For example, Sia writes “How can we find the impedance?”

For antenna impedance (and other antenna properties), there is no one simple answer. The impedance of an antenna varies with the geometry of the antenna, frequency used, and proximity to ground planes and other nearby conductive objects. That is what makes them fun to learn about for some people and evil black magic to others.

The best thing I can recommend is to do a Google search on “antenna impedance” and become familiar with the various standard geometry antennas that you will find (e.g. quarter wave, half wave dipole, folded dipole, yagi, loop, etc.). Each type has specific impedance, gain, directionality, and other properties, so you can make some design trade-offs. Since you asked about impedance, you may also want to Google “RF impedance matching”.

The actual math behind antennas is pretty esoteric (3-D partial differential equations) and is usually taught in the 3rd or 4th year of electrical engineering programs, and even then, only if RF is selected as the area of concentration. It is worth noting that specialized software is often used to solve these kinds of problems.

Since this is a stub article, at this point I will open the floor to reader comments!

Most modular office furniture can be set to any height, so you can easily adjust your cubicle to have one part of the desk at stand-up height. Some ergonomic furniture companies sell desks for this purpose, or you can also find small high tables sold for use in bars or patios, use a drafting table, or construct your own setup. Whatever you get, make sure it is at the right height or can be modified.

To find the perfect table top height for you, stand with your arms at your sides and your shoulders relaxed. Gently raise your hands in front of you by bending at the elbows until your forearms are level with the ground. Have someone measure the height from the ground to your elbow. Set your table top to this height so you can rest your level forearm on the table with your shoulder completely relaxed.

Next, you need a tall chair. Many office supply stores sell adjustable chairs in this height range or you may find a good bar stool or drafting stool. The ideal chair height is so that you will be somewhere between standing height to six inches lower.

Finally, you will need a footrest. Your tall chair may also have its own footrest, but it will generally be too far under you, so is better to have one that is more in front of you. You should be able to have your hips and knees at 90 degree angles and your feet on a firm surface. I was able to find a cheap plastic patio furniture end table and cut the legs as needed. It is also quite easy to make one out of wood. Even a sturdy cardboard box will do in a pinch.

Well, there you have it. Use good posture, alternate standing and sitting, take occasional walks and stretch breaks, and your back will thank you!

Answer:
Wow, this is a big but good question! For anyone considering undertaking a schematic capture and PCB layout for the first time, it can seem like a daunting task. A modern PCB serves not just as a means to interconnect components, but also as a mechanical structure, heat conductor, noise shield, and even as a circuit element, and must go through a complex automated manufacturing process to boot.

Perhaps it is best to start with a simple project to get your feet wet! Aside from that, it also helps to have some background knowledge about how electronic stuff works. Typically, at least a two year degree in electronics is recommended for anyone serious about PCB layout as a career. I can’t describe the whole process - that would take a book. What I can do is briefly touch on some of the basics to help you get started or even decide if PCB layout is something you really want to do.

Assuming you have a circuit design already and you just need to capture the schematic in OrCAD (or a similar tool) and do a PCB layout, I can propose some basic guidelines for how to approach it. As you mentioned, the design is partitioned into logical blocks like digital, analog, power, high speed, etc. It is good to keep the schematic and PCB layout also partitioned in this way. This will allow you to focus on the specific critical aspects of each type of circuitry throughout the process and avoid getting everything all tangled up.

If you are working on a medium to large scale project, there is generally a project team, so it couldn’t hurt to ask the Electrical and Mechanical Engineers who designed the thing and the Manufacturing Engineer who will build the thing and the Test Engineer who will test the thing what their design requirements are. The best way to do this is to have a PCB design kick-off meeting. Other team members may include the Customer, Quality, Reliability, Purchasing, Planning, and Safety representatives. As you go along, have the team sign-off at critical points.

Before drawing anything, start by reading the data sheets and application notes of any critical components to see what the recommendations are for PCB layout. A little bit of research up front can save a lot of time down the road!

