Simple modem for HF and VHF packet communications. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Data transfer
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The modem is designed to work as a package on HF and VHF. The circuit is based on a single-chip microcomputer AT89s1051. Demodulation of the input signal is carried out in software using a digital filtering algorithm with statistical processing elements to improve the noise immunity of reception. The precision comparator built into the microcircuit made it possible to do without an external operational amplifier.
LEDs V1-V4 are the simplest spectrum analyzer. When working on HF, this allows you to accurately tune in to a working station. LED V5 lights up when the received signal does not fall into the receive channel. When set correctly, LEDs V2 and V3 should glow brightly. A low level of illumination of the other LEDs indicates the presence of interference in the receiving channel. By adjusting the signal level or the input attenuator of the receiver, you need to achieve the lowest possible brightness of their glow. Statistical processing allows you to work with a significant noise level in the useful signal.
The comparator is tuned by resistor R3 in the HF reception mode (switch S1 is open). The input is a sinusoidal signal. By adjusting R3, you need to achieve a duty cycle of pulses at pin 3 of the processor equal to two. Gradually reducing the signal level, the procedure must be repeated until the maximum sensitivity is obtained. After that, you can apply a real signal and verify the presence of a demodulated signal at pin 11.
A sinusoidal transmission signal is generated using the simplest DAC on resistors R8-R11. The peculiarity of this solution lies in the formation of a signal without a phase break, which favorably affects the quality of reception of such a signal. Resistor R11 sets the modulation level for the radio.
On diodes V6-V9, resistors R14-R17, a circuit for interfacing TTL levels with the signal level of the COM port of the computer is assembled. The diagram shows the names of the port signals and the numbering of the 9-pin connector of the computer for working with the well-known BAYCOM package program. It allows you to work on both HF and VHF. If you intend to connect the modem to the TNC controller, the interface circuit can be omitted. In this case, the signals are connected accordingly: processor pin 8 to the TNC receive/transmit output, pin 11 to the demodulated data input to the TNC, pin 9 to the data output from the TNC. The SOUND, MIC, PTT signals are connected to the radio station, respectively, to the low-frequency output, to the microphone input, and to the receive / transmit control input.
If it is supposed to work only on VHF, all LEDs can be excluded, in this case the current consumption will decrease and will be about 10 mA.
The frequency of the quartz resonator can be any in the range from 9 to the maximum frequency of the microcircuit. When the resonator frequency is changed, constants are calculated for the operation of the digital filter and the two-tone signal generator, which are then written to the microcircuit.
The modem is assembled on a single-sided printed circuit board measuring 3x5 cm.
Publication: cxem.net
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