Wednesday October 18 , 2017

PCB Design Considerations

1-Time & Frequency Domain Analysis – Important facts for oscilloscope readings

 

First we have to learn more about the time domain analysis versus the frequency domain analysis and how pcb design can be affected by these parameters. Digital printed circuit board designers always measure the performance of PCB delay, rise and fall times under the meanings of the time domain. On the contrary the analog designers express delays in terms of frequency domain as most analog phenomena can be described well by the frequency domain.

 

  • f (Frequency) = 1/T
  • T(Time) =1/f
  • f(frequency) =1/(T |rise +T |fall)
  • f |knee =0.5*[1/(T |rise +T |fall)] = Knee frequency

Knee Frequency: the needed amount of time the voltage of the digital circuits can rise from 10% of a logic '0' to a logic '1' or rise from 10% of a logic '1' to a logic '0'. Also the knee frequency is related to the whole system bandwidth and the 3-db bandwidth.

 

You have to choose the right oscilloscope probe that match your circuit amplitude, load, capacitance and speed, probes are usually used to make sure that your circuit is isolated completely from the oscilloscope  so that you can get that most accurate reads, these probes are divided into voltage and current probes, voltage probes measures the voltage on the circuit and current probes measures the current, in the market you will find two types of voltage probes; passive or active , the current probes are the same as we need the active elements to amplify the weak signal for more accurate analysis, the passive voltage probe is commonly used by engineers everywhere and it named by the attenuation ratios as described below.

 

Voltage Probe

Load Added To Circuit

Capacitance Added To Circuit (in average)

Information

 

1X

 

1000 Kilo Ohm

 

100 PF

-Adds more capacitance, can affect the circuit operation negatively.

-Doesn’t attenuates the signal amplitude

 

10X

 

10000 Kilo Ohm

 

12.5 PF

-Commonly Used, Stick on the 10X voltage probe, suitable for the most circuits.

-Attenuate the signal amplitude by 10X with 500 volts maximum input.

 

100X

 

100000 Kilo Ohm

 

2 PF

-Used in very sensitive circuits.

-Attenuate the signal amplitude by 100X with 500 volts maximum input

 

High Volt.

 

Higher

 

Lower

-Attenuate the signal amplitude by more than 100X with maximum voltage input starting from 1KV.

 

 

Make sure that you are using a probe bandwidth that is the same as your scope or little faster.

 

2-Custom PCB Characteristics

 

Before starting the implementation of the electronic circuit and drawing on the printed circuit board, we need to set some parameters and constants:

 

Trace Width| minimum = from 0.1 mm to 0.25 mm

Copper Layer Thickness = from 0.2 mm to 0.1 mm

Material Dielectric Constant| typical = FR-4 PCB Sheets = er = 4.5

The Dieletric Constant for FR4 equals 4.5 in its inner layer is considered to work for frequencies up to ( 2GHz). You should decrease this value to be 4.2 if you work with high frequencies (up to 10GHz).

Material Tangent Constant = for FR4 is approximately 0.015

The Tangent Constant of the FR-4 PCB sheets makes it hard to design microwave circuits on it, as short distances can make undesirable loss. Microwave seekers should use the high speed sheets instead of FR-4, but FR-4 PCB sheets still able to cover these frequencies with some troubles.

 

 

3-Very Important Parameters

 

Any electronic circuit has many features we should consider before designing the printed circuit board, input and output resistances of the circuit is a very important parameter, also the inductance and the capacitance of the circuit can be affected by the size, diameter and thickness of  PCB holes and the transmission lines, that’s means you have to consider well the diameter of the holes, the spacing between holes and the spacing between copper lines. In microwave circuits these parameter is very important as the resonance frequency affected by the increase of inductance and capacitance of the circuit, also the resistance seen by antennas can be changed due to added resistance made by the copper in the printed circuit board and also the resistances added by other elements in the circuit; capacitors, coils, transformers and so on.

 

For example, the typical narrow trace in the printed circuit board can add up to 0.8 PF capacitance for every 1 CM in the printed circuit board, also the ceramic capacitors which are usually named the decoupling capacitors add more resistivity to the circuit that’s why some times we use several parallel capacitors to decrease the series resistance occurred. Also the wires in the printed circuit board which are longer than 2.5 Cm have an inductance of 80 to 90 nH, this change in the inductance can be a disaster in microwave designs.

 

4- Be Aware of Transmission Lines

 

The length of the line is the point of interest, for the typical transition time of 2nS each line has a length more than 10 Cm can be considered as a transmission line, also for fast transition of 1nS transmission lines calculations can be applied for every 5 Cm lines. 

 

5- Inductive and Capacitive Crosstalk

 

Inductive & capacitive crosstalk can occur in parallel lines which are very close to each other; especially in data buses, so try to eliminate any obstacles in the ground plane to minimize the effect of crosstalk.

Also you should always find the shortest path from VCC to GND to avoid too many problems.