What is high speed PCB Design ?

In this tutorial we are going to learn about What is high speed PCB Design ?

A circuit is considered high-speed “when the rise/fall time of a signal is fast enough that the signal may be change from one logic state to the other in less time than it takes for it to travel the length of the conductor and back. Generally one From this definition it is obvious that two critical factors determine if a circuit is operating at a high-speed: the switching time of the device(s) on the circuit & the length of the circuit. It is important to note that the “clock speed” of the circuit does not determine whether it is operating in the high-speed domain.

  • Use 45° angle or smooth curves, in order to minimize signal reflection. Sharp corners have a high field strength.
  • Avoid stubs or tees, vias, sharp 90° turns, all of which cause impedance discontinuities.
  • Minimize the number of signals that cross PWR domains. Each PWR plane should have HF decoupling caps between the planes to provide a return path for signals that cross from one domain to another.
  • Consider using buried capacitance pwr or gnd planes. This technology reduces HF bypass capacitor count. Unfortunately, this technology is expensive & of limited availability. The added cost can be partially offset by savings in HF capacitors.
  • Use all available pwr & gnd pins. This may seem obvious but it is not always done. Some schematic symbols define VCC & GND connections implicitly so they are not visible on the schematic. Therefore, it may not be apparent that some are missing. It is preferred to specify pwr & gnd explicitly on the schematic.
  • Consider board stackup order. It is preferable to have the gnd plane as close as possible to the components. Place the VCC plane towards the bottom side of the board.
  • Border the PCB with chassis gnd or place the VCC plane back from the edge of the board by three times the distance between planes.
  • Microstrip should only be used for short traces, traces with slow rise time signals & where driver & load are isolated from clock. (In practice, Microstrip is used for all signals &clks)
  • Stripline should be used when possible. It is especially desirable for clocks. (In practice it is rarely used on 4-layer boards.)
  • Keep clock chips or clock lines away from the edges of the board.
  • Minimize the trace length of clock lines.
  • Keep clocks away from I/O lines and connectors.
  • Avoid running traces under crystals, clock chips, or other “hot” circuits. (Hot in the EMI sense means noisy, HF/high energy, not high temp.) A good way to ensure this is to put a cross-hatched gnd plane on the surface under the oscillator/clock chip, which prevents crosstalk between the clock & signals.
  • To minimize crosstalk, use a trace spacing-to-height ratio greater than 2. Unfortunately, this is seldom practical due to space constraints. Usually, the designer must settle for approximately 1:1. Good signal integrity tools are important in this context.
  • Put line driver & receiver near the port they drive. Put filters as close to the connector as possible to prevent unwanted signals coupling into the output of the filter.
  • Use ferrites or LP filters on signals that go to an external cable.
  • Route diffl pairs together, so their lengths are matched & any common-mode noise is cancelled out.

Write some general guidelines for component placement?

In this tutorial we are going to about Write some general guidelines for component placement?

Component placement can also reason of EMI generated. The guidelines below are general Approaches to minimize EMI.

• Keep leads on TH components short. Through hole the components should be as close to the PCB as possible and trim leads if necessary.

• Place all components associated with one clock trace closely together. This reduces the trace length & Reduces radiation.

• Placement high-current devices should be  as closely as possible to the power sources.

• Minimize the use of sockets in HF portions of the board. Sockets introduce higher inductance & mis-Matched impedance.

• Keep crystal, oscillators, & clk generators away from I/O ports & board edges. EMI from these devices can be coupled onto the I/O ports.

• Placement of crystals so that they lie flat against the PC board. This minimizes the distance to the GND Plane & provides better coupling of EM fields to the board.

• Connect the crystal retaining straps to the GND plane. These straps, if ungrounded, can behave as an antenna and radiate.

• Provide a GND pad equal/larger than footprint under crystals & oscrs on the component side of the board. This GND pad should be tied to the GND planes with multiple vias.

What is the effect of 90° Trace corners in PCB Design ?

In this tutorial we are going to about What is the effect of 90° Trace corners in PCB Design ?

