Analog Telephone Terminal Circuits and Functions
1. TERMINALS PHONE
1.1 GENERAL
The so-called telephone terminal is the transducer that allows for communication between the two points being routed through the central telephone service.
In the basic telephone network, the terminal is essentially a modulator of the current rate phone line to sound pressure exerted by the speaker and a headset, with or without an amplifier, to hear.
Indeed, the telephone line has a voltage of 48 V, with a negligible current when the telephone handset is hung. However, when you answer, provides a current of some milliamperes and the voltage drops to only a few volts. In this situation, the terminal circuits modulate the flow of the line with the impulses provided by the microphone, bringing the current level of 30 or 40 mA. Its frequency band is cut to about 3 KHz, which is sufficient for verbal communication.
Whereas the telephone terminal at its most elemental, this can be represented according to Figure 1.
The pair of wires R (Ring) and T (Tip) that is the analog telephone line connected to the telephone terminal, which can be considered as basic functional three elements, namely:
- Telephony circuits
- Electrical / electronic charge of the telephone line coupled to the input transducer (microphone) and output (headphone or speaker).
S1
Switch on which rests the whole headset – microphone
This is open when the phone is hung up and closes the hook to put in communication with line telephony circuits, enabling communication.
Between ends of wire pair R and T is the flame detector circuit (not seen here), which is off the hook.
S2
Carrier switch dialing. In this simple example, marking occurs by the system known as pulses.
Indeed, the conventional system is dialing current pulses, which causes S2 to close or open the rhythm of the keystrokes on the terminal. Thus, by dialing 1 is a cycle of opening and closing, and therefore no power-line current, and marking, for example zero, there are 10 pulses, which are transmitted by the line as signaling for the station are identified and routed. General scheme of a telephone dialing disc (detailed line switch)
.- Hybrid coil is used to transform the four threads of communication needed (two for transmission and two receiving) in only two wire loop.
1.2 DESCRIPTION OF AN ANALOGUE TERMINAL
An analog terminal is set by the circuits shown in Figure 2, functional blocks whose description is as follows: 
Input circuit
The telephone line is commonly connected to a diode bridge to ensure the polarity, thus preventing any errors in the wiring of the rosette that damage the circuits.
Also associated to the input circuit voltage limiter comprises a zener diode, thus preventing the possible line overvoltages affecting the remaining circuits of the terminal telefónico.-Figure 3 shows the circuit in detail. 
Call Detector
Circuit that is connected to the line through the switch S1 inthe-hook condition. Its purpose is to detect the dial tone and activate their corresponding buzzer.
Upon lifting the handset, S1 switches to the voice circuits and detector (ring signal) is disconnected, interrupting his acoustic signal.
The detector consists of a filter of 25 Hz and a piezoceramic buzzer driver amplifier or loudspeaker. The tone that sends the plant is a sine wave of that frequency and 70 V rms level, pulses are sequenced 1 ‘active 5 seconds, 3 seconds idle.
Voice circuit
As stated in the introduction, the voice circuit comprises a current modulator is connected to the microphone and a current-voltage converter to drive elauricular.
Figure 4 shows a detail of the circuit. 
Based on the above figure, we see that the microphone is plugged into an amplifier and the output of this to a power generator coupled to the line, thereby obtaining the condition indicated. When the telephone terminal is received, the line current IV converter is coupled and, from there, the handset or speaker. The circuit shows the presence of two complementary signals, which are:
Pulse / Tone Circuit implementing the dialing signal generated in another circuit. It applies to the voice circuit to modulate the flow of the line with his information because it is in the modulator.
MUTE signal to cause muting of the amplifier circuit earpiece and microphone while it lasts dialing.Dial Generator
The dial is made to the telephone line number information that is called. Such information can be made by pulses (current pulses), which is called decadal mode and through the generation of tones (frequency), so called DTMF stands for Dual Tone corresponding Multifrecuency.
Both modes have in common the incorporation of an encoder attached to a keyboard matrix, the first generates the pulse sequence shown to introduce into the telephone line and the second two-tone frequency generated by each key pressed. Decadal mode, the pulses have a frequency of 10 Hz, T = 1 / 10 = 100ms asymmetric signal with a time of 67 ms opening and closing of 33 ms, with an interval between digits from 250 to 800 ms. In DTMF mode, each digit is represented by two tones, as indicated, corresponding to each group under the lines representing the keyboard and the other to the top group associated to the columns. There is a difference in level between the two tones of 2 dB. Each tone has a duration of 50 ms, with separation between them of 45 ms. Figure 5 shows a detail of the keyboard and the value of the tones 
From that moment, the encoder informs the IC control line that was pressed a key and as a result of this control signal generates pure mute silence the voice circuit. In addition, through a digital to analog converter (D / A), we obtain the sine waves to transmit over the telephone line. The frequencies of these signals have the value indicated in the corresponding row and column as Figure # 5.
