Circuit Diagram Recognition 8-Basic Methods and Steps for Analyzing Circuit Diagrams

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Circuit Diagram Recognition 8-Basic Methods and Steps for Analyzing Circuit Diagrams [Copy link]

There are many kinds of electronic equipment with different functions, and their circuit diagrams are also different in complexity and simplicity. A simple circuit diagram has only one unit circuit and a few components, while a complex circuit diagram often contains many unit circuits and thousands of components. When analyzing circuit diagrams, we should follow the ideas and methods from the whole to the part, from input to output, breaking the whole into parts, and gathering the parts into the whole. Use the principle of the whole machine to guide the specific circuit analysis, and use the specific circuit analysis to interpret the working principle of the whole machine. Usually, you can follow the following steps. 1. Understand the overall function and main technical indicators of the circuit diagram

The circuit diagram of a device is designed to complete and realize the overall function of the device. By understanding the overall function and main technical indicators of the circuit diagram, you can have a basic understanding of the circuit diagram at a macro level. This is the first step in reading and recognizing diagrams.

The overall function of the circuit diagram can be analyzed starting from the name of the device. For example, the function of a DC regulated power supply is to convert AC 220V mains power into a stable DC voltage output, as shown in the figure below;

[color=rgb(34, 34, The function of the superheterodyne radio is to receive the broadcast signal of the radio station, demodulate it and restore it to the audio signal for playback, as shown in the figure below,

The function of the superheterodyne radio is to receive the broadcast signal of the radio station, demodulate it and restore it to the audio signal for playback, as shown in the figure below,

[color=rgb(34, 34, The function of infrared wireless headphones is to modulate the sound signal of the audio equipment on infrared rays and transmit it, and then the receiver receives and demodulates it and restores it to sound, which is played through the headphones, as shown in the figure below.

The figure below shows the circuit diagram of the smart TV field strength meter. This section will take this circuit diagram as an example to specifically describe the method and steps for analyzing the circuit diagram.

Smart TV field strength meter, as the name implies, its function is to measure the field strength of TV signals, the main technical indicators are:

Measuring range: The measuring range is 1~60 channels, divided into three frequency bands: L, H, and U, with channel indication and 8 preset tuning functions.

[color=rgb(34, 34, Field strength range: The field strength range is 0~120dB microvolt, with automatic shift attenuator (40dB high frequency attenuator and 20dB medium frequency attenuator). Automatic level comparison: With automatic level comparison function, the level setting range is 0~120dB, and it has an audio prompt function. Second, determine the direction of the signal processing flow of the circuit diagram. The circuit diagram is generally drawn in the order of the signal flow being processed and according to certain customary rules. The analysis of the circuit diagram should generally be carried out according to the signal processing flow. Therefore, when analyzing a circuit diagram, it is necessary to clarify the direction of the signal processing flow of the diagram. According to the overall function of the circuit diagram, find the total input and total output of the entire circuit diagram, and then you can determine the direction of the signal processing flow of the circuit diagram. For example, in the DC regulated power supply circuit diagram above, the place where the AC 220V mains is connected is the total input, and the place where the DC regulated voltage is output is the total output; in the superheterodyne radio circuit, the magnetic antenna is the total input, and the speaker is the total output; in the infrared wireless headset circuit, the place where the audio signal is connected is the input of the transmitter, the infrared light-emitting diode is the output of the transmitter, the photodiode is the input of the receiver, and the headset is the output of the receiver. The direction from the total input to the total output is the direction of the signal processing flow of the circuit diagram. Usually, the circuit diagram is drawn by arranging the signal processing flow from left to right. 34)]In the circuit diagram of the smart TV field strength meter, since the function of the whole machine is to measure the field strength of the TV signal, the TV signal input terminal "IN" is the total input terminal of the circuit, and the level meter PA2 indicating the magnitude of the field strength is the total output terminal. The main signal processing flow direction of the circuit diagram is from left to right, which conforms to the customary drawing method.

Third, decompose the circuit diagram into several units with the main components as the core

Except for some very simple circuits, most circuit diagrams are composed of several unit circuits. By mastering the overall function of the circuit diagram and the direction of the signal processing flow, you will have a basic understanding of the circuit as a whole. However, to deeply analyze the working principle of the circuit, you must also decompose the complex circuit diagram into unit circuits with different functions.

