Question

1. Describe the process of Schematic Capture.

2. What is a Schematic Diagram.

3. Describe the guidelines for drawing schematics.

4. List the three main types of libraries for PCB design.

5. How are components classified, based on the leads.

Answer 1

1. The process of Schematic Capture.

Schematic capture is the process of creating a schematic diagram for an electronic circuit using various tools designed for the job. This can be done from as simple as using a pen and paper to using schematic capture software, including highly expensive electronic design automation suites or packages that can do everything from schematic capture, layout and simulation.

The schematic capture part of the design process is today undertaken interactively. Prior to the schematic capture of the design, the initial high level design must be undertaken. Then in years gone by, breadboards of the circuit would be made up and made to work before committing to the schematic stage. Now with highly sophisticated circuit simulation software, the circuit is designed interactively during the schematic capture stage and the circuit simulated using software rather than building a hardware version of the circuit. By using a computer based system for schematic capture, it is possible to enter very complicated circuits into a computer relatively quickly. It is also possible to undertake the design of the board and perform circuit simulations while the basic design is underway. In addition to this, many circuit capture systems provide a means by which the circuit revisions can be managed and configuration controlled properly. Where a circuit is being repeatedly updated, and there may be the possibility of several people working on different areas, this is of great importance. Elements entered into a schematic have a shape associated with them for the schematic. In this way a shape designed for a particular part will be pre-drawn and appear on the circuit every time that particular type of part appears on the circuit. When using an end-to-end design suite, the full shape may also include the PCB outline, pads and the like. In this way the part number for that part defines all the elements of the part for the design.

One of the big problems with computer based schematic capture systems now is that the circuits are often very large and they can become unreadable and difficult to follow. There is a trend to print the circuits out on A4 or letter sized paper, and each sheet may only have a few components.

2. Schematic Design

A schematic diagram is a picture that represents the components of a process, device, or other object using abstract, often standardized symbols and lines. Schematic diagrams only depict the significant components of a system, though some details in the diagram may also be exaggerated or introduced to facilitate the understanding of the system.

3. Guidelines for drawing schematics

• Symbols are important. Some are quite esoteric and rarely used outside specific disciplines. It goes without saying that you should use the right symbol, but a common example where most organizations deviate from this rule is with logic symbols.
• Being able to distinguish which nets connect and which cross over is critical. The “dot convention” has been developed to clarify connections.
• Always keep the specific standard spacings, because it helps schematic to look neater
• Another very important part of the schematic requiring attention are Reference Designators and the associated values. Part “references” are the letter/number combinations used to indicate the type of component and to uniquely identify each one (eg: R1, R2, R3, etc.) in design. References are frequently printed on a PC board “silkscreen” to permit easy location of components. References should be located next to the component with which they are associated. Develop a pattern and stick to it for the entire design (above or below horizontal components, left or right of vertical components).
• Draw your schematics so that signals “flow” from left to right. With amplifier circuits this is usually very easy. With larger digital designs it can be more challenging.
• Use busses when possible. All good CAD tools have the capability of grouping signals into a bus. If there is a 10-channel preamp, it could make sense to use a bus to connect those signals to the next stage. Avoid crossing busses if possible as this can be ambiguous.
• Use “global connectors” to minimize clutter. One global connector (or symbol) that all use but probably don’t think of in this way is the ground symbol.
• Use of explicit Net Names. Net Names are placed directly on a “wire” connection. They are global, so they can be used for connectivity (as long as you don’t misspell the name!). They are often used to name the signals on connector pins. Even if the signal doesn’t go anywhere else in the design, they can be very useful for clarifying the purpose of a signal.
• Show the ground symbols available in the Capture CAD tool used in the department. Below each symbol is the associated net name, which should be accessible with the symbol.
• Most CAD programs have tools for adding lines, boxes, arrows and text to a schematic. Use these tools to delineate circuit functions or add notes to be used when laying out, assembling, or testing the circuit.
• When you draw a schematic you should get used to adding connector symbols for all offboard signals and for power. Power has to get to your circuit somehow and while end up simply building the circuit on a “white board”, it still makes sense to document all of the inputs and outputs. All power and signal I/O should be twisted with a ground for low inductance and reduced susceptibility to EMI.
• Use the Title Block. Name your circuit and put your name on the circuit. It makes the drawing look more professional.

4. The three main types of libraries for PCB design.

• Integrated Library is library that contains schematic symbols, footprints and 3D models all compiled into the one single file. The only document that must be added to the integrated library package is the schematic library (or libraries). Integrated libraries are, by nature, inherently secure.
• Database Library is the library which contain the database of PCB parts which we already used before. The type of database library used will depend on how you want to handle your source symbol and model libraries. If the libraries are to be kept in a location on a hard disk or network drive, you would simply use a Database Library (DbLib).
• The component library and the data contained therein is a critical part of the PCB design and build process. Incorrect or incomplete data can result in incorrect parts or footprints being designed into the PCB, unnecessary CEM/EMS queries, and delay. So this librabry will help to provide us the exact components for the PCB design.

5. Classification of Components, based on leads.

Components such as capacitors, resistors, and inductors have only two leads, while some integrated circuits can have several hundred or even more than a thousand for the largest ball grid array packages. Integrated circuit pins often either bend under the package body like a letter "J" (J-lead) or come out, down, and form a flat foot for securing to the board (S-lead or gull-lead). There are two types of components,

• Active Components - Components in which lead content is very high. Active components rely on a source of energy (usually from the DC circuit, which we have chosen to ignore) and usually can inject power into a circuit, though this is not part of the definition. Active components include amplifying components such as transistors, triode vacuum tubes (valves), and tunnel diodes.
• Passive Components - Components in which lead content is low. Passive components can't introduce net energy into the circuit. They also can't rely on a source of power, except for what is available from the (AC) circuit they are connected to. As a consequence they can't amplify (increase the power of a signal), although they may increase a voltage or current (such as is done by a transformer or resonant circuit). Passive components include two-terminal components such as resistors, capacitors, inductors, and transformers.