Sunday, March 3, 2013

Reducing EMI in RTL SDR

Many people have noticed that even if there is not an antenna connected to the RTL SDR. FM Stations still come strong. This is not good at all because these are signals that are getting into the PCB via another way that is not the antenna. The most common tips for reducing EMI that are over the Internet for the RTL SDR is to use shielded USB cables, ferrite beads and a metal box. I was already using a shielded USB extension cable, and I mounted a ferrite bead near the RTL SDR. (At least 2 are recommended but I was just able to get one) The problem is that the USB cable is going to act as an antenna.

I also put aluminum foil inside the RTL SDR and it was touching the USB chassis ground. It seemed to help a bit but there was still a lot of noise. Any local FM station still goes through adjusting the gain. Near my home is a local FM transmitter that has always caused me problems, especially because I listen to air band. I use an AOF-128 bandpass filter and a coax stub filter and this still is not enough to attenuate completely the signal while listening with my PRO-95 scanner, so you can imagine how critical for me is reducing EMI in the RTL SDR for good performance.

I started to read about the USB specs and tips for reducing EMI while designing USB devices. There is a lot of information and different opinion about this. In general what I found out is that the shield on a USB cable is soldered with the male USB connector (in the host) but on the device is not but here comes the most important thing I found out. If in the USB device, the chassis is connected to ground, it is to a different ground, and not the ground to the PCB (To the chassis for example) or some designs have two grounds on the same PCB. Because that’s where it’s going to get the noise if the USB cable shield is connected to the USB chassis, so what I did on the RTL SDR, it’s to remove the male USB connector, and solder a cable with a male USB connector, and just solder the 4 cables (signal and power) and leave the shield floating. Then, where I cut the cable I put aluminum foil and then covered it with electrical tape.

This really reduced the FM broadcast interference. Even if the gain is set to maximum on the RTL SDR, the only station that comes some noise is the one that is my neighbor, but it’s really nothing to what was there before. I think if I got more ferrite beads and mounted the RTL inside a box it would be totally quiet.

 Hope this can help other people and I would love to hear comments.

10 comments:

  1. Hi Luis! PU3HAG Huelbe, from Brazil. I'm sending your URL to Reddit on RTL-SDR :) http://www.reddit.com/r/RTLSDR/

    Quick question: did you connect the aluminum foil to the some other point? Like, the cable shield (it if has one) or the dongle's chassis or ground?

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    1. Hello, that cable I used does have a cable shield, but is not connected to the RTL SDR in anyway.

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  2. Luis! I did this with my worst nosiest SDR, with ferrite and a cheap metal case from radioshack. I can't thank you enough. The difference was incredible. My worst is now my best. Thanks a bunch man!

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  3. Could you estimate the noise floor difference in db?

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    1. That's a really hard number to measure, but I can give you some empirical information. I have a FM Transmitter at around 500m of my home. Completely line of sight and it causes a lot of frontend overload and basically I can tune to it on the RTL SDR with the manually gain set to 0dB and AGC off. The signal was strong completely, but now with this mod I need to set the gain to at least around 20dB (without antenna) to listen to it.

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  4. Hi Luis,
    good article and even better idea!
    Following your article, I took apart my R820T dongle and put around the PCB an inner layer of paper and an outer layer of aluminium foil (which touches the USB shell and the MCX connector ground). Now without antenna and with gain at max nothing comes into my R820T!
    BTW I'm not using shielded USB cables (anyway the USB receptor shell is connected to the PC chassis, internally) or ferrite beads (with them I don't get any significant noise reduction) and it still doesn't pick up so much noise.

    Good work, man!
    Antonio

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  5. Luis thanks for the article. Can you post pictures of the inside so it would be easier for the less technical people.

    Thanks
    Mike

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  6. Hi, I was building an up converter. I am stuck on an issue that, I soldered wires from USB cable to up converter board and connected a USB female socket and dongle connected. From same port up converter also powered. Now when i open SDR# it says dongle not detected or some prob with driver. If I connect dongle in different port it works. Cant we power dongle and upconverter from same port?

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    1. I had the same problem! It's your USB cable creating a big voltage drop! Find a USB cable with a lower gauge wire. (My ebay discount usb cable did not cut it.) Also keep the terminating wires to the rtl short
      and to the up-converter.

      Test with your multimeter on the upconverter board (+5v rail and ground) and you will see the before and after.

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