Building a Simple Downconverter for the RTL-SDR

Over on YouTube Adam Alicajic, seller of the LNA4ALL low noise amplifier has uploaded a video showing how to create a simple downconverter using a 1.3 GHz local oscillator and an LNA4ALL. A downconverter extends the frequency range of the RTL-SDR to frequencies higher than the RTL-SDR’s 1.7 GHz limit.

Adam capacitively connects the 1.3 GHz local oscillator to the input of the LNA4ALL, which causes the input signal to be mixed with the input signal from the antenna. This moves a test 2.8 GHz signal down to 1.5 GHz, which is receivable by the RTL-SDR.

10 comments

  1. Pierre

    How does Adam manage to get such a low noise floor with a RTL-SDR dongle? Am I reading right? About -125 dB(FS) in SDR#?

    • Adam

      Hello Pierre,

      this value does not have any significant meaning unless you do not have a posibility to calibrate the scale, where the signal level will e represented with the real signal strength expressed in dBm.
      The dynamic range is not changed, just the side numbers. I prefer to have the real numbers in dBm to be able to make the calculations and relations easier.

      You can change the noise floor simply by editing the SDRSharp.exe.Config file with the Notepad.
      Open the file and find the following line:

      Instead of value -105 you probably have -60 or -50 ( I do not recall).
      Change this value and observe the numbers in your software.
      Do not forget to save the changes done in Notepad.

      Have fun,
      Adam

        • Pierre

          Haaaa… that was the trick! I think that I understand. I recall reading a forum thread somewhere which explains that SDR# shows in its spectrum analyzer the per-FFT-bin instantaneous strength of the signal as reported by the input device, in deci-Bel Full Scale (dBFS). So, simplifying, for RTL-SDR dongle providing over the USB stream I/Q samples at an unsigned 8-bits resolution (converted to a signed 7 bits sample by the osmocom driver), 0 dBFS in SDR# would correspond to an RF input signal strength reported by the osmocom driver to SDR# as peaking decimal -127 or +128 (or peaking at 128/SQR(2), perhaps? I need go back to my basics). Same reasoning for other devices (e.g. 16/24-bits soundcards, 12-bits Airspy, etc). And the SDR# fftOffset config setting allows you to “align” SDR# zero-dBFS to your specific device zero-dBm RF-input level (or which ever other logarithmic power scale of your choice), provided you can calibrate your device. I guess you came to -105 for your own dongle by measuring the dB-delta between your lab-calibrated signal generator dBm setting injected in your dongle and the dBFS reported by SDR# for that signal vertical bar in the spectrum analyser. I guess the default SDR# installation setting of -40.0 for fftOffset must correspond to an average RTL-SDR dongle dBm-dBFS calibration. Thanks for your answer! Pierre

        • Pierre

          If I was to build/ebay-buy a simple/cheap sine-wave generator, to experiment with injecting into my RTL-SDR dongle directly, what would be acceptable outputs for that sine-wave generator? In term of frequencies, I am clear, but in term of peak-to-peak voltage and output impedance, I don’t trust my too fresh radio ham theory not yet backed by practice. Thanks!

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  2. Craig

    This would be excellent for 2.4GHz video with appropriate software, thats what they were designed for anyway. Probably need new drivers too though.

    • Adam

      You wan’t to watch the ATV DVB-T on 2.4GHz? You do not need nothing better than the orriginal driver that you should have with the dvb-t stick.

      A simple downconverter and a VLC player and you are ready to go.
      Adam

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