What is Controlled Impedance?

In this guide, you will find all information you’re looking for about controlled impedance in PCB industry.

Keep reading to learn more.

What is the Impedance?

Impedance refers to the summation of resistance values and reactance values experienced in an electrical pathway.

You find the impedance value is expressed in ohms as are the individual parameters of resistance and reactance.

Resistance is a measure of a conductor’s opposition to the movement of current through it. Reactance is also a measure of opposition to the movement of current.

However, reactance arises due to the conductor’s inductance value and intrinsic capacitance and its interaction with shifting current and voltage values.

What is the difference between Resistance and Impedance in PCBs?

While resistance in the signal path contributes to the impedance value exhibited by the PCB, the two are different measures.

Both impedance and resistance manifest as opposition to signal movement in a circuit and are measured using the ohm unit.

Some major differences between resistance and impedance as experienced on a PCB include:

• Impedance occurs only in printed circuit boards utilizing alternating current only.

Resistance can be also be observed in direct current circuits alongside AC circuits.

• Resistance is an inherent reaction to oppose current movement through a conductor. Impedance is a developed response that opposes movement of current due to a conductor’s resistance, inductance, and capacitance.
• The impedance values for PCBs when exposed to electromagnetic waves can describe the energy stored and generated power.

Under similar conditions, the resistance value can only describe dissipated power.

• Resistance is only measured in real values whereas impedance can take on both real and unreal values.
• You also find that while both measurements have magnitude, only impedance can be provided a phase angle.

What is Impedance Control in PCBs?

Impedance controlled PCB refers to the management of impedance experienced in PCBs especially those utilizing alternating current with increased values of speed.

Impedance controlled PCB

Printed circuit boards with such characteristics experience inconsistent voltage and current supplies with frequent spikes.

Controllingthe impedance value insuch scenarios helps maintain the efficiency of a PCB.

Otherwise, the printed circuit board will suffer poor signal quality due to signal interference.

You find the board functionality greatly impaired as a result.

What are the Design Considerations in Controlling a PCB’s Impedance?

You realize that to control impedance in a printed circuit board, you have to make design accommodations.

The design alterations are targeted to adjust signal strength and to lessen the vulnerability of the PCB to noise interference.

The timing and speed of the signals can alter the voltage and current levels and therefore need to be controlled.

Moreover, when making design changes to tame impedance, the impedance values for the signal source and target need to be monitored.

Furthermore, the connecting peripherals such as cables also exhibit impedance which needs to be factored in the control efforts.

Also important is setting the tolerance level within which exhibited impedance can be accommodated.

In designing the PCB, carrying out simulations can help you identify the impedance levels likely to be experienced.

The simulations can be conducted with a platter of different material combinations to determine the least affected.

The dielectric properties of materials influence the impedance level.

Consequently, you can use materials with dielectric properties with the least susceptibility to impedance.

Furthermore, the material thickness can be varied as an extended measure.

How is the Characteristic Impedance of a PCB?

A PCB’s characteristic impedance is also referred to as surge impedance.

Signal transfer in a printed circuit board occurs along a transmission path.

Characteristic impedance manifests in this path as an amplitudinal ratio of an individual wave’s current or voltage value.

You find that the ratio is established for the wave given the lack of reflections in the opposite direction.

Furthermore, characteristic impedance can be provided as the impedance value at the source of a signal with an undefined path.

What Determines the Characteristic Impedance of a PCB?

To successfully control impedance such as characteristic impedance in PCBs, it is pivotal to establish a cause.

For characteristic impedance, you find the nature and type of materials alongside its size to be major influences.

The surface area rather than the length of the material is considered when it comes to size.

The charge transfer is enabled without having to be dispelled along the same line of action.

When the path is undefined, the characteristic impedance is determined as an extended non-reflective path.

What is the Transmission Line Concerning Impedance in PCBs?

The transmission line refers to the path of impedance action at a defined frequency on a PCB.

