60 questions about hardware design – electronic components、circuits…

60 questions about hardware design - electronic components、circuits…

Hardware design involves the creation of physical components and circuits used in electronic devices. It encompasses various aspects, including selecting and integrating electronic components, designing circuitry, and ensuring proper functionality. Here’s a brief introduction to key elements of hardware design:

Electronic Components: Hardware design begins with selecting appropriate electronic components such as resistors, capacitors, inductors, diodes, transistors, integrated circuits (ICs), and sensors. Each component has specific properties that contribute to the overall functionality of the circuit.

Circuit Design: Circuits are the building blocks of electronic devices. Hardware designers create circuits by connecting electronic components in specific configurations to perform desired functions. Common circuit configurations include amplifiers, filters, oscillators, and logic gates.

Schematic Diagrams: Engineers use schematic diagrams to represent circuits visually. These diagrams illustrate the connections between electronic components, their values, and their relationships within the circuit. Schematics serve as blueprints for building and troubleshooting electronic circuits.

PCB Layout: Once the schematic design is complete, hardware designers create a printed circuit board (PCB) layout. This involves arranging electronic components on a board and designing copper traces to connect them according to the schematic. PCB layout design is crucial for optimizing signal integrity, minimizing interference, and ensuring proper heat dissipation.

Signal Integrity: Signal integrity refers to the quality of electrical signals as they travel through a circuit or PCB. Hardware designers must consider factors such as impedance matching, signal propagation delay, and noise immunity to maintain signal integrity and prevent signal degradation.

Power Distribution: Efficient power distribution is essential for reliable operation of electronic devices. Hardware designers design power distribution networks (PDNs) to deliver stable power to all components while minimizing voltage drops, noise, and electromagnetic interference.

Testing and Validation: Hardware designs undergo rigorous testing and validation to ensure they meet performance specifications and functional requirements. Testing may involve simulation, prototyping, and real-world testing to identify and address any issues or shortcomings.

Compliance and Standards: Hardware designs must comply with industry standards and regulations governing aspects such as electromagnetic compatibility (EMC), safety, and environmental impact. Compliance with standards ensures interoperability, reliability, and safety of electronic devices.

hardware design - electronic components、circuits...Hardware is a highly complex system, and in the design process, one is bound to encounter more or less issues. This article summarizes 60 very basic problems for reference.

1. Please explain the difference between a ferrite bead and a filter inductor.

A ferrite bead consists of wire passing through a ferrite material. It has very low DC resistance and negligible impact on DC signals. At high frequencies (above several tens of megahertz), the impedance of ferrite beads increases significantly. High-frequency electromagnetic fields induce eddy currents in the ferrite material, converting high-frequency interference signals into heat and dissipating them. Ferrite beads are commonly used for power filtering in high-frequency circuit modules and for filtering high-frequency signal loops to suppress EMI interference.

An inductor consists of a coil and a magnetic core. It has relatively low DC resistance and high inductance. Inductors are mostly used for filtering in mid-to-low-frequency circuits, primarily focusing on suppressing conducted interference. Their application frequency is below several tens of megahertz.

2. Please explain the meanings of resistor, capacitor, and inductor package sizes: 0402, 0603, 0805.

These numbers represent package dimensions:
0402: 40*20 mil;
0603: 60*30 mil;
0805: 80*50 mil.

3. Please explain the meanings of the following letters representing capacitor tolerances: J, K, M, Z.

J: ±5%;
K: ±10%;
M: ±20%;
Z: +80% to -20%.

4. What parameters are related to the package size of resistors, capacitors, and inductors?

Resistor package size is related to resistance value and rated power;
Capacitor package size is related to capacitance value and rated voltage;
Inductor package size is related to inductance value and rated current.

5. What parameters should be considered when selecting resistors?

Resistor value, tolerance, power rating (calculate the maximum current and voltage it can withstand in the actual circuit), and package size.

6. What parameters should be considered when selecting capacitors?

Capacitance value, tolerance, voltage rating, and package size.

7. What parameters should be considered when selecting inductors?

Inductance value (including measurement frequency), tolerance, maximum current rating, and package size.

8. What parameters should be considered when selecting ferrite beads?

Impedance value (including measurement frequency), tolerance, maximum current rating, DC resistance (calculated for maximum DC voltage drop), and package size.

9. What parameters should be considered when selecting rectifier diodes?

Maximum rectification current, maximum reverse voltage, forward voltage drop, and package size.