Generally speaking, transistors, integrated circuits, etc. are the core components of each unit circuit. Therefore, we can use transistors or integrated circuits and other main components as a symbol to decompose the circuit diagram into several unit circuits according to the direction of signal processing flow, and draw a circuit principle block diagram based on this. The block diagram helps us master and analyze the circuit diagram.

For the circuit diagram of the smart TV field strength meter, the whole machine circuit diagram can be decomposed into the following 10 unit circuits based on core components such as integrated circuits.

1) High-frequency amplification and frequency conversion circuit composed of high-frequency head TDK-T34V, etc.

2) The first intermediate frequency amplifier circuit composed of integrated circuit IC3, etc.

3) The second intermediate-range amplifier circuit composed of transistor VT25, etc.

4) The detection circuit composed of crystal diode VD37, etc.

5) The buffer circuit composed of integrated operational amplifier IC4-1.

6) The level indication circuit composed of transistor VT26 and microampere meter PA2, etc.

7) Preselection tuning circuit composed of integrated circuits IC1, IC2, transistors VT1~VT24, etc.

8) Automatic attenuator circuit composed of integrated operational amplifiers IC4-2, IC5, IC6, etc.

9) Level comparison circuit composed of integrated operational amplifiers IC4-3, etc.

10) Sound prompt circuit composed of integrated operational amplifiers IC4-4, etc.

Among them, unit circuits 1~6 constitute the main signal channel, and 7~10 are auxiliary circuits. The block diagram of the whole machine is shown in the figure below.

Fourth, analyze the basic functions of the main channel circuit and its mutual interface relationship

For simpler circuit diagrams, there is generally only one signal channel. For more complex circuit diagrams, there are often several signal channels, including a main channel and several auxiliary channels. The basic functions of the whole circuit are realized by the unit circuits of the main channel. Therefore, when analyzing the circuit diagram, the functions of the unit circuits of the main channel and the interface relationship between the unit circuits should be analyzed first. For example, the main channel of the intelligent TV field strength meter includes 6 unit circuits: high-frequency conversion, first intermediate frequency, second intermediate frequency, detection, buffering and level indication, as shown above the dotted line in the figure above. The functions of the unit circuits of the main channel are analyzed below.

1. High-frequency conversion unit circuit

The function of the high-frequency conversion unit circuit is to receive TV signals and convert them into intermediate frequency signals. The input is a high-frequency signal and the output is an intermediate frequency signal.

2. Intermediate amplifier unit circuit

The function of the two-stage intermediate amplifier unit circuit is to amplify the intermediate frequency signal. Both the input and output are intermediate frequency signals.

3. Detection and buffer unit circuit

The function of the detection and buffer unit circuit is to perform peak detection on the signal. The input is an intermediate frequency signal and the output is a DC signal.

4. Level indication circuit

The function of the level indication unit circuit is to indicate the magnitude of the electric field. The input is a DC signal, and the output end is indicated logarithmically by a level meter. The working process of the main channel circuit is as follows: the TV signal is received, amplified and frequency-converted by the high-frequency head TDK-T34V, and the intermediate frequency signal is output from the IF terminal (pin 1). After being amplified by IC3 and VT25, it is detected by VD37, buffered by IC4-1 voltage follower, and driven by the meter PA2 to indicate the field strength level.

V. Analyze the function of the auxiliary circuit and its relationship with the main circuit

The function of the auxiliary circuit is to improve the performance of the basic circuit and increase the auxiliary function. After understanding the basic functions and principles of the main channel circuit, the functions of the auxiliary circuit and its relationship with the main circuit can be analyzed.

The part below the dotted line in the above figure is the auxiliary circuit of the intelligent TV field strength meter, including four unit circuits: pre-selection tuning circuit, automatic attenuator circuit, level comparison circuit and sound prompt circuit.

1. Pre-selection tuning unit circuit

The function of the pre-selection tuning unit circuit is to select the measured signal, which can be pre-set and has a channel indication. This auxiliary circuit improves the convenience of instrument operation.

2. Automatic attenuator unit circuit

The function of the automatic attenuator unit circuit is to realize automatic shifting during the measurement process to expand the measurement range. This auxiliary circuit improves the automation level of the instrument.

