A Homebrew One Transistor Upconverter for the RTL-SDR
Recently Qrp Gaijin wrote in to us and wanted to share his experiences on building a one transistor diode-ring mixer upconverter for his RTL-SDR. An upconverter for the RTL-SDR translates low HF frequencies ‘up’ into ones that are receivable by the RTL-SDR. This is a different method to the direct sampling mode used in the V3 dongles to achieve HF reception.
Qrp Gaijin’s post goes into some detail about his circuit and shows the schematic as well. He also shows the results with an active loop antenna, RTL-SDR and the upconverter in some videos. In the email to us he also notes that his upconverter is still a work in progress as the LO is quite noisy and he suspects that it may be too weak to drive the 1N4148 based diode ring mixer. There is also no filtering on the circuit yet, so there is some broadcast FM breakthrough.
Another project he worked on was attempting the direct sampling mod on a standard RTL-SDR. However, Qrp Gaijin’s method is slightly different to most attempts as instead of soldering the wires into the Q-branch holes he simply uses hot glue to hold them mechanically in place. This may be an idea to consider for those who want to attempt the mod on a standard RTL-SDR, but don’t have any soldering tools or experience.
Agreed that a commercial mixer would be better — in terms of physical balance, diode matching, transformer specs, and shielding. My circuit was a just a junkbox build based on parts I had on hand, in order to do some first tests with HF up-conversion and the RTL-SDR. I’m encouraged with my HF reception results so far so I’m looking forward to improving my circuit and maybe even upgrading to a better dongle at some point in the future (with a higher-resolution ADC for greater dynamic range).
Thanks for the comments, you have given me some ideas with your design, I’m going to try an SBL-1 with a 100MHx DIL Osc I have and see what happens, I hope I can find an SBL1-1-X in my spares box which operates to 100KHz.
If you are happy with >1MHz you could try using a MiniCircuits SBL-1 instead of the 1N4148’s and associated toroids. Should be better matched, but not sure about the Osc level for it ?
I updated my blog article with some new findings: (1) The LO signal is actually clean, and the noisy CW signals were fixed by reducing the software sampling rate from 2.8 MS/s to 2.4 MS/s, and (2) Perhaps the LO drive is actually sufficient because a level-7 mixer (such as a homebrew 1N4148 diode ring mixer) can be driven with only +3 dBm, and the performance is only slightly degraded. In practice, I’m very happy with my HF upconverter now. The only thing left to do is to add some filtering to cut down interference from broadcast FM signals.