10. What parameters should be considered when selecting MOSFET transistors?

Minimum gate-source voltage Vgs(th), maximum gate-source voltage Vgs(max), maximum drain-source voltage Vds, maximum drain-source current Id, on-state resistance Rds(on), power dissipation, and package size.

11. How should the rated voltage of output filtering capacitors for DC power supplies be chosen based on the actual operating voltage?

The rated voltage of the capacitor should be slightly higher than the DC output voltage. Typically, selecting a capacitor rated at 1.2 to 2 times the DC output voltage is sufficient.

12. What is the phase relationship between voltage and current at both ends of an ideal capacitor: in-phase, out-of-phase, voltage leads current by 90°, or current leads voltage by 90°?

Current leads voltage by 90°.

13. Please list some brands of diodes you know.

DIODES, FAIRCHILD, FH (Fenghua), FormosaMS, IR (International Rectifier), MCC (Micro Commercial Components), MOTOROLA, ONSemi (ON Semiconductor), PHILIPS, RECTRON, ROHM, TOREX, TSC (Taiwan Semiconductor), VISHAY, WTE (Won-Top Electronics), XUYANG.

14. Please list the uses of zero-ohm resistors.

1) Bridging jumpers on the circuit;
2) Optional configuration circuits;
3) Debugging reserved positions;
4) Fuses;
5) Single-point connections to different grounds.

15. Please briefly explain the working principle of a varistor.

When the voltage across a varistor exceeds a certain amplitude, the resistance of the varistor decreases, allowing surge energy to dissipate and limiting the surge voltage to a certain amplitude.

16. Please briefly explain the working principle of a PTC thermistor as a fuse in a power circuit.

When the input voltage of the power supply increases or the load becomes too large, causing an abnormal increase in current, the PTC thermistor quickly increases its equivalent resistance due to temperature rise. As a result, the output voltage drops, reducing the output current. Once the fault is removed, the PTC thermistor returns to normal temperature, and its resistance becomes small again, restoring the power circuit to its normal operating state.

17. Common surface-mount capacitor materials include X7R, X5R, Y5V, NPO (COG), and Z5U. Which type of capacitor has the most stable capacitance value and dielectric loss?

The most stable capacitance value and dielectric loss are found in NPO (COG) material capacitors.

18. For a certain ferrite bead with parameters of 100R @ 100MHz, please explain the meaning of these parameters.

At a frequency of 100MHz, the impedance value is 100 ohms.

19. Please list the functions of pull-up resistors.

1) Port level configuration during power-on reset;
2) Pulling up OC and OD gates to ensure high logic levels;
3) Increasing the high logic level of output ports;
4) Increasing the driving capability of output pins;
5) Reducing input impedance to prevent electrostatic damage;
6) Enhancing the electromagnetic interference resistance of the bus;
7) Matching resistors to suppress reflected wave interference.

20. What is the function of a common-mode inductor?

To suppress common-mode interference.

21. What do the numbers 0.25um, 0.18um, 90nm, and 7nm refer to in semiconductor or chip fabrication?

These numbers represent the technology nodes or process nodes used in semiconductor or chip fabrication, also known as process technology nodes. They have physical significance such as “half pitch,” “physical gate length,” and “process line width.”

The semiconductor industry typically uses parameters such as “half pitch,” “physical gate length” (the length of the gate of a MOS transistor), and “junction depth” to describe the integration level of chips. The smaller these parameters are, the higher the integration level of the chip.

For example, if a chip is manufactured using 90nm technology, the half pitch is 90nm, and the physical gate length of the transistors is 37nm. Half pitch refers to half the distance between interconnect lines inside the chip, which is also half of the lithographic pitch.

As each new technology node has historically been used for manufacturing DRAM chips, the latest technology node often refers to the half pitch of DRAM.

Additionally, in technical articles, there are two expressions similar in meaning to “half pitch”: “line width,” “line spacing,” and “feature size.” If the line width is equal to the line spacing, then the half pitch is equal to the line width and line spacing. They represent different expressions of the same data.

22. What is the typical forward voltage drop of a green light-emitting diode (LED)?

Around 2 volts.

23. What kind of voltage should be applied to a varactor diode and a zener diode under normal operating conditions: forward, reverse, forward for the former and reverse for the latter, or reverse for the former and forward for the latter?

Both should be subjected to reverse voltage.

24. If an LED indicator light doesn’t specify a color, which color would you choose among red, green, yellow, orange, blue, and white, and why?

According to convention, red is used for power indicator lights, and green is used for signal indicator lights. These two colors of LED lights have the most mature technology and are the least expensive.

