DIY "FM TRAP" or 88-108 MHz Band STOP filter

Among the few "blockers" the most called are the broadcast WFM transmitters. Simple way to solve the problem is to filter the received signals using the "FM TRAP". So called "FM TRAP" is nothing more than a simple Band stop filter for the frequency range of 88-108Mhz. They became really available and so cheap this days that making them at home is more time consuming then money saving. 

But for the guys want to learn how they are designed, and does not want to spend on the shipping filters round the globe here is the receipt and how to guide. On the other hand, if you will need the band stop filter for some frequency other than FM broadcast you will know how to make it at home instead of spending the $$ on the e-bay. Basic idea was simple approach to the problem and solving the same using the freeware software and calculators. The second goal was using the standard components and values that are common and can be found easily. The coils will be wound using the magnet wire and this will reduce the cost of the filter too. At the end, you are free to use any type of the coaxial connector you prefer, so you will not depend only on the standard F-connector.

Let's start. After we decide the range that we want to "block" (88-108 MHz) we need to decide how sharp or step filter we need and the attenuation in the stop band. Yeah, you probably think that there is nothing to decide, we know the FM band (88-108 MHz) If you use this corner frequencies (-3dB) in your design the attenuation of the signal at the beginning and the end of the target range will be low, just 3db, in real life even lower. As we want the uniform attenuation across the whole FM band we need to choose the wider filter range. To make the things simple, I choose the Chebyshev type filter, 3 pole, first element series with the center frequency of 100MHz and bandwidth 55Mhz with the ripple 0.1dB. Of course, the impedance should be 50 ohms or 75 ohms if your system is using 75 ohms for the standard. Setting the mentioned parameters on a free online web filter calculator: http://www.changpuak.ch/electronics/chebyshev_bandstop.php and pressing the calculate the results will appear in the lower window. The required capacitors are 2x 56pF and 1x 20pf. 56pf is the standard value while the 20pf is not. Instead of 20pF you may use the 18pf or 22pf or two parallel 10pF. I used 22pF in my project. 

As we will make the coils using the magnet wire, the values are not critical at all. We can calculate the required inductance based on the wire we have and the former we want to use. I wound mine coils using the 0.35mm dia magnet wire on the drill bit dia 5mm used as a former. Of course, all three coils are air wound. To calculate the required number of turns you can use several on-line calculators. I prefer the following one: http://hamwaves.com/antennas/inductance.html

It is smart idea to check the stop band frequency characteristic and the other performance of calculated filter before taking the soldering iron in hands. Free software that can do that is the RFSim99 that can be found on the web. No installation is required, just running the exe file. Let's draw our filter and simulate it. The result is really good, but only in the perfect world. In the real world the attenuation in the stop band will not be so deep but the curve can give us idea how wide the filter will be and the center frequency. We can tweak the capacitor/inductance values to tune the response of the filter, but calculated values will give us the best results. 

Once we are good with the design we can build the filter. I used double sided FR-4 laminate 1.6mm thick but the others are good too. The PCB is just 1cmx3cm and the traces were cut using the exacto knife. Instead of the SMD type capacitor, the standard lumped parts may be used hence the bigger size PCB and complete filter. The filter can be even made without PCB, all parts soldered in the air style inside the metal container and coaxial connectors. If you stick to the calculated values the filter should be very close to the design and no tuning should be required. If you have the possibility to measure the filter on some network analyzer then you can tweak the filter to the required response as I did. 

I stretch the center coil (5 turns) a bit to obtain uniform attenuation across the required range (at least 30dB) rather than having on some frequencies attenuation of 50dB and a bit lower not more than 25dB. Obtained result can be seen on the following photo. The -3dB stop band range is from 79.5Mhz to 123.3MHz. The -10dB stop band range is from 85MHz to 119MHz. The attenuation on the required range 88MHz-108MHz is 30dB or better with the maximum attenuation of -44.6dB at 103.4MHz. 

If you compare this results with the design done in the RFSim99 simulator the figures are lower but the shape of the filter is the same. This is to be expected due to simple simulation paradigm used and no electromagnetic simulation. More accurate predictions and calculations are possible using very expensive software and this is not the scope of this post. To recapitulate done, we designed the 3 pole stop band filter with the attenuation of 30dB plus in the range of 88-108MHz. The attenuation outside the designed range is really low, where the insertion losses up to 1.7Ghz are bellow 1dB and flat response over the range as can be seen from the lower picture.

What you need to design this "FM trap"? You need:

2x 56pF capacitors (SMD 0805)

1x 22pF capacitor (SMD 0805)

2x 45nH inductance (2.5 turns, 0.35mm wire inner dia 5mm)

1x 126nH inductance (5 turns, 0.35mm wire inner dia 5mm)

Additionally, if you want to have a neat unit then 2 coaxial type connectors, prefer the female SMA type or the combination of the female and male connectors. You can save on connectors if the coaxial cable (preferred Teflon type) is soldered directly to the filter input/output with the braid to the ground shield. The shield/metal box is important. You want to shield the filter properly not allowing the FM signal to penetrate the system using the air wound coil as antenna. If the signals are really strong that can degrade the quality of your design. On the other side, the dongles should be shielded too as the radio signals are passing through the plastic housing without significant attenuation. i made my shield using the empty motor oil can. the shielding is tailored using the simple scissors. Watch your fingers :-)

Designing filters using the lumped elements can work up to 500Mhz. Anything above that will be difficult to create using the lumped elements where instead the micro-strip line technology or some other approach should be used. Good luck and have fun with building the filters.