Multifunctional microcircuits of the MC34118 series for telephone sets. Reference data

Encyclopedia of radio electronics and electrical engineering / Reference materials

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ICs MC34118P and MC34118DW (Motorola) are designed for use in high-quality speakerphones with voice switching. They are produced in a plastic rectangular case of two design options - MC34118R for traditional mounting - 710-02 (DIP) and MC34118DW for surface mounting - 751F-05 (SOIC) - with 28 pins (Fig. 1, a and b, respectively).

The domestic analogue of the MC34118 microcircuit is EKR1436XA2.

The microcircuit contains a microphone amplifier (MU), controlled receiving and transmitting attenuators, two background noise identifiers (IFN) for receiving and transmitting paths, an automatic gain control (AGC), an attenuator control unit, an output paraphase linear amplifier, an amplifier for forming a high or low filter frequencies. In addition, there is a set of auxiliary units - four level detectors (DU), a frequency dial detector (DCHN), a blocking receiving attenuator for the time of dialing, a supply voltage midpoint generator (FST), a bias voltage generator (UCM) for DFS, etc.

The pinout for both design options for the microcircuit is the same. It is listed in the table.

Main technical characteristics at Tacr.av = 25 °С

• Supply voltage, V.....3,5...6,5
• Current consumption, mA, at a supply voltage of 5 V.....5
• Current consumption in the blocked state, μA, at a supply voltage of 6,5 V ..... 600
• Input resistance at the blocking output (pin 3), kOhm ..... 90
• Gain factor of microphone and paraphase amplifiers, dB.....40
• Input impedance of the microphone amplifier, kOhm.....90
• Open-loop gain of microphone and paraphase amplifiers OS, dB, at a frequency less than 100 Hz.....80
• The highest gain frequency of the microphone and paraphase amplifiers, MHz ..... 1
• "Dead zone" of blocking voltage of MU and blocking of a microcircuit, V.....0,8...2
• Input voltage of attenuators (terminals 9 and 21), mV, not more than.....350
• Input impedance of attenuators, kOhm.....10
• Maximum load current, mA, microphone amplifier (pin 10).....1
• attenuators (pins 8 and 22).....2
• paraphase amplifier (terminals 6 and 5) ..... 5
• Harmonic coefficient of the end-to-end path, %, no more, in the receive mode..... 0,5
• transmission.....0,8
• Crosstalk attenuation between the paths of reception and transmission, dB..... 52
• Output voltage of the artificial midpoint shaper (pin 15), V, at a supply voltage of 3,5 V ..... 1,3
• 5 V ..... 1,8-2,4
• Output impedance output 15, Ohm.....400
• The maximum current of the external load of the voltage driver of the artificial midpoint (pin 15), mA ..... 0,5
• The level of limitation of the output voltage of the two-phase amplifier at the output of NTO- (pin 6), V, from above (at an output current of -5 mA) ..... 3,7
• bottom (+5 mA) ..... 0,25
• The level of limitation of the output voltage of the two-phase amplifier at the output of NTO + (pin 5), V, from above (at an output current of -5 mA) ..... 3,7
• bottom (+5 mA) ..... 0.45

The microcircuit is characterized by a very high temperature stability of the parameters.

A typical circuit for switching on a device powered by a telephone line is shown in fig. 2.

(click to enlarge)

The required gain of the microphone amplifier (MU) A1 in the operating frequency band (about 40 dB) is set by the appropriate choice of an external resistor RMF (R6) connected to pins 10 and 11. When a DC voltage of more than 12 V is applied to pin 2 (MUT), the amplifier is blocked and its gain is reduced to -39 dB. If the blocking function is not supposed to be used, pin 12 is connected to a common wire (pin 28). The coefficient of non-linear distortion of the amplifier in the frequency band of 0,3 ... 10 kHz does not exceed 0,15%.

Two amplifying stages A10, A14 form a two-phase amplifier (terminals 5-7) necessary to match the transmission path with a two-wire telephone line connected through an external isolating transformer T1. Just like in a microphone amplifier, the required gain is set by choosing the resistor RHF (R13) connected to pins 6 and 7. The coefficient of nonlinear distortion of a paraphase amplifier does not exceed 0,3% in a frequency band of 0,3 ... 10 kHz.

Transmitting and receiving attenuators A7, A9 (pins - inputs 9 and 21, outputs 8 and 22, respectively) are used to switch from reception to transmission and vice versa in half-duplex mode. Duplex mode cannot be implemented in loud-speaking telephones due to the strong acoustic coupling between the loudspeaker and the microphone, which leads to self-excitation of the path.

The attenuators are controlled by signals generated by a special unit, which analyzes the signals from the line and microphone coming to its inputs through level detectors (DU) A2, A4, A11, A13 (as well as other signals) and automatically switches the telephone channel from reception to transmission and back. Thanks to this, when talking, you do not need to hold the handset in your hands, press the phone to your ear and speak into the microphone. Thus, the "Hands-free" function is implemented, which distinguishes this phone from traditional ones.

In the absence of a conversation (standby mode), the attenuation of each attenuator is -20 dB. In the reception mode (the subscriber at the near end listens) and transmission (the subscriber speaks), the difference in attenuation of the attenuators reaches 52 dB.

To suppress the acoustic noise of the room and noise in the line in the transmitting and receiving paths, background noise identifiers (IFN) A3, A12 are provided. They distinguish speech signal, which has sharp changes in amplitude, from background noise, which has a constant level. As a result, with a speech signal, the attenuator gain is +6 dB, and in the presence of background noise - 20 dB.

The amplifying element F A15 (pins 1 and 2) with external resistors and capacitors form a high-pass filter that protects the receiving path from mains interference with a frequency of 50 Hz (and its harmonics) on the line wires. If it is necessary to correct the frequency response of the receiving path in the upper region of the audio frequency range, a low-pass filter can be assembled. The input impedance of the filters is 1 MΩ, the output impedance is less than 50 Ω. Filter schemes, characteristics and main calculated ratios are shown in fig. 3, a, b.

Automatic gain control (AGC) A5 is designed to ensure stable operation of the receiving path in conditions of high signal attenuation in the telephone line. When the voltage in the line decreases to 3,5 V (recall that the microcircuit is powered from this line), the AGC acts through the A8 control unit on the receiving attenuator, reducing the current consumption and thereby preventing a further decrease in voltage.

Some of the most important typical characteristics of the MC34118 chip are shown in fig. 4-6.

The amplitude-frequency and phase characteristics of the microphone and paraphase amplifiers are shown in fig. 4.

The transfer characteristics of the level detectors of control (here - a fragment of the real circuit of the level detector) for different ratings of the input circuit elements - in fig. 5.

The dependences of the transmission coefficient of the receiving and transmitting attenuators on the control voltage are shown in Fig. 6.

Loud-speaking subscriber reception is provided by an external power amplifier 34118 MC34 connected to the MC34119 chip. This microcircuit is produced in a plastic rectangular case of three design options - 626, 751 (SO-8) and 948J (TSSOP); respectively MC34119P, MC34119D and MC34119DTB. The domestic analogue of the MC34119R microcircuit is K1436UN1.

The MC34118 microcircuit is designed to work together with a dialer based on the MC145412 microcircuit, which has a memory device for 10 numbers, which, when the device is disconnected from the line, supports an external 3 V power supply.

Author: V.Khmartsev, Moscow

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