The simplest radio modem for PC. Scheme, description

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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
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The simplest radio modem for PC. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / Data transfer

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This radio modem scheme is "rolling around" all over the Internet in large numbers, but I will repeat it anyway. This radio modem is terribly easy to create and operate, which is interesting not only for beginner radio amateurs. Despite its simplicity, it works at speeds up to 9600 baud.

To work with this radio modem, a 286/16Mhz computer with the appropriate software, a VHF or HF transceiver with a PTT control socket is sufficient.

The simplest radio modem for PC. Radio modem scheme

This modem connects to the RS-232 port and is powered directly from it (RTS and DTR signals) and does not require additional power for the operational amplifier. The diode bridge serves to provide power regardless of the potentials on the RTS (Request to Send) and DTR (Data Terminal Ready) contacts.

The LF signal from the output of the transceiver is fed to the input of the operational amplifier. The minimum level of this signal must be at least 100 mV. The output signal from the operational amplifier is fed to the DSR (Data Set Ready) pin.

When transmitting, a PTT signal is applied to the corresponding input of the transceiver. A positive voltage from the RTS line puts the transistor T1 into a saturated state. At this time, a TD (Transmitteg Data) signal is applied to the microphone input of the transmitter, which causes carrier modulation. The digital signal is changed to a sinusoidal one with the help of an RC filter, and the PT1 potentiometer helps to set the appropriate signal amplitude at the output, depending on the sensitivity of the microphone input of the transmitter.

The simplest radio modem for PC. Radio circuit board

The entire radio modem is mounted inside the plastic housing of the HD25/HD9 adapter (female-male) using the printed circuit board shown above.

Required radio components

Resistors	Capacitors	Diodes	Chip	Transistor
R1 82 kOm	C1 100 nF	D1-D5	ULY7741	BC107
R2 82 kOm	C2 100 nF	1N4148
R3 1,5 kOm	C3 2,3mkF-16v
R4 4,7 kOm	C4 2,3mkF-16v	replacement	replacement	replacement
R5 4,7 kOm	C5 10 nF	KD521	TL061	KT315
PT1 22 kOm	C5 10 nF	KD522	KR574UD1	other npn

Publication: cxem.net

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