Using the RTL-SDR as a Transmitter

Back in July of last year we posted about a video from oh2ftg where he showed how he was able to get his RTL-SDR to act as a crude transmitter by using the RTL-SDR’s leaky oscillator.

Now another RTL-SDR experimenter, Oscar Steila (IK1XPV) has had a similar idea to use the RTL-SDR as a transmitter, and has taken the idea further than OH2FTG did. 

Oscar decided to take a standard RTL-SDR dongle and modify it so that it outputs a signal from the mixer output of the R820T tuner chip. To do this he removes some unneeded components from the PCB, and wires pin 5 of the R820T to the MCX antenna port through a 100pF capacitor. Pin 5 is connected to the mixer output from inside the R820T chip.

TX mod for the RTL-SDR.
TX mod for the RTL-SDR.

After performing the hack the RTL-SDR is able to output a signal anywhere between 500 MHz to 1500 MHz 1.8 GHz to 3 GHz (see why). To control the output frequency you simply need to tune to the frequency you want to transmit at in SDR# (after setting an offset to account for the R820T’s IF offset). This tunes the mixer in the R820T and causes the output frequency to change.

In the future Oscar hopes to take this idea further by creating a specific tuning application for the generator and finding a way to possibly FM modulate the output.

Using SDR# to tune the TX RTL-SDR, and using another instance of SDR# and RTL-SDR to receive the 1GHz signal.
Using SDR# to tune the TX RTL-SDR to 1 GHz, and using another instance of SDR# and another RTL-SDR to receive the transmitted 1 GHz signal.

Update: Oscar has revised the frequency range from 500 – 1500 MHz to 1.8 GHz – 3 GHz. More information about his new tests can be found at http://www.steila.com/SDR/RFgenmod/index.html.

8 comments

  1. Alex

    I would like to know if it would be possible to use this transmitter, as an ADSB transmitter? Has anyone tried to do it?
    I know that rtl chips are used in order to track aircrafts with ADSB signals, what about creating a code to use it as a transmitter?

  2. Max

    So what will be the output frequency if you tune 10, 100, 400 Mhz ?
    I’ve read on the datasheet that the typical xtal for R820T is 16 Mhz. How will the Output frequency change if you change the xtal with 16 Mhz?
    Sorry for the silly questions but i have not realized yet how the circuit works exactly 🙂

    • OH2FTG

      I have not completely forgotten about this RTL transmission thing. But I got some more pressing projects like a 3400MHz transverter and beacon.
      The long term goal is to make a RTL-to-RTL QSO over distance of 1km using modified sticks for TX and RX.
      Oscar should be able to add a low FM tone by switching between tow frequencies, creating FSK, like my original video shows, it can then be received on a normal rig.
      I was planing on exploring the various ports of the RTL for the best LO leakage while still preserving receive capability for tranceiving with a single stick. tons of work to do and sticks to modify.

Post a comment

You may use the following HTML:
<a href="" title=""> <abbr title=""> <acronym title=""> <b> <blockquote cite=""> <cite> <code> <del datetime=""> <em> <i> <q cite=""> <s> <strike> <strong>