4800bps PSK modem. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Data transfer
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In a packet radio network, phase (two-level) modulation at a rate of 4800 baud is successfully used.
There are two types of modems:
- on loose elements (in the 561GG1 demodulator) the modem is designed for connection to the TNC2 controller
(click to enlarge)
- single-chip modem on MS AT89C2051 by Atmel was developed as BAYCOM (it can also be connected to the TNC2 controller).
(click to enlarge)
Radio station requirements:
- the frequency band of the receiving path for the IF is not lower than 15 kHz (highly desirable is a good linearity of the frequency response in the passband);
- linear characteristic of the HF path RX-TX in the frequency band 300 Hz - 8 kHz;
- the switching time of the PRM-TX radio station is as short as possible (preferably no worse than 100 ms, TXD 10).
It is recommended to use radio stations like "Mayak", "Transport", "Palma" with minor modifications. Of the imported ones, only those that have a digital input for 9600 bps are suitable. The refinement of the stations consists in the fact that from the modem the signal to the input of the modulator should be applied directly to the VCO varicap (without filters). The LF output from the PRM should also be removed before the pre-correction filters. Preference should be given to stations with electronic switching RX-TX (including antenna switch on pin diodes).
When mass access is working, the speed in such a channel is approximately 230 bps. When optimizing the channel parameters for point-to-point operation, the speed can reach 300 bps or more.
The single-chip version exists in the form of a manufacturing kit (dock, flashed Atmel, board). It has a program DSD (in the BAYCOM version it is used only to indicate a free channel). The board is slightly larger than a matchbox. Works without tuning, there is nothing to adjust!
Publication: cxem.net
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