Multifunctional microcircuits of the MC34118 series for telephones. Reference data

Encyclopedia of radio electronics and electrical engineering / Application of microcircuits

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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).

Rice. 1a. The appearance of microcircuits

Rice. 1b. The appearance of microcircuits

The domestic analogue of the MS34118 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), blocking the receiving attenuator for the time of dialing, a supply voltage midpoint generator (FST), a bias voltage generator (U_cm) for DCHN, etc.

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

Table 1

Main technical characteristics at Tacr.av = 25 °С

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. The required gain of the microphone amplifier (MU) A1 in the operating frequency band (about 40 dB) is set by the appropriate choice of the external resistor R_MF (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 decreases 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%.

Rice. 2. Typical circuit for switching on a device powered by a telephone line

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 R_HF (R13), connected to terminals 6 and 7. The coefficient of nonlinear distortion of the paraphase amplifier does not exceed 0,3% in the frequency band 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 gain of the attenuators is +6 dB, and in the presence of background noise -20 dB.

Rice. 3a. Filter schemes, characteristics and basic design ratios

Rice. 3b. Filter schemes, characteristics and basic design ratios

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

audio frequency range can be assembled low-pass filter. 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.

Rice. 4. Amplitude-frequency and phase characteristics of microphone and pair-phase amplifiers

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 pair-phase amplifiers are shown in fig. 4, and the transfer characteristics of the level detectors of control (in Fig. 6 - a fragment of the real circuit of the level detector) for various 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. 7.

Rice. 5. Transfer characteristics of remote control level detectors

Rice. 6. Fragment of a real level detector circuit

Loud-speaking reception of the subscriber 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 MC34119R, MC34119D and MC34119DTB. The domestic analogue of the MS34119R microcircuit is K1436UN1.

Rice. 7. Dependences of the transmission coefficient of the receiving and transmitting attenuators on the control voltage

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 city; Publication: cxem.net

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