FlightAware Discussions

ADS-B Filter Question

I apologize if this has been asked before, but can someone please explain to my why the RTL-SDR cannot be programmed through the software to filter/notch out frequencies above or below the 1090mhz area?

It seems to me that this would be rather easy to program, no? If a mechanical filter can do it, why can’t the software?

Thanks.

If they are very strong then they can overload your SDR which causes very poor performance, even on other frequencies.

If a signal is strong enough, it may cause overload, i.e. many (unwanted) signals show up on the spectrum display that are not present at the antenna input. Also, if we listen to a desired signal, another signal (if strong enough) may cause a reduction of the S/N ratio of the desired signal.

https://www.rtl-sdr.com/measurements-on-rtl-sdr-e4000-and-r820t-dvb-t-dongles-image-rejection-internal-signals-sensitivity-overload-1db-compression-intermodulation/

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SWEEP/SCAN BY NOISE GENERATOR

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does this block diagram of a “typical” dongle help? you can see there is filtering but if the input signal has too much energy and the overload occurs pre filters than the in-line filter (ie before the dongle) makes life much easier for the dongle
http://aaronscher.com/wireless_com_SDR/rtl_sdr_info.html

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The dongle is programmed to filter out frquecies above and below the designated frequency of operation (1090Mhz +/- 1.5 Mhz for Adsb), but the Cell/Mobile signals are so strong compared to ADS-B signal that this filtering is insufficient, and additional filtering is required to eliminate these XXXL Cell/Mobile signals.

Please see the relative intensities of 1090 Mhz and Cell 850 Mhz & 950 Mhz in scans below:

scan by rtl_power-1a

scan by rtl_power-1b

scan by rtl_power-1c

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You can find the relative strength of signals in your area by scaning using method given here:

Do I Need A Filter?

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Thank you all for the quick replies. I appreciate all of the replies and am going through them. I do have a few things I can do to improve the signal, including optimizing the coax to decrease losses.

Looks like I may need to upgrade some hardware for the best signal.

Thanks again.