ADS-B uses Pulse-Pause-Modulation (PPM), the Bandwidth is 1 MBit/s and the length of the frame is 56 or 112 Bit, lasting 56 µs or 112 µs. To make things easier, we double the Bandwidth to 2 MBit/s and use ASK.

As we can set the Baudrate of the Serial Ports very precise, we used Pin 11 (SPI, MOSI) for that. With 1 Bit / 0.5 µs, we can reproduce the preamble, which takes 8 µs or 16 Bits, very precise. To represent the payload, we use 2 Bits to depict a LOW = 01 and a HIGH = 10. In order to avoid confusion, we use the word Dits to represent 2 Bits. (2 Dits = 1 Nibble).

A complete Packet

Block Diagram of our Modulator

The Teensy 4.0 is mounted on the bottom side. The complete circuit is powered via the usb port. It draws about 150 mA at 5.0 V.

The Synthesizer consists mainly of the following components (Minimum Equipment List) :

DEVICE	REMARKS
XLL535A90.000000I	Standard Clock Oscillator 1090.000000 MHz LVDS 3.3V, 50 ppm
ETC1-1-13TR	Signal Transformer 4.5-3000 MHz, IL 2 dB, Impedance Ratio 1:1
ADE-5+	RF Mixer, Level 7, 5 to 1500 MHz, used for the ASK
SF2321D	Low-loss SAW Filter, CENTER = 1090 MHz, BW = 30 MHz
TEENSY® 4.0	Teensy® 4.0 Development Board by PJRC
PCB	ADS-B-TX-2.zip from this website

Setting the Function Generator (DS345) to 2 MHz Squarewave, 0.2 Vpp with 0.1 V Offset and injecting this baseband into the IF-port of the Mixer, we can see the following Spectrum :



Resulting Spectrum of the Modulator

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