Arguments against 90° corners fall into two categories:

Impedance mismatch: A 90° corner is, necessarily, wider than the rest of the trace. This results in a decrease in Zo, the intrinsic impedance of the trace, & therefore causes an impedance mismatch at the corner. This, in turn, causes reflections, signal distortions, & noise along the trace. Zo of the trace varies with trace width, but is approximately a 15 – 20% decrease in Zo at that point. The distance over which the effect is felt is equal to the trace width, W. Thus, the impedance goes from nominal to about 20% below nominal in a distance of W/2 & then returns back to nominal in another W/2. For most traces, this is VERY quick.

EMI:  90° corners postulates that electronic fields become concentrated at the sharp corners, causing destructive Emi radiation from that point that manifests itself as EMI.

What is Fan in and Fan out Via?

In this tutorial we are going to learn about What is Fan in and Fan out Via?

In the case of SMD’s attached to double-sided/multilayer boards, each component pad is usually connected by a short length of track to a via which forms a link to other conducting layers, & this via is known as a fan-out via. Generally fan-out via is generally also taken to include any vias that fall inside the device’s footprint (under the body of the device). Some designers attempt to differentiate these vias from those that fall outside the device’s footprint by referring to them as fan-in vias, but this is not an industry-standard term.

What is an FR4 Board?

In this tutorial we are going to learn about What is an FR4 Board?

The most commonly used insulating base material for PCB’s. FR4 is made from woven glass fibers which are bonded together with an epoxy. The board is cured using a combination of temperature & pressure which causes the glass fibers to melt & bond together, thereby giving the board strength & rigidity. The first 2 characters stand for “Flame Retardant”. FR4 is technically a form of fiberglass, & some people do refer to these composites as fiberglass boards/fiberglass substrates, but not often.

How to calculate the resistivity of copper tracks?

In this tutorial we are going to learn about How to calculate the Resistivity of copper tracks?

PCB DESIGN  of finer lines, distributed resistance of copper is becoming increasingly important. The

formula for calculating resistivity in copper traces is given by the following equation:

R=(0.679 X 10-6 ohm/inch) / (width X thickness inches)

Example: In fine-line designs, using 0.5 oz. Copper, a .005 trace, 5 inches long, the resistivity will be:

( (.679 X 10-6) / (5 X 0.7 X 106) ) X 5 = 0.97 Ohm

What is a Gerber file and explain the RS274X Gerber format?

In this tutorial we are going to learn about What is a Gerber file and explain the RS274X Gerber format?

Gerber is the standard photo plotting command language. It is supported by virtually all modern photo plotting equipment in use today. The command structure & format of a Gerber file (the name Gerber borrowed from the popular photo plotter maker Gerber Scientific Instruments Co.,) is actually a subset of the EIA RS-274-D standard for numerically controlled machines. Each Gerber file contains commands & data that instruct the photo plotter on where to expose the film when generating PCB artwork.

How Many Types of Capacitor ?

In this tutorial we are going to learn about How Many Types of Capacitor ?

There are many different types of capacitor material because no material provides the best performance in every regard. Here is a quick list of some of the characteristics. You can find worse and better units in each type.

Aluminum Electrolytic:

1. Cheap per farad

2. Small per farad

3. Drifty

4. Limited life

5. Leaky

6. Poor at mid and high freq. Some designers won’t use them in a high fi audio path. Generally seen in

power supplies.

7. Tolerance: poor

8. temperature range: restricted

Tantalum Electrolytic

1. Cheap, but not as cheap as Aluminum

2. Small, but not as small as aluminum

3. Drifty, but better than alum

4. Better life than Alum

5. Less leaky than alum

6. Good up to high audio freqs.

7. Tolerance: better than alum

8. temperature range: better than alum,

PolyEster (mylar)

1. Cheap, but not as cheap as aluminum.

2. Small, but not as small as aluminum

3. much less drifty than electrolytic

4. much longer life

5. very low leakage

6. usable up to RF

7. Fairly tight tolerence available

8. temperature range: good at low temps, can melt at high temp

Ceramic: There are many kinds, I’ll call them low, medium, and high.

