RTL-SDR upconverter

Some time ago I purchased TECSUN PL-600. It was nice radio to start with. Then I got FUNCube Dongle, and it was great fun to play with SDR. I connected both devices: FUNCube dongle to AM IF of TECSUN receiver. This was my first SDR setup able to receive HF band. As we all know, this is the band where most fun is!

After that, Remco encouraged me to experiment with diode mixers.

This is initial version of the converter: something I started with, and it was performing nicely since January until September:

I used SCM-1 mixer, almost any Mini Circuits mixer would do. Choices are: SBL-1, SRA-1, ADE-1 or RMS-1.
100nF ceramic and 16µF electrolytic capacitors near generator should be enough. It could be then powered directly from USB. Here it was powered from “general 12V supply”, so I wanted good noise isolation.

My test board:


  • 70-to-125MHz filter is a disaster
  • there should be capacitor between generator and attenuator (was added later)
  • capacitor between generator output and mixer should be gone (was later moved to generator)

The best generators for LO (local oscillator) were those in metal cans. Plastic ones (I tested only two EPSON) had too large noise, visible around every stronger signal.

In final version 3.3V 100.000M CO19025 was used. It worked on 5V supply for almost 4 months non-stop.
SARONIX 75.0000MHz was used before, also in metal can, and it was also good.

Antenna used was K9AY with bistable relays as direction switches (material for another article).

Overall performance was good. Converter covered all bands, worked with both FUNCube Dongle PRO and RTL-SDR. The signal was too broadband though, and sometimes RTL-SDR overloaded. Mostly annoying were strong AM stations, and LO leaking through the mixer. To solve “AM stations” problem I built another input filter: band-pass 3500 to 5500 kHz. Believe me, most interesting things happen between 3 and 6­ – 7 MHz (sample). Also, next version of converter allowed me to receive weaker stations, but that’s another story.

The Borg style clock

Few days ago I discussed with a friend about making Nixie clock. After a while we concluded that nothing compares to neon discharge light, not even 1970-s LED matrix displays. And green colour looks Borg-ish. That gave me the idea…



Yep, those are green 5×7 1970′s LED matrix displays I had in my junkyard. And this will be The Borg style clock. I don’t like those tiny SMD transistors though. They simply don’t belong here.

Nice thing about using matrix displays is that they can display text or graphics, not just numbers. They have no driver, so it’s possible to address any particular pixel. There’s also drawback: it’s tricky to make them work. More effort, more possibilities.

After another evening:


More photos here. That’s just a part of driver! Most part however: three stacked 74ALS373, de-soldered from some old board. There are 5 segments, 7×5 each, which gives 7×25 LEDs. Cathodes are connected as rows, anodes are columns. The last left alone resistor is 25th column, left to be connected to microcontroller’s GPIO. Rows will be connected to ground through transistors driven by shift register.

Now I’m waiting for replacement transistors. I just hope I didn’t make any mistake there!