Along this line, the impedance value is established as an interaction of the voltage and current values of a waveform.

Additionally, a reflected wave formation can be transmitted along this path.

When this happens, the reflected wave travels along the transmission line in a direction opposite to the impedance.

However, the length of the transmission line does not influence the impedance characteristics.

How is a Lossless Line related to the Transmission Line in Controlling Impedance in PCBs?

A lossless line is essentially a transmission line.

However, a lossless line contrary to a transmission line has no instance of loss attributed to dielectric properties.

Additionally, it does not exhibit any form of line resistance.

Consequently, for a lossless line in a PCB transmission path, the conductive levels respond perfectly.

Similarly, the substrate layers exhibit ideal dielectric properties.

In this regard, frequency has little effect on impedance along the lossless line.

As a result, the resistive element of the PCB can be expressed as pure.

How is Impedance Controlled on PCB Traces?

Impedance triangle

PCB traces are modifiedtomanageimpedance levels, especially when used for high-frequency transmission of signals.

The concern is to ensure matching impedance values for both the transmitting and receiving ends.

The length of the conductive path influences the PCB’s frequency levels.

Consequently, altering the trace parameters will determine the board’s impedance values.

These parameters include the trace length, its thickness, the spacing in between the traces, its width, and even height.

You find a matching of impedance on a circuit board is best handled on a bare board.

A populated printed circuit board will pose difficulty in impedance matching due to the different tolerance values of components.

Additionally, the different thermal properties possessed by components may result in different responses to temperature changes.

Consequently, the effect of impedance may take an inconsistent nature.

Since these components are connected to the conductive track, you can wrongfully attribute impedance problems to components.

Therefore, whereas you can replace the components, it will become a costly affair when several components are involved.

What applications require impedance control on PCBs?

Not all PCB applications require controlled impedance.

Impedance control is principally required for applications where the speed of signal transmissions is fundamental.

However, many present technologies regardless of the industry consider speed an essential aspect.

Some of the common applications requiring PCBs with controlled impedance include:

• Analog and digital circuits used for telecommunications.
• Printed circuit boards in devices used to process graphic and video signals.
• Electrical circuits for control engineering processes.
• Household appliances such as RF communicators, mobile phones, and televisions.
• Control modules for automated processes.
• Electrical appliances such as cameras, printers, and gaming consoles.

How is Impedance Controlled in a Multilayer PCB?

For multilayer PCBs, impedance control is dependent on the stacking approach.

You will find the arrangement of the conductive layers in a multilayer circuit board useful in controlling impedance.

Multilayer PCBs have both conductive and non-conductive layers.

Multilayer PCB

The conductive levels in a multilayer PCB are designated as signal planes, ground planes, and power planes.

The ground and power planes provide paths for current to and from the components.

To ensure signal quality with limited interference, the power and ground planes are typically layered adjacent to the signal planes.

You find when the VCC and ground are stacked next to signal planes, they act as shields thwarting impedance manifestations.

Also, adjusting the thickness of the substrate used between the conductive layers provides a buffer against signal interference.

What is a Controlled Dielectric in PCB Impedance Control?

Impedance control in PCBs includes measures put in place to curb the movement of electric flow due to inductance, resistance, and capacitance.

Some of the techniques used include adjusting the trace features such as trace width and thickness.

Also important in controlling impedance values in a PCB are the laminate features such as thickness and dielectric properties.

You can adjust the characteristics of the substrate material in a PCB to achieve control over the circuit board’s overall impedance.

When this happens, you have executed dielectric control.

What is a Test Coupon in Impedance Control of PCBs?

A test coupon is a clone PCB with the same construction form of a fabricated PCB used for impedance testing.

A test coupon is used due to the impedance value being dependent on various aspects.

These include the trace parameters, layer configuration, and laminate properties.

You find carrying out an impedance test on each manufactured PCB to be a costly affair.

Consequently, a test coupon modeled on the actual PCB being manufactured offers a cheaper solution.

