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Impedance, Electrical Circuits and PCBs -What You Need to know

Impedance, with its complex mathematical formulae and terminology can be mystifying, I get it.  Here we strip back the terms and pare down the formulas to let that dangling penny drop.  So, lets dive in and get to grips with something you’ve probably have heard of but sometimes struggle to get your head around. 

So - What Exactly is Impedance?

OK, so in DC circuits, where the current flow is steady, there is one type of opposition to this flow which is called “Resistance”.  AC circuits can experience resistance too, but they also can experience another form of opposition to the current flow, “Reactance”.

 In AC circuits, then it is the combination of these two forms of opposition to current flow that is called impedance (impedance is signified by the symbol Z). So essentially, in AC circuits, opposition to current flow is Resistance + Reactance = Impedance (Z).  Just like DC circuit resistance, AC circuit impedance is measured in ohms (Ω).

Why is it Important to Understand Impedance?

It is all about efficiency. Theory shows that maximum signal transfer happens when all impedances along the signal path are matched.

In all circuits which operate under high frequencies and process high speed digital signals, it is important to control impedance. Impedance control is fast becoming a standard requirement for PCB manufacture and is has become critical in areas such as:

  • Telecommunications
  • RF applications
  • High speed digital applications

Impedance controlled circuits can be found in the electronics you use every day:  your mobile phone, TV, computer, printer and even your car!

Impedance Control: An Example

Let’s take the cable connecting your TV to your antenna\satellite dish and look at it through the lens of impedance.


The purpose of this cable is to transfer the signal from one device (in this case your satellite dish) to another (in this case your TV). To obtain the best possible signal, the impedance of the satellite dish must match that of the cable and the impedance of the cable must match that of the TV. If the impedances don’t match, then only a portion of the signal gets transferred down the line to the TV. The rest of the signal gets sent back along the line to the antenna where it gets resent and another portion of  it may now get to the TV but obviously later than the original signal.  This impedance mismatch causes interference in the signal resulting in a blurred picture on our TV or even some double imaging if the mismatch is particularly bad.

Now transfer this scenario to PCBs and imagine the consequences if the correct signals were not

reaching their destination at the required time due to a mismatch in impedance.

Basically nothing would work as well as it should. Controlling impedance then sounds like a pretty good idea, but how do you go about it?

How Can Impedance Be Controlled?

In our TV/satellite cable example the antenna is the source, the TV the load and the coaxial cable the conductor.  The cable has conductors and insulators and the dimensions of these along with their electrical characteristics are measured to carefully control the electrical impedance of the cable. If we want the signals in our PCB to

transfer along the path from a signal source to a load via a conductor or track efficiently, we need to control impedance. So what can I do to control impedance?

For the signal to get from the source to the load the impedance’s along the line must match. Therefore, the output impedance of the source, the impedance of the track and the input impedance of the load must match.

Ways to help this happen:

  • Match component impedance:  This information should be found in the component data sheet. Once you have this information you now know the required impedance value of the track you need to connect these two components.
  • Track characteristics: track length, width and thickness, its proximity to other tracks, ground planes, laminate thickness and dielectric constant (Er) are all important factors to consider.  For tracks located on outer layers the thickness of the solder mask must also be taken into consideration.
  • Use a Microstrip: This is a trace on one layer separated from a plane on another layer by a material (such as the substrate).  Remember the antenna example? A substrate acts like the cable insulator and the plane acts like a shield. Microstrip configurations are popular because you can use impedance calculators or field solver software to give you recommended trace dimensions based on the impedance you require and the parameters\configuration you are using. 

Different configurations and combinations of these Microstrips can be used:

Surface:           Trace on one side and a copper plane on the other.

Coupled:         Where you have more than one trace on one side and a copper   plane on the other.

Embedded:      Where the trace(s) are sandwiched between the substrate with a copper plane on one side.

Stripline:          Where the trace(s) are sandwiched between the substrate with a copper plane on both sides.

Using these methods, where you calculate all the parameters in order to control the impedance is often referred to as “Controlled Dielectric”.  This is sufficient for a lot of designs, as long as you specify the parameters to your manufacturer and they stick to the specifications.

 You can calculate your impedance for free here.

 

Specifying Impedance in your PCB Design

If you are designing a product where the impedance is critical then you will need to clearly specify the impedance value required on the specific traces. These may be designs with tight tolerances or ones that cannot be designed using common Microstrip configurations.

It is important to note that when you define a trace as having an impedance control requirement, it is the impedance of the trace that is critical and not the size of the trace. By defining a trace as impedance controlled you are giving permission to the PCB manufacturer to vary parameters such as the size of the trace or thickness of the laminate in order to achieve the desired impedance. The manufacturer will probably go with the recommended parameters from the field solver first, then test the PCBs impedance and if necessary make appropriate changes and produce the PCB again until the required result is achieved.

Measuring Impedance

Testing the impedance values within your PCB is not always easy. Various factors such as accessibility, via connections, branches off tracks and length of traces can make getting an accurate reading difficult

  • Test Coupon: is manufactured on the same panel as your PCB and will contain the same trace characteristics as the traces requiring impedance control on your PCB.
  • A Time Domain Reflectometer (TDR) is the most common method of measuring impedance. Basically this sends a signal along a line and then measures the part of the signal that is reflected back when an impedance mismatch occurs.

 

Sherlock Ohms 

So there you go, now you are informed!  The next time impedance comes up in conversation (we know you WILL shoe horn it in) at work or with your mates you are the guy that has a clue, (smugly puffs on pipe) it’s elementary my dear Watt-son…

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The research was carried out for the example was helped greatly by that great paper written by Polar Instruments.

 

 

 

The resident PCB Nerd at Mint Tek Circuits, Tony simplifies advanced aspects of PCB design and technologies. Follow him @MintTekCircuits
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