Modems for RTTY and SSTV. Scheme, description

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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
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Modems for RTTY and SSTV. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / Civil radio communications

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Today it is already difficult to establish who invented the simplest modem for RTTY and digital modes of information transmission (SSTV, PACKET and FAX), the diagram of which is shown in fig. 1.

Modems for RTTY and SSTV

With slight differences, his description was published in amateur radio magazines in many countries of the world. The receiving path of this modem does not differ from the node, the description of which was given in the note "We accept the "digit" and the image" ("Radio", 1999, No. 3). The cascade on the operational amplifier DA1 performs the functions of a comparator. If the level of the signal coming from the output of the ultrasonic receiver exceeds several tens of millivolts, then the output signal will be close to the supply voltage of the op-amp on the positive or negative bus (depending on the polarity of the input signal). In other words, this stage effectively converts the tone signal into a sequence of pulses with the levels necessary for their further processing by the computer. The optimal output signal levels of the modem are set automatically, since the power for the op-amp is taken from the signals of the computer's COM port. So, when receiving, when there is no request to send (RTS - Request To Send), there will be a voltage of about +10 V on the RTS bus, and a voltage of about -10 V on the DTR (Data Terminal Ready) bus. When switching to transmission, the situation changes to opposite, but the bridge on diodes VD1-VD4 will ensure the invariance of the polarity of the supply voltage to the op-amp. Capacitors C3 and C5 smooth out possible power surges during the transition from reception to transmission, and vice versa.

The computer-generated transmission signal (TXD bus) is fed to the microphone input of the transceiver through a two-section low-pass filter with a cutoff frequency of about 3 kHz. The filter limits the spectrum of the modulating signal to the values accepted in the voice path of an amateur radio station. The optimal level of the modulating signal is set by a variable resistor R3.

And finally, the cascade on the transistor VT1 controls the switching of the transceiver from reception to transmission. When a voltage of +10 V appears on the DTR bus (see above), this transistor opens and connects the PTT (Push To Talk) line of the transceiver to the common wire.

In the modem, you can use almost any general-purpose op amp (analog of op amp 741 - K140UD7), almost any low power silicon npn transistor (analog of transistor VS107 - KT342B) and any low power silicon diodes with a permissible reverse voltage of at least 25 V (for example, KD503A) . The recommendation to replace the transistor VT1 applies to semiconductor transceivers with a supply voltage of 12 ... 14 V. In lamp and lamp-semiconductor transceivers, the voltage on the PTT line can be several tens of volts, so it must be controlled and the transistor VT1 must be installed with the appropriate allowable collector voltage. emitter.

If the modem is supposed to be used only for RTTY operation, then it is advisable to introduce additional filtering in the modem to increase the noise immunity. A version of such a modem, based on the development of N1FB (Paul Pagel, Re-exploring HF / VHF digital and image modes on the cheap. QST, 1997, No. 4, p. 69), is shown in Fig. 2.

Modems for RTTY and SSTV

In the receiving path of the modem, an additional RC filter with a cutoff frequency of about 1 kHz and an L1,7C1 bandpass filter with an average frequency of 2 Hz and a -1100 dB bandwidth of about 3 Hz are introduced (compared to Fig. 600). In the transmission path, the cutoff frequency of the low-pass filter has been lowered to approximately 250 Hz and a signal level limiter (VD5VD6) has been introduced. The inductor in this modem is the primary winding of the UZCH output transformer from the transistor receiver. If it differs from 0,6 H for the existing transformer, then it is necessary to select a capacitor C2 of such a capacity to provide a resonant frequency of about 1100 Hz.

See other articles Section Civil radio communications.

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