AFSK Modem

The modulator and the demodulator (pdf < 10 kb). For the easier tuning of the modem it could have been built with potentiometers, but it was easier to reach a smaller size this way (I didn`t want to use SMD potentiometers). Unfortunately, even like this, the size is still too big, because I couldn`t get SMD boxed ICs
AFSK Modem - schematic

. Basically the size is determined by the two RCA and the DSUB-9 connectors. At testing, connecting the modem`s input and output without a radio, the modem works perfectly. With YEASU FT-xxxx and similar radios a good and stable connection can be achieved. It is especially good for remotely controlling devices with a simple radio. It is also useful for remote measurement data collection, remote control of models and robots. Unfortunately the original design with the XR2206 and XR2211 pair didn`t work because the temperature dependence of both IC are too big. They work just fine at room temperature, but they become unstable at temperatures of -10. +50 C. For the solution of the transmitter side problems a program-controlled AFSK modulator, running on AVR MCU, was built. It`s operation is very simple. The sin wave is assembled from 11. 25 degree (32 steps) peaces as the function of the FSK input. Thus, for example, for the generation of a 1200 Hz wave from 32 samples, the samples need to be delivered to one of the 8 bit ports of the AVR with 38400 Hz (2200 Hz *32 = 70400 Hz) frequency. To this port a simple R-2R (8 bites) D/A is connected with an RC low-pass filter at the end. Learning from the trials above, an entirely quartz controlled modem was built. Even though it is a non-standard modem, it still has a wide range of applications, and it is completely temperature independent. It uses three frequencies 950 Hz, 1200 Hz,...

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