25. Please briefly explain the working principle of a TVS (Transient Voltage Suppression) diode.

When the voltage across a TVS diode exceeds a certain amplitude, the device rapidly conducts, dissipating surge energy and limiting surge voltage to a certain level.

26. What are the two most popular types of programmable logic devices today, and what are their differences?

FPGA (Field-Programmable Gate Array) and CPLD (Complex Programmable Logic Device) are the two most popular types of programmable logic devices today. FPGA is based on a lookup table structure, while CPLD is based on a product term structure.

27. Please list the types of diodes you know.

Switching diode (small-signal diode), Schottky diode, rectifier diode, zener diode (voltage regulator diode), transient voltage suppression diode (TVS), varactor diode, light-emitting diode (LED).

28. Please briefly explain Kirchhoff’s Current Law (KCL) and Kirchhoff’s Voltage Law (KVL).

Kirchhoff’s Current Law (KCL) states that the total current entering any junction in a circuit is equal to the total current leaving that junction. Kirchhoff’s Voltage Law (KVL) states that the algebraic sum of the voltages in any closed loop of a circuit is zero.

29. What type of circuit is generally used for the input of analog integrated circuits: common-emitter circuit, common-base circuit, differential circuit, or common-collector circuit?

To suppress thermal drift and improve accuracy, a differential input circuit is generally used.

30. What is feedback? What is the purpose of feedback?

Feedback is the process of taking a portion or all of the output signal (voltage or current) of an amplifier, comparing it with the input signal (by addition or subtraction), and using the resulting effective input signal to control the output. Negative feedback can be used to stabilize output signals or gain, and can also extend the bandwidth, making it particularly suitable for automatic control systems. Positive feedback can create oscillations and is suitable for oscillator circuits and waveform generator circuits.

hardware design

31. What ability does the β parameter of a transistor reflect: current controls current, current controls voltage, voltage controls current, or voltage controls voltage?

The β value reflects the ability of the base current to control the collector and emitter currents, so it belongs to the ability of current controlling current.

32. Why is a pull-up resistor necessary for the outputs of open-drain (OD) and open-collector (OC) gates?

Because when MOSFETs and transistors are turned off, the drain (D) and collector (C) are in a high impedance state, resulting in an undefined output level. Therefore, in practical applications, a pull-up resistor is needed to pull the output to a defined level.

33. What are the conditions for amplitude and phase for a circuit to generate self-oscillation?

The amplitude condition is: |AF| ≥ 1

The phase condition is: φ A + φ F = 2nπ (n = 0, 1, 2, …)

34. Please list three typical ESD models.

Human Body Model (HBM), Machine Model (MM), Charged Device Model (CDM).

35. Please briefly explain the difference between DC-DC converters and LDOs.

DC-DC converters achieve voltage regulation by switching, chopping, converting magnetic energy through inductors, and filtering with capacitors to achieve a relatively smooth voltage output.

DC-DC converters have a large output current capability, strong load capacity, and high conversion efficiency. However, due to the switching action, they may produce high-frequency radiation.

LDOs regulate the output voltage by adjusting the voltage difference between the input and output of a transistor or MOSFET. The basic components are the regulating transistor and voltage reference components. The voltage conversion process is continuous and smooth, without any switching action.

LDO circuits feature minimal output voltage ripple but have weaker load capacity and lower conversion efficiency compared to DC-DC converters.

36. In what situations is a charge pump boost circuit generally used? Can a charge pump handle applications with large currents, and why?

A charge pump boosts voltage by charging and discharging capacitors through switches. Since the circuit lacks inductive components for energy storage, its driving capability is weak, and it can only be used in low-current applications.

37. What are the common EMC test items?

1) Conducted emission interference test
2) Radiated emission interference test
3) Conducted immunity susceptibility test
4) Radiated immunity susceptibility test
5) ESD immunity susceptibility test
6) Electrical fast transient pulse group immunity susceptibility test
7) Surge immunity susceptibility test
8) Power frequency magnetic field immunity susceptibility test
9) Harmonics and flicker test
10) Voltage dips, short interruptions, and voltage variation immunity susceptibility test

38. Please list the electrical safety certification standards for electronic products in various countries that you know.

United States: FCC, Europe: CE, Germany: TUV-GS, China: CCC, Taiwan: BSMI, Japan: VCCI, Australia: C-Tick.

39. Which six hazardous substances does the RoHS directive restrict in the use of electrical and electronic equipment?

It restricts the use of six hazardous substances, including lead, mercury, cadmium, hexavalent chromium, polybrominated biphenyls (PBB), and polybrominated diphenyl ethers (PBDE).