The similarity extends to the number of planes and trace characteristics.

Why is a Test Coupon Useful in Establishing Impedance Control for PCBs?

Using test coupons is preferred over testing individually printed circuit boards for a variety of reasons.

Some of the motives behind using test coupons include:

• For a given printed circuit board, you find accessing the inner layers difficult. Consequently, testing their traces for impedance values is problematic.

A test coupon design is however exploded to allow easy layer access.

• With a printed circuit board the layer interconnection separates the signals planes from the VCC and power planes.

Therefore, you find the lack of interconnection presents a deficiency that may reflect in imprecise measurement outcomes.

• Conductive tracks on PCBs are patterned to fit the layer surface.

However, testing on individual tracks is conducted on when they are laid out straight.

To extract straight tracks from PCBs will be destructive. Test coupons on the other hand are furnished for the very purpose.

• A PCB with two or more conductive layers employs the use of via network to provide interlayer connections.

You find vias sophisticates the testing process making it an impossible task.

Test coupons eliminate vias instead of creating continuous paths.

How can you Measure Controlled Impedance?

Measuring of impedance can be guided by the combination of a variety of systems and processed.

Common approaches to impedance measurement include using a network analyzer and a test system for controlled impedance.

You can also carry out a laboratory test using a time-domain reflectometer.

The impedance measurement process using a network analyzer is a byzantine undertaking that requires high skill levels.

However, the test system for controlled impedance using a time-domain reflectometer is more commonly used.

The time-domain reflectometer can be safely and reliably conducted without the need for special skills.

Furthermore, it offers high throughput with simple interpretation by plotting a graph pitting the impedance values against the coupon length.

To use atime-domain reflectometer, a stepped electrical signal is sped through the test coupon.

The signal is rushed through a cable whose impedance is controlled.

Varying impedance values areregistered by the reflectometer through capturing reflections.

Controlled impedance PCB

How do Differential and Coplanar Configurations compare in Impedance Control in PCBs?

Differential and coplanar configurations entail track formations on the conductive layers of printed circuit boards.

These formations are furnished to provide limited signal interference on the conductive surface.

You find using differential configuration on a PCB entails a pairing of conductive tracks between the board’s populates.

Having two-track paths instead of one minimizes the interference produced.

Additionally, a double-track design provides better shielding from interference.

The coplanar configuration is provided such that rather than a field creation on the surface, the effect is felt aerially.

The field results from the conductive track interaction with the plane.

With this design, the non-conductive layer exhibits less signal loss at elevated frequency values.

You find that the coplanar configuration augments the dielectric properties of non-ceramic substrate materials.

For instance, when used on boards employing FR – 4 substrates, these boards can sustain operations at increased frequency levels.

What Guides the Trace Pairing in a Differential Configuration for Impedance Control in PCBs?

Just having two conductive tracks parallel to each other is not enough for effective impedance control.

As a result, the trace pairing in differential configuration has to be laid in a certain way.

The subsequent matching of the traces is essential in achieving the desired control of impedance.

The following instructions guide the laying of the conductive tracks:

• The conductive path pairing should be identical with similar length, width, and spacing parameters. The spacing in this case is with other track pairings.
• The space between the double signal path formations requires very close tolerance. The smaller space, the better.
• You also find the spacing provided for the twin-track formation should be maintained across the length of the conductive pattern.

What is Single-ended Impedance on a PCB?

Single-ended impedance refers to the impedance taken for a determined track length.

To establish single-ended impedance, the required impedance value to ensure control is determined.

You find the conductive path requirements, the layer count, and material composition as major determinants.

Single sided PCB

The PCB structure is then furnished according to the required impedance.

To determine the single-ended impedance, an individual path is identified from the assembly. You note that the selected conductive path is uncoupled.

The single-ended impedance is subject to the laminate thickness, its dielectric properties, and the path features such as width and thickness.