1.3 Description of the circuitry of the telephone terminal PHONIC-100
The telephone terminal PHONIC-100 is based on a business phone that has been didactificado in order to facilitate the recognition, understanding and measurement of the major circuits involved in performance analog telephone. Such circuits are:
Input circuit
Ring Circuit
Voice circuit
Circuit Marking
Such circuits are, mostly, in the integrated circuit A52533, with few features configurable external bridges to suit the requirements of the country’s telephone line is marketed.
The pinout (fig.7) block diagram (fig.8) and its operation are described below:
THE CPU pinout 
IC1 BLOCK DIAGRAM
In its most schematic, coach PHONIC-100 is composed of the elements contained in its block diagram (Fig. 9), which are described further in later sections.
According to that block diagram of the telephone line input corresponds to the R (Ring) and T (Tip), where S1-1 and S1-2 switch that hung beneath the receiver, which is represented in this diagram hanging position.
Rectifier bridge DB1 is responsible for ensuring the polarity for the other circuits, regardless of having R and T. For stages with transistors, operation is as follows:
Q1, 2 transistors function responsible for hanging / hook (connect the circuit telephony) as well as part of the pulse dialing when this mode is selected.
Transistor Q3 responsible for the modulation of the line current for the transmission of voice and sharp, both pulses (cutter offset) and Tone (DTMF).
Q4Transmisor amplifier tones applied to the buzzer to buzzer.
For its part, IC1 has the following functional blocks:
– Tone Generator for the ringer:
Generates three frequencies that are F1 = 800, F2 = 1067, F3 = 1.333 Hz This sequence forms the melody of the bell, which is composed of four repeats of the sequence F1, F2 and F3.
– Amplifier receipt, transmission and marketing. Form the voice circuit, according to the (fig.4).
– Encoder pulse / tone and control:
Combination of the encoder dial, reading the keyboard and control system.
After presenting the simplified block diagram is described below each of the circuit, shown in the following block diagram the main components. The details of the complementary components must be consulted in the overall scheme.
Input circuit (power)
The description of the functional blocks is as follows:
The handset is powered by the telephone line. Being a DC voltage is necessary to ensure the correct polarity for-performance electronic components, for which we appeal to the rectifier bridge DB1.
According to fig.10, where food continues to pick up the following route:
Ql telefónicaà DB1à Line AR9 à Pin27 ICl
The above terminal 27 (LI) of IC1 corresponds to your diet when you are off hook, since switches S1, DB1 closing the circuit – Q1.
In such conditions, the terminal 4 runs out of tension, with the pin 27 on which feeds the voice circuits (see block diagram of the integrated fig. N ° 8).
In on-hook, S1-1 switch, closing D81-R14, which means the path of the stream is as follows:
Arl Hotline R14 Apin AR3 Ã DB1 Ã 4 IC1.
BELL CIRCUIT
D2 and C34 to maintain regular seencargan VDD voltage that feeds the circuit RAM marker (callback function), also feeds the circuits for the timbre (tone generator and frequency discriminator timbre).
As indicated above, the current tone (timbre) is formed by a 70V AC voltage with an effective rate of 25 Hz This voltage is filtered by the network formed by C1-R2 (see fig.12). The zener diode D4 is responsible for setting the sensitivity (threshold) of the ring, and delivers the necessary impetus to pin 21 of IC1 (FCI). Inside IC1, the comparator circuit detects ring on the frequency of these impulses and activates the tone generator (melody) of the bell, with output terminal 8.
This melody affect the base of transistor Q4 ringer amplifier, which, through the ringer volume switch S3, feeds the buzzer ring, thus producing the required acoustic warning informs us that it is receiving a call .
VOICE CIRCUIT
The voice circuit is directly related to the dial circuit as shown in Figure No. 12.
As can be seen, the audio from the microphone modulates the line current in the same way it does tone dialing. For both functional and educational purposes will be described separately, although some of the associated circuitry are common.
Under the general scheme, the off hook condition, S1-1 closes a circuit that biases Q1, under such conditions, IC1 is fed through the terminal 27 (LI) (see fig.8) and the sound received is applied Reception entry, terminal 28 (RI).
The transmit audio is captured by the microphone and delivered to the input differential mode Ml and M2 preamplifier. This signal, a vezamplificada is uncoupled continuously and finally delivered by the terminal 25 (C5) of IC1 (audio output).
The transistor amplifier Q3 transmission, has a behavior equivalent to a variable resistor connected between the line input (LI) and mass. (V55). The result is that the current flowing through the telephone line is modulated by changes in conduction of Q3, being proportional to the variations of sound pressure incident on the microphone.
DIAL CIRCUIT
As described in previous chapters, there are two ways demarcation, which are:
– For pulses (cutter offset)
– Tone (DTMF)
These modes are described in the following sections:
Pulse dialing
The pulse dialing is the oldest and slowest of the two systems and, consequently, is that, for compatibility, all stations recognize the basic telephone network.
