Wednesday, December 9, 2015

DIY Low-pass 200 MHz filter

Every while I am receiving requests for some custom filter design. Time permit I apply to the call and sometimes this projects are simple and fun to build. This is one can be handy and simple to build using lumped elements with the standard off the shelf values. There was a request for the 200 MHz Low-pass filter, so here it is, maybe somebody else may find it useful in his project.

This is standard Low-pass Chebyshev 5 pole filter. The values are chosen to meet the standard capacitor values 18pF and 33pF. This should be easy to find. The coils are wire wound (home-brew) 65nH. I used the speaker wire 0.4mm dia. where 6 turns on the 3mm drill bit will bring us  pretty close to 65nH. Quick simulation gave us the wanted result with the corner frequency of 200Mhz (-3dB). Simulated attenuation at 400 MHz should be -39.2dB and -57.8 at 600MHz.

As I have some extra spare boards left from my ADS-B and L-band filters i decide to use them for this project. The boards are made of FR-4 1.2mm. They are SMA connector ready so just the coils and capacitors should be added. Extra parts mounted are Bias-T parts so the filter can be used also to power remote LNA through the coaxial line. Above simulation is done without Bias-T circuitry.

Filter from the picture was measured and the results are very close to the simulation. Small difference at the lower frequency is due to Bias-T creating a notch filter. When the Bias-T circuitry was simulated too, the S21 and S11 were quite close to real figures. 

Corner frequency was shifted to 184MHz and this can be tuned to 200MHz simply by stretching a bit both coils. I was quite happy with the results where attenuation on 400MHz is 40dB and 54db at 600MHz. This filter can protect the receiver from the strong broadcast TV stations, GSM cell tower signals but also from the quite spread telemetry on 420-470MHz band. Quick test of the filter can be seen on the following video.


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