You also find the permittivity and thickness of the glazed finish used over the conductive pattern influence the single-ended impedance.

Can a Lack of Impedance Control in PCBs Interfere with Signal Integrity?

Signal integrity failings can amount as a result of mismatched impedance.

In matching impedance, the input value of the impedance needs to correspond to the output value in a transmission path.

Furthermore, the matching process should be handled with awareness to prevent the development of parasitic impedance.

Failure to correctly match the impedance can result in signal deficiencies such as over and under-shooting.

Furthermore, the edges of the signal waveform can develop ringing or cascades.

A resistor adjustment can be used to rectify the signal anomaly if unrelated to the tool of measurement.

What Approaches are used for Impedance Matching in PCBs?

There are two common approaches you can use in matching impedance in oriented circuit boards.

Impedance matching is useful in ensuring the quality of signals transacted in PCBs is unblemished.

The two methods are series and parallel termination matching.

In series termination matching, the impedance value of the input needs to be lower than that along the path of transmission.

To match the impedance, a resistor is used between the input source and path of transmission.

When this happens, the impedance values at the output can be matched with that at the input.

You find the resistance value contributed by the resistor is summed with the impedance in the transmission path.

Consequently, any scattered signal is stifled by the resistor load at the termination of the transmission path.

The series termination matching is favored for its reduced power demands.

Additionally, you find no need for several load connections in the circuit.

As a result, the recorded impedance value is not added to with only the resistor value to contend with.

Parallel terminal matching is used when the PCB’s input signal source exhibits a lower impedance value than the transmission path.

Accordingly, to match the input and output loads, a parallel load of resistance is connected to the path.

The expected signal dispersion at the end of the line is thereby limited.

You find a one or two load sources can be used depending on the path impedance.

A single load source will match the impedance while two load sources will each double the path’s impedance.

Executing a parallel match is a lot easier.

What is the Difference between Odd Mode and Even Mode Impedance in PCBs?

Odd mode impedance is the impedance taken for a single trace line that is in a double formation.

To correctly measure this value, the untested line should be signal driven with a similar magnitude and of reverse polarity.

The differential impedance value is usually twice the odd mode impedance.

The even mode impedance also takes the measured value of a single trace line in double formation.

However, in this case, both lines are drivers.

You find the common-mode impedance value is half that of the even mode.

The common impedance is a measure of both lines in a driven pairing formation.

When are the Odd and Even Impedance Values Important?

Consider noise generation when a signal driven line pairing is established on a PCB.

There is a need to sustain signals with decent quality and with little or no interference at all.

Accordingly, the paths of signal travel have to be appropriately terminated.

Correct termination will involve establishing an odd mode impedance on the input signal.

Contrarily, the noise will be matched with the even mode value.

Additionally, you can utilize two grounded resistors to establish an even mode impedance for line termination.

An extra resistor connected in series with the other two can augment the required impedance.

When a third resistor is used, it is shielded from signals with even mode characteristics.

You find the even mode signal through the line pairing to be similar preventing charge flow through the resistor.

Contrarily, when matched with the odd mode impedance, it is kept at zero.

Otherwise, a parallel arrangement where the resistor value is half the even mode impedance value can be made.

Here, a simultaneous way to terminate the transmitted signals is created.

You find this approach applies to the even mode as well as the odd mode.

Can Controlling Impedance Lines Reduce Ringing and Reflection in PCBs?

For an operational PCB, signal transfer within the board is expected to be absorbed as a load.

Impedance controlled printed circuit board

 Nonetheless, the signal transfer is not a perfect process leading to stray energy releases.

You find these releases travel back along the conductive path to their point of origin.

When this happens there is a random resonating outcome that is uncharacteristic.

You can remedy this situation by controlling the impedance of the trace lines by making them shorter for instance.

Otherwise, you are bound to experience signal interference whose magnitude varies with the size of the reflection.

A short electrical path subdues the reflected signal due to the waveform of the original transmission.

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