Printed Circuit Boards (PCBs) have been the focus of scientist and engineers to bring novel ideas on how to improve the quality of end electronic product. As PCBs play the key role in functionality and performance of any electronic product or device so the perfectly designed PCB layout is highly important. There are many factors that a design engineer must consider while designing a PCB layout and these factors are driven by the requirements of end product.
Like number of layers PCB, size and dimensions of PCB, number of electronic components to be soldered upon PCB, types of components, routing techniques and many other PCB design factors. Among them one of the most important aspect is the “Impedance Matching”. The PCB that is dedicated for the electronic product that is to be used for High frequency application like RF or microwave electronics, then the most critical part of the PCB layout design is to control the impedance of the circuit.
What is Signal Reflection.?
As we are familiar with the phenomena of reflection that when a light ray is incident on the mirror then the light is reflected from mirror’s surface. Another example is water, when light enters the water some of the light is refracted while some is reflected. The same phenomena is with electrical signal. The signal reflection is the phenomena where the source transmits the electrical signal in the signal trace to the receiver/sink and some part of the signal is reflected back from receiver/sink back to the source. This reflected signal can cause signal distortion and oscillation in the circuit.
Why the Signal is reflected..?
The reason for the signal reflection from receiver to the transmitter is the transient impedance caused by the discontinuity in characteristic impedance of signal trace. If the characteristic impedance is uniform through starting from source or transmitter to sink or receiver then there will be no signal reflection. The discontinuity in characteristic impedance of signal trace can be caused by variation in signal trace width, thickness, distance between the trace and the corresponding reference plane and dielectric constant of the substrate material of PCB.
Effects of Signal Reflection:
Fortunately, the signal reflected from receiver is always less in strength then the main signal, hence the reflected signal is again transmitted and then again reflected with lesser strength, and hence in this way the signal is diminished slowly but will cause a temporary oscillation.
Overshooting and Undershooting:
If the delay time between the signal transmission and reception is short and the signal transmission is faster and if the previous reflected signal was not given time/delay to diminish and next signal is transmitted then this will cause the signal “peaks” to accumulate and will cause reflected signal overshooting thus complete failure of the circuit will happen. Similarly if the signal “valley” are accumulated this will cause reflected signal undershooting thus weakening the main signal to cause false clocking or misinterpretation of digital data lines like SDI, SDO, SCLK etc. This overshooting or undershooting can completely destroy the protective diodes at the signal ends.
The reflected signals from receiver end if are strong enough then they can possibly change the logic state of digital circuitry hence the circuit will behave in unanticipated manner. Distorted signal are sensitive towards noise.
How is Signal Reflection Calculated.?
As an example scenario refer to the diagrammatic representation of the signal trace between two points A and B on the PCB.
In the above diagram signal (Vi Voltage and Current Ii) Vi incident from source A to sink B. The characteristic impedance from A to B was continuous but from point B the impedance of signal trace changed hence changing the voltage Vo and current Io transmitted onwards. The characteristic impedance from A to B is Zi while from point B it is Zo. Keeping the picture in view above, for point B looking from left, we can write using ohm’s law as
Now looking at point B from right, line impedance is now Zo then we can write for Vo as
Now there are two cases
Case-1: The Impedance is not discontinuous and Zi = Zo
In this case if Zi = Zo then we simply get Vo = Vi means the transmitted voltage is same as incident voltage Vi and no signal is reflected.
Case-2: Discontinuous Impedance Zi ≠ Zo
Now here the incident signal is not completely transmitted onwards because of discontinuous non-uniform impedance of signal trace. Hence some part of the incident signal is reflected back as “Vr”.
Hence we can write
Now since the reflected current flows in opposite direction so it will be minus from the incident current hence
The reflected signal is travelling along the signal trace part with impedance Zi therefore we can use ohm’s law
Put equations 1, 2 and 5 in 3 we get
This term with the impedance is called the reflection coefficient “Rc”.
The value of Rc can be from -1 to 1. But ideally Rc should be zero
Methods to Reduce Signal Reflection:
- The transmission rate or speed of the signal can be decreased so that the oscillation of reflected signal can be minimized and stabilize the circuit’s signal trace
- The PCB thickness is relatively kept low so as to reduce parasitic capacitances
- Shorten the signal transmission trace length by carefully arranging number of layers in multilayer HDI PCBs. This can effectively decrease the parasitic inductance to reduce cross talk between signals
- The number of turns or bends in the signal trace should be kept as low as possible. The signal trace should be in straight line but if the bend is necessary then the bend arc should be at 45O. This helps to reduce EMI radiation
- Route all important signal lines on same plane to minimize unwanted through holes.
- The separate ground and power planes should be used for separate regulated power supply for noise reduction in multiple power supply circuit system. This will enhance signal integrity.
- Apply correct routing topology
The Parallel Topology:
In this structure arrangement, the source is simultaneously feeding the signal to more than one sinks or receivers. All the nodes/receivers connected in parallel or star fashion are synchronized with source. However the separate termination resistance is required for each node/branch and must be compatible with characteristic impedance.
The Series Topology:
Here the one transmitter or source is connected in daisy chain or series fashion the output of one is connected to input of other. A simple series resistor can be placed close to the driving/transmitting/source end to make the impedance at the receiver side compatible with characteristic impedance. The daisy chain branch length should be kept as short as possible. However the signals received at different receiving ends are not synchronized with main transmitter.
Signal Trace Termination:
There are two ways to terminate the signal trace. Either from the source end or from the sink end.
A simple series resistor placed between source and load and close to the source will do the job.
There are 4 ways to make signal termination at load / sink end.
Single resistor parallel termination:
Differential Pair Termination:
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