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RTL-SDR Transmitter experiments

So, the LO leaks from the popular "RTL-SDR" dongles. This generally unwanted, but why not abuse it as a transmitter?

So we did.

Here's the original video from 2014:

And here's a quick and dirty RTTY QSO on 1270MHz with rtl-sdr sticks at both ends:


1.7.2016 New and better software released! 

This time we transceive! A tx only beacon software is also included. 
For enchanced TX power, consult this site:

I have crudely modified one stick for TX and currently get -45dBm measured output power on 1270MHz. 
Further modifications will likely increase this to a level where amplifying it to the milliwatt level will not result in a huge lump of noise. 
One problem with the amplifier is that you don't really want to amplify all the QRM from USB, 28.8MHz xtal and digital lines. So getting rid of low frequency gain in MMIC's would be useful here. 

More pictures and possibly new video will be added later. 

Some pictures of my current modified stick.

There is a bias-t in the tuner VDD line with some additional filtering. Some 100nF ceramics under the electrolytics and to ground. VCC also got extra filtering in the form of 1uF and 100NF to ground right at the usb connector. 

The coupling cap from tuner VDD to coax is a 100pF 0603 ceramic cap. The VDD line bias-t has a handformed conical choke and behind it three caps, 180pF, 1nF and 100nF. 
The wire connected to L6 is a fix as I accidentally cut the trace. The red wire on top deals with the cut DC trace.

Original article content:
The modulation heard in the video was achieved by changing the frequency, this resulted in FSK and is the reason why it can be heard on the FM handheld.
Later on a more sophisticated software with a FSK morse beacon was written. The frequency of 1270MHz was chosen because it was high to ensure more leakage and we had the Kenwood TH-55 handheld available and it had the highest frequency coverage of the equipment at hand. We also tried 433.550MHz and this had a range less than a meter.

With the modified drivers the frequency can be changed 300 times a second, so tones of 300Hz are possible. This also opens the door for various digital modes using MFSK or plain old FSK. Unfortunately the tuning steps of the RTL-SDR don't seem to fit any widely used amateur digital mode, with the possible exception of RTTY with some weird shift.

Interestingly the 4th "harmonic" of the frequency at the mixer leaks the best, this is likely result of the LO actually being on a higher freq inside the tuner and then divided to get the I and Q LO's
So the strongest output is at <transmit frequency>/4 - 3.57 MHz.
Also quite interestingly there is strong LO leakage at the VCC input of the tuner. One crude way to achieve voice transmission would be to injecting low level audio to the tuner VCC line, but this would require some mods to the tuner to tap it and that is not as easy and fun as this.

The modified RTL-SDR drivers and the beacon code will be released later when the author of them (not me) feels that the code is ready for release.

For those who cannot wait for the code to drop, the dongle leaks without anything done, it is just not modulated. The leaking LO should be visible +-3.57MHz form the receiving frequency indicated in the software used. This applies to the Rafael Micro R820 tuner, other tuners ware not tested.

The next step for me is to build a small amp with few MMIC's in cascade to boost the output to a few mW level for further experiments. As is the most use we see for this is as a simple weak signal source for testing receivers. With the amp we might be able to actually hold a contact longer distance than we can shout.

Credit for the idea, preliminary work and the code used goes to OH2EAT. I just filmed the video and put it on youtube.