This pulse dialing is based on large variations in current, which is responsible Q3 (see fig.13), which, in this type of bearing, acts as a switch that shorts out the line before each pulse. These pulses have a duration of 100 msec (10 Hz), staying Q3 in saturation (shorting the line) 33 msec and 67 msec in court. Each digit is represented by as many shorts as has the number that represents, so that the No. 1 corresponds to a short circuit and the No. 0 ten.
For the telephone to recognize the beginning and end of each digit is introduced between digit range from 250 to 800 msec.
We will now see how the signal is generated which acts on the base of Q3, to which refers the block diagram of IC1 (fig.8).
The marker circuit is connected matricia1 keyboard, which consists of rows (R1, R2, R3, R4) and columns (C1, C2, C3, C4) so that pressing a key bind for the row and column , which triggers the following actions by the circuit marker:
– Informs the control logic that has been pressed a key to this run the corresponding action (increase or decrease the volume, redial, etc).
– Save in RAM corresponding to the digit key pressed. This RAM is responsible for storing the last number dialed for later use in the callback function.
-Activate MUTE to mute the microphone and the headset play confirmation tone having pressed the key.
-Acts on the DTMF generator, a digital to analog converter (D / A). Depending on how the MODE terminal is selected, the circuit will score different types of information to that digital to analog converter, which generates pulses with the voltage information (dialing decade) or sine-tones (DTMF). In this case (dial decade), the converter generates voltage pulses, the output of ICl terminalC5 which, after crossing the line adaptation block, affecting the transmission base of transistor Q3.
Tone dialing
Tone dialing is the most recent and rapid, but not all stations recognize the telephone network in this way.
For example, if after dialing the subscriber’s number will still receive the invitation to dial tone, it is certain that the phone is connected to an old plant that does not recognize this type of marking, the only possibility dialing Decades (pulse). Whenever possible, it is more advantageous tone dialing because of its speed.
In this section concerning tone dialing only shows the differences from pulse dialing as the rest of the identical operation.
As can be seen in Figure 13, the transmission transistor Q3 is connected through R9 in parallel with the telephone line.
In tone dialing, the behavior of Q3 is also a power modulator, which acts on the flow of the phone line module with the frequencies, corresponding to the pressed key. Each key is represented by two frequencies, a low frequency group and one of the top group. Their values are standardized and correspond with those given below. 
The operation of the integrated circuit TC1 only differs from that described in the pulse dialing as follows:
– The DTMF generator circuit receives information from one side of the circuit marker frequencies must be generated and other tension control that adjusts the modulation level to be adopted for these frequencies. Such a control voltage is applied to terminal 2 (MFL).
Subscriber Signaling
Subscriber signaling is signaling that occurs between the user of the telephone network and the nearest stations. In the basic telephone network, can be done by tones (both within the voice band and out) or by changing the voltage or line current.
The different types of signals that are involved in a process of communication are:
Signs of supervision or state: are used to perform a service request (eg making a call), and to maintain and close the established connection.
The situations in which signals are used in this class are:
The subscriber picks: In this case, it closes a switch so that current begins to circulate through the loop. The plant senses the increased flow and prepares to receive the destination address information.
The subscriber hangs up: the previous switch opens, leaving the previous current circulating through the loop. The power supply detects the decrease and close communication.
Malicious Call Signal: Used a special signal to alert the telephone that a call was received, which source you want to know.
Signal direction: used to send information about the location of the called subscriber (your phone number, etc.)..
The two forms of transmission of these signals are:
Sending Information decade: the digits that make up the destination number are transmitted by ticks. These pulses are nothing more than the previous switch fast switching. The 0 is represented by 10 pulses.
Sending multifrequency information: each digit is composed as the sum of two tones of different frequencies.
Management or pricing signals: used to send information to the terminal on the cost of the call which is ongoing at the time. This information is not used in all terminals.
In Spain, you can send in two ways:
Pulses of 50 Hz common mode for the case of public terminals in establishments (eg bars). The common mode means that the pulses are applied to both strands of the loop simultaneously.
12kHz pulses in differential mode for the telephone booths.
Signs of information: they indicate how the process is the call for the user.
The different ring tones that the user can hear are:
Invitation to dial tone: indicates the subscriber can begin to dial the destination phone number. 400Hz are used permanent.
Busy tone: 400Hz signal, being active 200ms and 200ms idle.
Ring Tone: 400Hz signal, being active 1.5s and 3s inactive.
Congestion tone: 400Hz signal, presenting the following structure of active and inactive times:
200ns on, off 200ns, 200ns on, off 200ns, 200ns on, off 600ns, 200ns on, 200ns …
Ringing tone indicates to the terminal that is taking place a call directed to it, for it to activate audible. It sends a 75V signal at 25Hz, while active 1.5s and 3s inactive.