1200 Baud AFSK Modem   
- With XR2206 FSK modulator and XR2211 FSK demodulator
- AVR MCU Software AFSK modulator and XR2211
- Entirely AVR based modulator and demodulator ( non-standard solution)

2002.01.21.  XR2206 and XR2211

AFSK Modem's circuit diagram : 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. 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.

XR2206 datasheet (192kb, pdf)
XR2211 datasheet (243kb, pdf)

Standard AFSK modem built with XR2206 and XR2211 ICs

2002.04.09.  AVR software AFSK modulator

 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.
A sample program for AT90S2313.

2002.05.29.  TriTone Modem   -   a digital  solution (AT90S2313)

  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, 1500 Hz. Selecting frequencies that are so close to each other made the programming of the demodulator much easier, and we saved a little bit on bandwidth (it is important because of the use of radios for the transmission). The 950 Hz is the so called Guard signal. When there is no transmission the receiver uses this frequency to check the existence of the connection with the transmitter. The 1 and 0 bits are delivered by the 1200 Hz and 1500 Hz frequencies respectively. All frequencies are created digitally (DDS). The modem has a standard RS232 connector, the settings are: 1200 bps, 8,N,1. In the heart of both the receiver and the transmitter there is a AT90S2313 MCU with 4 MHz clock. The transmitter generates a rectangular signal which goes through an RC low-pass filter before going into the radio's input. On the receiver's side there is an RC low-pass filter also. The signal goes through this first before getting into the 2313 comparator, from this the MCU re-assembles the data by measuring period times. Because of the limited frequency transmission capabilities of the radio, it transmits sin wave signals.
For his help in building the modem, I have to say thanks to Laca.
The modulator's program for At90S2313, and it's circuit diagram (gif)
The demodulator's program and it's circuit diagram (gif).

2007.12.13.  TriTone Modem II  -  Full-duplex  (ATmega48)


Back to the "projects" page