compensate for the variables in the PCB material but here is how the tuned prototypes looked:


I think it matches the objective design quite well. Normally it is quite difficult to get a lot of attenuation at the GPS 1575 Mhz signal and still maintain a low loss at the 1280 Mhz Video TX frequency. This tuned stub notch filter approach achieves that with only 0.4 dB loss at 1280 Mhz. In addition, the stub notch at 2440 Mhz keeps the second harmonic of 1280 Mhz from interfering with 2.4 Ghz R/C control systems like FASST. Although Spread Spectrum systems at 2.4 Ghz are usually quite resistant to interference, they can be overwhelmed by a very strong signal near the operating frequency. When you have a 1 Watt 1280 Mhz Tx next to the FASST RX on the flying platform, there will most often be some range reduction if a filter is not used on the 1280 Mhz Tx.

Rusty has just announced the availablility of these filters at his website:


For anyone who's interested in the notch filters that Mike designed, I've listed them on my site now.


At the moment, I've only got 11 because I ran out of connectors. I've got an order in for about 100 more, but all these things come from China, and that free shipping takes a few weeks.

I think I've added all the important comments on the web page, but if there are other questions, ask Mike :P



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