Next, it is great to start by drawing a block diagram to use as a guide along the way. Ideally, the block diagram should be part of a hierarchical schematic that drives the design, but it can also be a separate sketch. It should have a logical flow e.g. from left-to-right and top-to-bottom. It should be the type of thing that can be used to show people how the thing works and updated as the project moves along.

Now you are ready to fire up OrCAD or whatever design tools you are going to use.

Draw any new schematic symbols needed
� To save time, first check for built in libraries or ones from part vendors or user groups
� Rather than start from scratch, modify an existing library part when possible
� Use a left-to-right and top-to-bottom flow of signals on the symbols
� Typically you place power pins on top and GND on bottom
� Double check all the symbols for correct pins! A mistake here can make a lot of mistakes in the PCB times the number of PCBs - You don’t want to go there.

Capture each section of the circuit into the schematic
� Again, use a left-to-right and top-to-bottom flow of signals on the schematic
� Each block can be on a page
� Draw critical circuitry like you want it to be laid out on the PCB (i.e. short lines where you want short traces etc.)
� Note any special design rules like length limits, required widths, or controlled impedances
� Name signals in a way that makes it easy to understand the schematic
� Add test points to critical signals (and lots of GND test points)
� Double check schematic against design source with a highlighter etc.
� Get approval from team

Draw any required new PCB footprints and pad stacks
� To save time, first check for built in libraries or ones from part vendors or user groups
� Follow vendor guidelines if available, else use industry standards
� Consult with the manufacturing engineer about process specific requirements

Proceeding to PCB Layout
� Draw board outline with critical cut-outs, mounting holes, and keep outs
� Sketch “rooms” corresponding to each block onto the board outline
� Decide on number of layers, copper thickness, and board stack-up arrangement. This is a function of board density, the sheer number of signals and power planes needed, the controlled impedance scheme, heat conduction requirements, etc.
� Consider routing adjacent signal layers with horizontal / vertical routes to obtain perpendicular overlap. This reduces cross talk and eases routing.
� Place critical components and get approval on the initial placement from the team
� Route critical traces and planes, and then route the rest
� Do a clean up pass to fix any odd things, improve silk screen, etc.

Here are some bullets to keep in mind when doing PCB layout for each type of circuit:

Low Speed Digital
� Not very susceptible to noise
� Generates a medium amount of noise, so keep it away from the most noise sensitive circuits
� The most active signals are the most noisy
� Other than that, layout is not too critical and this can be placed around the board as kind of a “filler” in places where nothing else will want to go and can be easily auto-routed.

High Speed Digital
� Same as low speed digital, plus the following:
� There can be a very high number of interconnecting traces
� Generally uses controlled impedance traces
� Requires controlled, minimized, or matched signal delay times and proper location of termination devices
� Can generate some serious heat - added copper planes to conduct heat away and heat sinks will need to be designed in

Analog
� Usually very sensitive to noise pickup
� Keep away from noise generators
� High impedance nodes and nodes followed by a lot of gain are generally most sensitive to noise
� Look for recommended grounding and shielding in application notes
� May also be sensitive to surface leakage currents, temperature, etc
� Power amplifiers may generate significant heat - added copper planes to conduct heat away and heat sinks will need to be designed in

Power
� Can generate a lot of noise (esp. switching power supplies, class D amplifiers, etc)
� At the same time, some nets are very sensitive to noise (i.e. sense and feedback nets)
� Can generate a lot of heat - added copper planes to conduct heat away and heat sinks will need to be designed in
� Switching nodes are sensitive to layout parasitic inductance and capacitance
� Keep high AC currents confined to small loop areas to reduce noise
� May involve high voltage and safety issues, special spacing and insulation requirements

RF
� Uses controlled impedance traces
� May require traces to be designed as antennas (requires RF design background)
� Receivers are very sensitive to noise
� Transmitters generate noise and dissipate heat proportional to the power involved
� Again, added copper planes to conduct heat away and heat sinks will need to be designed in
� Requires special attention to shielding and grounding

It is embedded below and can also be opened in its own window.

