After experimenting with telemetry systems based on dedicated FSK transceiver chips and with analog image transmission through SSTV, it is time to try a new, more challenging project: an high speed IQ modulator based transmitter for digital television: DATV. If this works we will be able to focus our future stratospheric telescopes live
I will leave image acquisition and compression for another time, now I will focus on data transmitter.
The I/Q waveforms are generated using a microcontroller and two high speed DACs with differential current outputs. I/Q waveforms are amplified and fed into an I/Q Modulator. The circuit will also have a dedicated LO with a frequency of ~2.35GHz. Block diagram of IQ modulator based transmitter can be found on the data sheets of all major RF IC manufacturers
Fig 1: IQ modulator based transmitter block diagram
Using an IQ modulator offer a great flexibility in choosing the modulation techniques, not only multi level digital like QPSK or 16-QAM but it can also be used for simpler or analog modulations like FSK or FM [1]
Fig 2. FM modulator using IQ modulator based on reference [1]
To accelerate the development of this project I got 2 evaluation boards from Analog Device and I build some fast and dirty circuits around a STM32F7 MCU
Fig3: Test setup
After writing some firmware and soldering parts on the test PCB it was time to check if the signal looks good on the spectrum analyzer. To make sure I have a working testing setup, I generated I/Q samples from Matlab and exported them in Stm32IDE.
Fig4: spectrum analyzer measurements
To receive the signal and check it is correctly decoded I will use ADALM-PLUTO and “DVB-S GUI AMSAT”
Fig 5. RX software
Apparently we are having some sort of constellation. At least we are good up to this point.
After some heavy RF circuit design I now have full-fledged modem
Fig 6. S Band Modem
Andrei Hapenciuc 