40. Please explain the working principle of a WatchDog.

The WatchDog has two important signals: a clock input and a reset output. When the circuit is operational, the CPU sends clock signals to the WatchDog, essentially “feeding” the WatchDog.

If a system failure occurs and the CPU fails to send continuous clock signals, the WatchDog then outputs a reset signal to the CPU, resetting the system.

41. There are three basic transistor amplification circuits: common emitter, common collector, and common base. Please briefly describe the characteristics of these three basic amplification circuits.

Common emitter: The common emitter amplification circuit has both current and voltage amplification capabilities. It has a moderate input resistance, high output resistance, narrow bandwidth, and is suitable for general amplification.

Common collector: The common collector amplification circuit only has current amplification. It has a high input resistance, low output resistance, and voltage-following characteristics. It is often used as the input and output stages of multistage amplification circuits.

Common base: The common base circuit only has voltage amplification. It has a low input resistance, and its output resistance and voltage amplification are comparable to the common emitter circuit. It has good high-frequency characteristics and is suitable for wideband amplification circuits.

42. What are the several methods of interstage coupling in multistage amplification circuits? Which coupling method has the most severe zero offset? Which coupling method can realize impedance transformation?

There are three coupling methods: direct coupling, RC coupling, and transformer coupling. Direct coupling has the most severe zero offset, while transformer coupling can realize impedance transformation.

43. Explanation of terms: coupling, decoupling, bypass, filtering.

Coupling: The interconnection between two separate circuits or two parts of the same circuit that were originally isolated from each other. It allows energy to be transmitted from one circuit to another or from one part of a circuit to another.

Decoupling: Preventing the exchange or feedback of energy from one circuit to another, to prevent unpredictable feedback that affects the normal operation of the next-stage amplifier or other circuits.

Bypass: Filtering out the high-frequency components from a signal containing both high-frequency and low-frequency components through electronic components (usually capacitors), allowing only the low-frequency signal to enter the next stage without the high-frequency signal.

Filtering: Filtering is the process of removing specific frequency band components from a signal. It is an important measure to suppress and prevent interference.

44. What are race conditions and glitches?

In logic circuits, due to different gate input signal delays, the arrival times of the signals at the gates are inconsistent. This situation is called a race condition. Glitches occur when spikes (momentary errors) are generated at the output due to races caused by inconsistent delays.

45. What is the difference between passive filters and active filters?

Passive filters are composed of passive components such as resistors (R), inductors (L), and capacitors (C). They are designed to have very low impedance at a certain frequency, diverting the corresponding harmonic current. Passive filters can be divided into two categories: tuned filters and high-pass filters. Passive filters have simple structures, low cost, and high reliability, making them widely used passive harmonic mitigation solutions.

Active filters are composed of active devices (such as integrated operational amplifiers) and resistors (R) and capacitors (C) but no inductors (L). They are small in size, lightweight, and do not require inductors. Active filters are actually amplifiers with specific frequency responses.

Integrated operational amplifiers have high open-loop voltage gain and input impedance, low output impedance, and provide certain voltage amplification and buffering effects after constructing active filter circuits. Integrated operational amplifiers have limited bandwidth, so the working frequency of active filters cannot be too high.

46. What are the components of a phase-locked loop (PLL)?

A phase-locked loop consists of a phase detector, loop filter, and voltage-controlled oscillator (VCO), with some PLLs also including a 1/N frequency divider.

47. What is the logic 0 and logic 1 voltage range of the RS-232C standard? What are the working voltage levels of CAN and RS-485 transceivers?

The RS-232C electrical standard is negative logic, with a voltage range of +5V to +15V for logic 0 and -5V to -15V for logic 1.

The working voltage level of CAN transceivers is typically 5V, while that of RS-485 transceivers is typically 3.3V.

48. Explanation of terms: UART, USART, USRT.

UART: Universal Asynchronous Receiver/Transmitter, capable of asynchronous communication.

USART: Universal Synchronous/Asynchronous Receiver/Transmitter, capable of both synchronous and asynchronous communication.

USRT: Universal Synchronous Receiver/Transmitter, capable of synchronous communication.

49. What are the components of the character frame format in asynchronous serial communication?

The character frame format consists of a start bit, data bits, parity bit (optional), and stop bit.

50. Briefly explain the differences between USB HOST, USB Slave, and USB OTG.

USB HOST is the host device responsible for controlling and accessing data, such as a computer PC. A USB HOST can only connect to USB Slave devices.

USB Slave is the peripheral device, controlled by the host, capable of inputting and outputting data, such as USB devices like flash drives, external hard drives, MP3 players, keyboards, mice, game controllers, network adapters, printers, card readers, etc.

