The old-fashioned approach: modified SAT tuner for ADS-B

Please check out the web, you will find numerous sites that will tell you how to turn a surplus SAT tuner into an ADS-B received. Don’t fiddle around with old analog stuff, get one that has a proper demodulator (preferably, TDA8012), and a build-in PLL. The PLL – it’s easy enough to find out the pins going to the SCK, Data In, Data Out lines – it should be a TSA5055 running off a 4 MHz Xtal reference.

Step (1) – get the PLL to work. Just have a look at the TSA5055 datasheet, and clock in some divider data that make sense, and check with a receiver, or spectrum analyzer, if the LO is giving some useful output. The TSA5055 has a fixed:512 reference divider, therefore, with a 4 MHz reference clock, the phase comparator is working at 7.8125 kHz. For ADS-B reception, the main divider is set at around 12666 divisions, plus the :16 build-in prescaler, equals 1583.25 MHz LO frequency. Keep in mind, the SAT Tuner IF chain has a 480 MHz SAW filter, with about 27 Mhz bandwidth – therefore, I suggest to tune the receiver a bit to the edge (like 1103.25 MHz in my case, for 1090 MHz reception) – this will improve the shape of the output signal.

Step (2) – disable the AGC circuit of the TDA8012 by lifting Pin 9 of this circuit off the board (use a SMD soldering iron, and good lighting!). I connected Pin 9 through (unused) Pin 2 of the tuner to the outside world (33 k to VCC).

Step (3) – Now, some soldering – you will need a low pass and comparator circuit to convert the video, Pin 6 of the TDA8012 to digital output. The low pass is a simple 1 k/82 p RC network, the comparator a MAX903 (which is fairly high speed can can handle the 1 µs pulses, with fast risetimes at the output). Note that the MAX903 is open-collector, so you will need a 10 k pull-up resistor at the output.
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Step (4) – connect to antenna, check with a scope (best a digital storage scope, triggered by an ADS-B “frame start” pattern) for signal at pin 6, and tune the LC tank circuit of the TDA8012 a bit, until the signal is clear. Just a bit of bending of the coil with a non-metallic tool is all that is required, if any.
Reception quality can be optimized quite a bit by adjusting both the LO frequency (around 1580 MHz), and the TDA8012 LC tank oscillator.

Step (5) – add a bit of peripheral circuitry – a MAX232 for the serial interface.

Decoding is done with a bit of assembler and C code, using a ATMega8-16, run at 16 MHz. Frames are being checked real-time, and only the “good” ones forwarded the PC software, via a 19200bps (slow!) serial link.

Antenna!

You might ask, how to receive 1090 MHz? – fortunately, enough, not all that difficult. It’s old news: the best receiver is a good antenna. No need for anything fancy here, a bit of copper wire (silver plated, but any wire will do), and a piece of coax will do.
If you want to fabricate one for yourself – the coax needs to be connected about 10-12 mm, from the lower end of the loop. Short pole is 55 mm, long pole 205 mm, in my case – with the shield soldered to the short end (doesn’t really matter too much). The coax has a few turns, acting as a simple balun (you might want to put some heat shrink tubing over the semi-rigid coax, to prevent shorting of the loops).
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FYI, I found that it works well in the attic. So there is no need of weatherproofing, and the neighbors aren’t getting too curious because of numerous antennas sticking out of your home….. The roof isn’t all that strong, plywood and tar paper; both keep out the rain, but not the RF waves, fortunately enough…

SimonsDialogs – A wild collection of random thoughts, observations and learnings. Presented by Simon.