In this video I demonstrate the function of front end filters that I constructed for use with my RTL 2832 SDR to reduce intermod from strong local AM and FM stations.
The toroids I used for the 1.8 MHz high pass filter are all FT50 type 61 material. I am using 24 gauge magnet wire and they have 10, 16 and 13 turns on them to get a computed (not measured) inductance of 6.9 uH, 17.7 uH and 11.7 uH respectively. I am using the Mini Ring Core Calculator by DL5SWB to determine the number of turns required for each inductor. Note that the inductance values listed for the toroid inductors are only calculated an not measured as the 1.8 MHz high pass filter was my first attempt at filter design and construction and I did not have a meter that would measure inductance at the time. You really want to measure them if possible as the toroids do have some variance in the inductance they give for a known number of turns of wire.
I use hi-q ceramic capacitors with 5% tolerance in 50 v and 100 v ratings in in various brands that I ordered from Allied Electronics (www.alliedelec.com/).
I am using FT50-61 toroid cores that I ordered from Kits & Parts (www.kitsandparts.com)
NOTE: The FM traps are no longer available from MCM. If I manage to find another source for them I will link them here.
The FM traps from MCM Electronics (mcmelectronics.com) I ganged together are part # 33-341 and cost about $3 from their website.
The aluminum case is from MPJA (www.mpja.com) and can be found at this link here: http://www.mpja.com/350in-X-137in-X-118in-Die-Cast-Aluminum-Box/productinfo/16283+BX/
The HF high pass filter was designed using Elsie design software. SDR Sharp is the SDR software. The student version of Elsie filter design software that was used to design all of my filters. This can be found at www.TonneSoftware.com
Note: When I switch either the FM or AM filter in, you will notice that the signal decreases slowly. This is due to the setting I use for the signal decay rate in SDR Sharp and not an effect of the filter itself.
