Design Tips

Welcome to Design Tips! In a past issue, we discussed how easy it is to create common mode noise on differential signals with small amounts of skew, rise/fall time mismatch, etc. In this issue, we will discuss how easy it is to create common mode noise from asymmetries with ground-reference vias on PCBs! At first glance, it might seem that a ground-reference via would not have any impact on a differential pair of vias on a PCB. However, this is not necessarily true! This common mode noise

created by asymmetry can then create a significant amount of EMI problems.
     Please send me your most useful design tip for consideration in this section. Ideas should not be limited by anything other than your imagination! Please send these submissions to bruce.arch@ieee.org. I’ll look forward to receiving many “Design Tips!” Please also let me know if you have any comments or suggestions for this section, or comments on the Design Tips articles.

 

Via Asymmetry Causes Common Mode
Conversion from Differential Mode Signals


By Bruce Archambeault, PhD, IEEE Fellow, IBM Distinguished Engineer, bruce.arch@ieee.org
Sam Connor, IBM Senior Technical Staff Member, sconnor@ieee.org

 

From an EMC perspective, the differential currents generate insignificant EMI emissions. If external, unshielded cables have only differential currents, very little emissions will be generated. However, if common mode currents exist on these same unshielded cables, the common mode currents are equivalent to the single ended signal currents and can cause significant EMI emissions.
     A previous Design Tips article showed how quickly common mode noise is created when there is skew between the two differential signals, or rise/fall time mismatch, etc. Any asymmetry in the path of the differential signal can cause significant common mode conversion of the original differential mode signal. This Design Tip will show how asymmetrical ground-reference vias placed near to differential signal vias will cause a significant increase in the common mode conversion.

Fig. 1. Single-ended signal via with nearby ground-reference via.


 

Single-Ended Signal Via

First, let’s look at what happens when a single-ended signal passes through a via and changes reference planes. We’ll assume that both planes are considered ground-reference and have many vias connecting the two planes together. We’ll create a signal via that passes through the two planes and noise will be coupled from the signal via to between the two planes. This noise can then propagate and couple onto other vias or I/O connectors and escape the shielded enclosure. Figure 1 shows a simple geometry where the nearest ground-reference via is moved from very close to the signal via to further away. Figure 2 shows how the location of the ground-reference via can affect the amount of noise coupled to between the planes. When the ground-reference via is close to the signal via, the amount of noise coupled is lowest.

 

Fig. 2. Effect of distance between signal via and ground-reference via. Fig. 3. Differential vias with nearby ground-reference via.

 

 

Differential Vias
If we now take the case of a pair of vias with differential signals with no common mode noise on these vias, there will be little noise coupled to between the planes as long as there is no nearby asymmetry. We’ll now place a ground-reference via nearby to one side of the differential pair, as shown in Figure 3, and we’ll move it further from the differential via pair. Figure 4 shows the transfer function of the noise coupled to between the planes from the intentional differential signals. Note that as the ground-reference via comes closer to the differential via pair (and increases the asymmetry), the amount of noise coupled to between the planes increases!

 

Fig. 4. Noise coupled between planes due to ground-reference via asymmetry. Fig. 5. Common mode noise conversion due to ground-reference via asymmetry (single plane pair transition).


     We can also observe how much common mode noise is created on the differential signal vias. This common mode noise could be conducted along the differential signal path until it arrives at an I/O connector and exits the shielded enclosure. If the I/O cable is unshielded or poorly shielded (as with many high speed differential cables) the amount of EMI emissions will increase significantly.

Fig. 6. Common mode noise conversion due to ground-reference via asymmetry (ten plane pair transition).

     Figure 5 shows the common mode conversion for a single plane pair transition. In typical PCBs, there may be many plane pairs as the signal travels from the top to the bottom of the PCB. Figure 6 shows the common mode conversion as the vias travel through 10 plane pairs.

     Consider a 2 Gb/s differential signal travelling through the asymmetrical differential vias with a ground-reference via only 50 mils from one of the differential vias. From Figure 6 we see the fundamental frequency of 1 GHz will have a common mode conversion of 240 dB. This means if the intentional differential signal is one volt, then the common mode noise will be 10 mV. If we use the rule-of-thumb that above 1 GHz an external cable must have no more than 1 mV of noise in order to meet Class A emissions requirements, then the common mode noise is 20 dB too much, and will require at least 20 dB of shielding to meet the limits.

     From the differential signal’s common mode conversion point of view, it would seem that we want the ground-reference via to be far away from the differential via pair. However, this would only help reduce the amount of common mode created by the asymmetrical ground-reference via. Any other common mode noise on the differential signal pair created because of other issues, such as skew, rise/fall time mismatch, etc. will need to have a close ground-reference via so that the other common mode noise will not be coupled to between the planes. Clearly the best option is to use a nearby ground-reference via (or more than one) to ensure symmetry!

     Another example is taken from a real-world PCB. A screen shot of the PCB layout is shown in Figure 7. The insert shows the initial asymmetrical placement of the ground-reference via, and the new location where the symmetry is enforced. Figure 8 shows the impact of moving the ground-reference via to a symmetrical location. More than 40 dB reduction in the common mode conversion was realized!

 

Fig. 7. Real world pcb geometry with asymmetrical ground-
reference via.
Fig. 8. Comparison of mode conversion (SCD21) with GND return via symmetry and asymmetry.


Summary
Common mode noise will be created by a variety of causes. Via placement and symmetry of nearby ground-reference vias is another source of significant common mode noise for differential signals. It is important to closely control the symmetry of ground-reference vias near to differential pair vias.     EMC

 


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