3. Level comparison and sound prompt unit circuit

The function of the level comparison and sound prompt unit circuit is to compare the measured signal with the set level and issue a prompt sound. This auxiliary circuit adds additional functions to the instrument.

VI. Analysis of DC power supply circuit

The DC working power supply of the whole circuit is a battery or a rectifier-regulated power supply. The power supply is usually arranged on the right side of the circuit diagram, and the DC power supply circuit is arranged from right to left, as shown in the figure below. R and C1, C2 in the DC power supply circuit form a decoupling circuit to eliminate harmful coupling that may be formed through the power supply circuit, which is very common in circuit diagrams composed of multi-level unit circuits. The DC power supply of the smart TV field strength meter includes +12V working voltage and +30V tuning voltage. The +30V tuning voltage is sent to the tuning terminal Tu (pin 4) of the high-frequency head through the tuning potentiometer (RP1~RP8) in the preselector to tune the frequency. The +12V working voltage is sent to the three frequency band control terminals BL, BH, BU (6, 7, 9 pins) of the high-frequency head through the frequency band selection switch (S1~S8) in the preselector to select the frequency band, and is also sent to each unit circuit as a working power supply.

Seventh, detailed analysis of the working principle of each unit circuit

Based on the overall analysis of the above circuit diagram, each unit circuit can be analyzed in detail to clarify its working principle and the role of each component, and calculate or calculate the technical indicators.

The following still takes the circuit diagram of the intelligent TV field strength meter as an example to introduce the working principle of each unit.

1. High-frequency conversion unit circuit

This circuit uses the electrically adjustable high-frequency head TDK-T34V as the high-frequency and frequency conversion part of the field strength meter to amplify the input TV RF signal and convert it into an intermediate frequency signal for output.

2. Two-stage intermediate frequency amplifier circuit

The integrated circuit uPC1651 is used as an intermediate frequency amplifier with high gain, stable operation and simple circuit. Transistor VT25 and other components form an intermediate frequency amplifier, which is a resonant amplifier. The primary of intermediate frequency transformer T1 and C7 form a parallel resonant circuit, which is the collector load of VT25. The primary of T1 is equipped with a tap to increase the Q value of the resonant circuit. 3. Detection circuit Crystal diode VD37 and C10, R54, C11, R55 and other components form a peak detector, so that the level indication is only related to the peak value of the synchronization signal, eliminating the measurement error caused by the different brightness of the TV signal screen. 4. Buffer circuit The integrated operational amplifier IC4-1 constitutes a voltage follower, which is located between the detection circuit and the level indication circuit, thereby increasing the load impedance of the detection circuit to further ensure the high Q value of the second intermediate frequency amplifier resonant circuit. 5. Level indication circuit The transistor VT26 is a logarithmic element, which enables the meter PA2 to directly indicate the logarithmic level of the TV field strength. 6. Preselection tuning circuit This circuit uses the octal counting distribution circuit CD4022 (IC2) to control the 8-bit preselector (including S1~S8, RP1~RP8) to realize the preselection tuning of the high-frequency head. The meter PA1 indicates the selected channel. The light-emitting diodes VD1~VD8 indicate the working status of the preselector. 7. Automatic shift attenuator34)]This circuit uses the method of controlling the gain of the high-frequency head and the second intermediate frequency amplifier to realize three 20db attenuators (2 high-frequency and 1 intermediate frequency). IC4-2, IC5-3, and IC5-4 form three Schmitt level comparators with an upper threshold level of 60dB and a lower threshold level of 40dB. The access or exit of the three attenuators is automatically controlled according to the output level of IC4-1. The light-emitting diodes VD38, CD39, and VD40 indicate the access status of the three attenuators respectively.

8. Level comparator

[color=rgb(34, 34, IC4-3 constitutes a level comparator to compare the measured field strength level with the set threshold level. The threshold level can be set within the range of 0-60dB (adjust RP9). S1~S12 are level value-added switches. When turned on, the set values can be increased by 20dB, 40dB, and 60dB respectively. 9. Sound prompt circuit When the measured field strength level reaches the set threshold level, the controllable oscillator IC4-4 starts to oscillate, and the buzzer HA gives an audible prompt.