Features:

• Results update as you type
• Several choices of units
• Units and other settings are saved between sessions
• Blog format allows user comments

Note: To go below 0 AWG, for example 00 AWG, enter -1 and so on.

Inputs:

Wire Size

AWG mm

Optional Inputs:

Wire Temperature		Deg.  C F
Wire Length		m cm ft inches
Number of Wires in Bundle

Results (per each wire):

Resistance		Ohms
Single Wire Ampacity		Amps
Wire Bundle Ampacity (per wire)		Amps
Copper Diameter		mils mm AWG
Copper Area		mils^2 mm^2
Copper Weight		kg g lbs oz

Tips:

• Type a number into any cell below and it will be converted into the other two formats.
  
• Updates occur automatically as you type.

User Inputs and Results:

Hex:
Decimal:
Binary:

Option Scale Factors:

A:
B:
C:

Scaled Result = Decimal_Value*A/B + C

Scaled Result:

As an example, the Scale factors can be used for an 8-bit A/D converter. With a 2V full scale, A=2, B=255, and C=0. The scale factors may be changed as desired.

At higher frequencies, things get a little strange. Current takes the path of lowest inductance which is also the path of smallest loop area. This results in lower energy being stored in the magnetic field. So current will be concentrated along surfaces so it can be closer to the currents in the return path. In a related phenomenon called the proximity effect, at high frequencies, a PCB trace's return current in the ground plane will hug along the trace no matter how crooked that trace may be. Although this is not the shortest distance, it results in the smallest loop area.

So, at high frequencies, current does not flow evenly throughout the entire cross-section of the conductor but is more concentrated at the surface. The higher the frequency, the more the current is concentrated on the surface. This results in higher I^2*R losses at higher frequencies.

The current density varies exponentially as a function of depth from the surface of the wire. However, it is helpful to think in terms of a skin depth. The skin depth is defined as the depth at which a hollow conductor carrying DC would have the same loss as a given solid conductor carrying AC. In other words, the conductor is behaving as if it has been hollowed out. Making a conductor thicker than the skin depth does not have much effect on lowering losses. One may design a very thick conductor only to find that most of the copper is not being used.

Although the skin effect is generally associated with RF and microwave circuitry, it often plays a significant role in switching power electronics, transformers, motors, and high power AC transmission lines. Whenever designing conductors for anything other than DC, it is advisable to be aware of the skin depth (a.k.a. depth of penetration).

For copper at 100 deg. C, the skin depth is given by:

Skin_Depth = 7.6/SQRT(Frequency) [cm]
where, Frequency is in [Hz]

References:
[1] http://www-s.ti.com/sc/techlit/slup125.pdf
[2] http://en.wikipedia.org/wiki/Skin_effect
[3] http://en.wikipedia.org/wiki/Skin_depth

User Inputs:

Frequency

Hz KHz MHz GHz

Results:

Skin Depth

cm mm um inches mils oz/ft^2

Thus, a recommended stack up for a high-speed four-layer board is; the first layer is a signal layer, the second layer is power, the third layer is ground and the fourth is a signal layer. This results in placing most of the routing on the fourth layer closest to the ground layer, and allowing a higher component density on the first layer.

Next, the spacing between signal and ground layers and trace widths can be planned using [1]. To provide the lowest possible power distribution impedance, the spacing between power and ground planes should be as close as practical.

Routing Guidelines:

1. Route high-speed signals on the bottom (adjacent to ground) whenever possible
2. Route high-speed signals using a minimum of vias and corners
3. Maintain the proper trace width at the corners by rounding or beveling
4. Avoid stubs on traces whenever possible
5. Route all traces over continuous planes (VCC or GND), with no interruptions
6. Decouple power and ground planes with good ceramic capacitors
7. Minimize the trace length and loop area seen by decoupling capacitor currents

[1] PCB Stack-Up Design And Impedance Calculator

Right-Click, Download and Save the Excel tool here:
layerstack_planningoriginalipc.xls

Istvan Nagy’s home page:
[Sorry, link had to be removed due to content issues.]

Do you have a favorite stack-up and impedance calculation tool? Let us know about it in the comments below.