Scheme of a universal frequency radio modem for 300/600/1200/2400/4800 baud. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Data transfer

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The modem is intended primarily for use in packet radio systems, but can also be used in general purpose radio systems and wired systems.

The modem does not require a mandatory hardware packet communication controller, it works with any software controllers such as L2, TFPCX, TFX. When using the Kiss-mode emulator, TFKISS can use the TNC software controller of TCP / IP programs such as KA9Q, TNOS, JNOS. When using the ISO OSI standard (X25/AX25 protocol) of the subscriber program of the fuel dispenser and the program of the Host-mode FBB, the speeds are doubled due to software compression by 2 times. Similarly - when working according to the ARPA standard (TCP / IP protocol). The modem includes 3 domestic microcircuits of the 561 and 1401 series (K561GG1, K561IR2, 1401UD2B). In total - 59 elements. Functionally, the modem consists of two completed parts-blocks: a receiver and a transmitter. The receiver and transmitter sections operate completely independently. The total current consumption of the entire modem does not exceed 3,2 mA in the +5 V circuit. The modem on the TCM3105 chip contains two cases and has 419 parts. With almost the same number of elements in both modems, the undoubted advantage of a modem based on domestic element base is the possibility of obtaining higher nominal rates for broadband voice frequency channels.

(click to enlarge)

At speeds of 2400/2800 Baud, the proposed frequency modem is also close in characteristics to the duplex FFSK, GMSK, 4-L FSK microcircuits FX469, FX589, FX909, FX919. It has high sensitivity and high failure rate statistics in interference conditions. But unlike the listed single-chip radio modems, it does not require a direct current connection to the transmitter varicap and from the output of the receiver's frequency detector. This limitation is a major barrier to working with radios that do not have these modem pins. In addition, the modem does not reproduce scrambling errors, as in the GMSK modem, which does not allow the latter to work with signals below 8...9 points on the S-meter.

The presented frequency modem can have five regulators:

- regulator of the middle frequency of the transmitter;
- transmitter deviation controller;
- regulator of average frequency of the receiver;
- receiver deviation controller;
- Receiving signal duty cycle control and reception band switch (2,5/5 kHz).

The modem scheme is shown in the figure. The receiver is built according to the PLL scheme. At the input, a zero detector of the D1.1 microcircuit is installed. From the output of the zero detector, the signal is fed to the frequency detector of the PLL of the D2 chip. The output of the PLL is fed to a second-order low-pass filter on the D1.2 chip. At the output, a threshold device is installed on a D1.3 chip. The PLL loop constant is determined by the R4 C4 chain. Capacitor C7 determines the frequency. Resistance R11 sets the center frequency, and R12 sets the frequency deviation.

The receiver requires the threshold element to be adjusted by setting the resistance value R14 to obtain a duty cycle of 2. The transmitter is based on continuous phase FM modulation. There may be the following types of ratios without phase break: 0,5/1; 0,75/1,25; 1/1,75; 1/2 subcarrier cycles.

The resistance R23 and the diode VD2 provide the key mode of operation of the control element of the transmitter VCO (microcircuit D1.4.), which is the transistor VT2. The resistances R26 and R 17, together with the capacitors, determine the frequency parameters.

Resistance R24 and capacitors Cll, C12 determine the frequency parameters of the transmitter.

The speeds and frequencies are shown in the table, where FH is the lower frequency, Fcp is the average frequency value, FB is the upper frequency.

The table is compiled in such a way as to ensure minimal switching when going over 1200/4800.

At 300/600/1200/2400 baud the modem operates in a typical voice frequency channel, at 4800 baud - in a non-standard voice frequency channel (double width).

The VCO of the transmitter is made on a D1.4 chip. Its output signal goes to a ring counter. The ring counter performs the functions of forming a stepped sinusoidal signal modulated in frequency with a continuous phase (16-step sinusoid without phase break at the output of the resistive adder). The spectrum of the output signal thus obtained has a minimum width of 1,15 of the deviation band. The VCO operates at 16 times the deviation frequencies listed in the table and is controlled by the input bit stream.

Transistors VT3 and VT4 are used to shunt the output signal and control the transmitter on. On the elements VD1, VD3, a parasitic power supply circuit from the RS232 computer is assembled, and on the DD4 chip, a voltage regulator of 5 V is assembled.

The LED VD1 (AL107A) is an indicator of the hangup of the RS232 port, an indicator of the reception and transmission. The indication is carried out on the basis of a partial change in the luminosity of the diode. The following display modes are available. If the program is unloaded from RAM, and the indicator is on, then the port is hung. A general glow means that the modem is working and the TNC resident program is loaded into RAM. When transmitting, the brightness of the diode VD1 decreases by 50%, and when receiving - by 30%. By changing the parameters of the resistances R11, R12, R24 smoothly or discretely using switches, it is easy to obtain the required transmission rates in a regular voice-frequency channel or in a double-width channel. To obtain five speeds, these resistances can be made in the form of a line of three to five resistances mounted on a hinged switch or on a board. It is most convenient to perform a number of these resistances based on the resistances of SP 19-1. The inaccuracy of setting the nominal frequencies of 20% does not have a significant effect on the operation of the circuit.

Reducing the capacitance value of capacitor C8 by half gives double the bandwidth. The design of the modem may be as follows. The modem is made on a double-sided board 75 mm wide and 78 mm long. The board is soldered into the ends of two 25 pin connectors. The case has a U-shape and covers the height and length of 3 mm connectors. The lower flat part of the housing is inserted between the thrust flanges of the connectors. The case is fastened without screws due to latches from below into the sidewalls of the U-shaped case. Switches and variable resistances are installed on the underside of the board. For access to them, holes are provided in the lower flat part of the body.

Publication: cxem.net

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