Low

1. Cost: high per farad

2. Size large per farad

3. Drift: among the best

4. Life: long

5. Leakage: very low

6. Usable frequency : RF

7. Tolerance: very tight

8. temperature range: wide

9. Voltage: very high available

Medium

1. Cost: High per farad

2. Size: Large per farad

2. Drift: better than electrolytic, worse than polyester

3. Life: long

4. Leakage: low

5. Usable frequency: RF

6. Tolerance: moderate

7. temperature range: wide

High

1. Cost: High per farad

2. Size; Large per Farad

3. rift: very

4. Life: long

5. Leakage: low probably (not sure)

6. Usable frequency : low RF

7. Tolerance: medium to poor

8. temperature range: restricted high and low

This is not an exhaustive list of types or significant parameters.

PCB design tips for Differential pair routing?

In this tutorial we are going to learn about PCB design tips for Differential pair routing?

Because some device technologies utilizes differential techniques, its worth explaining some of the advantages & key layout aspects of differential circuitry & comparing them to similar single ended op amp circuitry. Differential circuitry is superior to single ended circuitry for a number of reasons. The CMR of differential inputs lets balanced circuitry reject common mode interference, including GND noise, that would be amplified by single ended circuits. Also, a differential circuit’s balanced properties usually reduce non-linearities & improve distortion. ICs with differential outputs have inherent common-mode output noise that is cancelled if the DAC is followed by a differential-input amplifier or filter. Because a differential signal’s 2 conductors carry a balanced signal, reduced EMI generation & reduced susceptibility to magnetic pickup are additional benefits of differential circuitry. Even “quasi-differential” circuitry, with its GND taken adjacent to a single ended source but shipped to a differential load as if it were the II half of a differential signal, is superior to single ended circuitry. This is because the small common mode interfering currents between the source & load are still reduced by the differential input. for examples of these concepts. Generally the single-ended connection in standard op – amp, although it could also apply to single ended connections into Devices gain blocks if the GND referenced signals were instead referred to 2.5 volts. In general, the rules fall into one or more of these five categories:
Planes: There should be a continuous power system plane underneath the differential pair.
Length: Care must be taken to ensure that differential traces are of equal length.
Spacing 1: Care must be taken to place the traces as close together as possible.
Spacing 2: Care must be taken to ensure that the spacing between traces is constant everywhere along the length of the traces.
Impedance: Differential impedance rules must be applied.

Make D > 2S to minimize crosstalk.
Where
S: Space between the two traces of a Differential Pair
D: Space between two adjacent differential pair
2) Route the 2 traces of a differential pair as close to each other as possible after they leave the device
to ensure minimal reflection.
3) Maintain a constant distance between the 2 traces of a differential pair over their entire length.
4) Keep the electrical length between the 2 traces of a differential pair the same. This minimizes the
skew and phase difference.
5) To minimize impedance mismatch and inductance, avoid using vias.
To minimize the crosstalk, keep the traces for the differential pairs as short as possible. If the pair has to
be routed for an extended length, observe the following guidelines.
· Keep the differential pairs as far away as possible to eliminate cross talk between the pair.
· In PCB design ground strip should be on the top layer to separate the differential pairs.
· Use vias to connect the top layer GND strip to the bottom plane extensively(every several
hundred mils)
· Keep the distance of the differential pairs constant when routing the signal.
· Keep the distance of the differential pairs to the ground strip as a constant.
· Try to match the length of the differential pairs.
· the reciever pair closer to the Reciever pins of IC chips rather than to the transformer. This is because the crosstalk current depends on the physical layout of the signals & the characteristics of the board. The voltage strength of the crosstalk signal is equal to the impedance of the load times to the crosstalk current. The higher the input load, the higher the crosstalk created on the same board. Because it is impossible on the board to match the input impedance. Its better to keep the trace from the termination to the input as short as possible.

Why Use Buried Resistors PCB ?

In this tutorial we are going to learn about Why Use Buried Resistors PCB ?

Buried resistor technology replaces discrete resistors with thin film planar resistors laminated within multi layer PCB. Space previously occupied by discrete resistors may now be used for additional components, trace routing/eliminated to create smaller, denser boards. Using buried resistors can significantly shorten signal paths. This results in reduced lead inductance, shorter signal paths, & improved impedance matching. Assembly costs can be reduced when buried resistors replace enough discrete resistors on a board.