USB OTG (USB On-The-Go) is a specification that allows devices to act as either hosts or slaves dynamically, enabling direct control and data transfer. It can switch between host and slave modes using ID signals.

51. Please list the logic levels you know.

Low-speed: RS232, RS422, RS485, TTL, CMOS, LVTTL, LVCMOS, ECL, PECL, LVPECL

High-speed: LVDS, GTL, PGTL, CML, HSTL, SSTL

52. Please list the balanced differential interface standards you know.

RS422, RS485, RJ45, CAN, USB, LVDS.

53. What are the three elements of electromagnetic interference?

The three elements of electromagnetic interference are the source of interference, the propagation path of interference, and the sensitive device affected by interference.

54. Please explain what crosstalk and ringing are.

Crosstalk: Crosstalk refers to one signal being interfered with by another signal, typically through electromagnetic field coupling. Mutual inductance and mutual capacitance between signal lines can induce noise on the lines. Capacitive coupling induces coupled currents, while inductive coupling induces coupled voltages.

Ringing: Ringing occurs due to impedance mismatch in signal lines, leading to signal reflections and superposition, resulting in oscillating waveforms.

55. What signals have you encountered that require controlling single-ended impedance to be 50 ohms or 75 ohms? What signals have you encountered that require controlling differential impedance to be 90 ohms, 100 ohms, or 120 ohms?

Generally, high-frequency signal lines are around 50 ohms to 60 ohms. 75 ohms is mainly for video signal lines. USB differential impedance is 90 ohms, and Ethernet differential signal lines have a differential impedance of 100 ohms. RS422, RS485, and CAN differential signals have a differential impedance of 120 ohms.

56. There are two principles for the routing of differential lines: equidistance and equilength. However, it may not be possible to fully satisfy both in practical routing. So, which one takes priority, equilength or equidistance?

Equilength should take priority. The differential signal is based on the intersection of the signal’s rising and falling edges as the signal transition points. Unequal routing lengths would offset this intersection point, significantly affecting the signal timing. Additionally, uneven routing introduces common-mode components to the differential signal, reducing signal quality and increasing EMI.

Small deviations in equidistance don’t significantly affect the differential signal. Though inconsistent spacing may alter the differential impedance, the coupling between differentials is not significant, resulting in a small range of impedance variation, typically within 10%. This is equivalent to the reflection caused by a via and doesn’t noticeably affect signal transmission.

57. Why should the reference ground plane for high-frequency signal lines be continuous (i.e., high-frequency signal lines cannot hop islands)?

A continuous reference ground plane provides a signal return path for high-frequency signal lines, ensuring the return path closely follows the signal line, minimizing the area of the current loop. This helps to reduce radiation and improve signal integrity.

If the reference ground plane is discontinuous, the signal will seek its own minimal path, which might coincide with other signal return paths, leading to mutual interference. Moreover, hopping islands with high-frequency signals can alter the signal’s characteristic impedance, resulting in signal reflection and ringing.

58. What is a semi-cured sheet?

A semi-cured sheet is a material in PCB composed of dielectric and adhesive materials, typically consisting of fiberglass and epoxy resin, with a dielectric constant of approximately 4.0 to 4.5. At room temperature, the semi-cured sheet is solid. When heated to a high temperature, the semi-cured sheet becomes adhesive, bonding the copper foils on the upper and lower sides together, serving as the intermediate dielectric.

59. What are through-holes, blind vias, and buried vias? What size can be mechanically drilled and what size must be laser drilled? Can laser micro-vias be directly drilled onto component pads, and why?

Through-holes penetrate the entire PCB, blind vias connect from the surface to inner layers of the PCB, and buried vias are embedded within the inner layers of the PCB.

The processing capabilities of most PCB manufacturers are as follows: holes equal to or larger than 8 mil can be mechanically drilled, while holes equal to or smaller than 6 mil must be laser drilled. For micro-vias equal to or smaller than 6 mil, when there is insufficient drilling space, it’s permissible to have some vias drilled on the PCB pads.

60. What are the two parasitic parameters of vias, and how do these parameters affect circuits?

Vias have two parasitic parameters: parasitic capacitance and parasitic inductance.

Parasitic capacitance elongates the signal’s rise time, reducing the circuit’s speed. Parasitic inductance weakens the contribution of bypass capacitors, diminishing the filtering effect of the entire power supply system.

Resource: https://mp.weixin.qq.com/s/dsA25iwKTmRFeadG98lREQ

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