Searching for Clarity: Signal Integrity Issues in PCB Design

January 9, 2018 Altium Designer

 TV with antenna getting no signal

 

 

The clarity of TV signal transmission these days is amazing. I can still remember the days when I had a set of rabbit ears perched atop my 19” RCA. It was a challenge to position them just right to get the clearest picture, not to mention having to adjust them for every channel. Sometimes it’d take all night just to find the channel I wanted, so I’d sit down for five minutes and then head to bed. And if the weather was bad, forget about it.

 

While an occasional snow storm might make my TV signals a little choppy, that’s nothing compared to the old days. Network designers in the latest TV signal technologies such as high-speed cable, internet, and satellite have to consider transmission paths, obstructions, and interference when they’re planning and developing our modern TV needs. But signal transmissions aren’t strictly for the network designers—you can have the same types of problems with your PCB design. Since you don’t have any rabbit ears to play around with on them, protecting against signal integrity issues is critical for keeping your PCBs running smooth and without static.

HDMI, or Composite? Know the Signal in Signal Integrity Issues

Whether it’s cable, satellite, Netflix, Hulu, Amazon, Youtube—smart TV, flat-screen, plasma—there are hordes of ways to receive the signals you need to watch the television you want to see. Similarly, most PCBs will include several signal types. Some of the most common are:

 

Power Supply Signals: Depending upon complexity, you may have several different levels of power supply Signals. For example, most processors require signals in the 3-5V range, while amplifiers may require bias voltages of up to 15V range.

 

Data Signals: Data may be either analog or digital. For analog, typical ranges are +/- 10V. For digital, the range may be 0-5V or +/ 5V depending upon signal format.

 

Control Signals: These are usually for switching devices on or off and are typically 0-5V signals.

 

Communication Signals: These may have signal strengths as low as in the microvolt level. These are typically RF and the frequency may vary over a wide range.

 

In addition to signal strength, in the case of analog signals, frequency is also an important consideration that must be incorporated into your PCB design. This is especially important for PCBs that incorporate communication devices where the frequency levels can be very high.

Output Dimensions and Routing Considerations: How Wide?

For very simple PCB designs, you can usually get away with designing routes to accommodate the extremes of current and voltage. For example, using a single trace width throughout that can handle the maximum ampacity. Your board may work fine for these cases, although, this is far from the best practice. If you are like me, these are far and few between and we usually want to optimize our boards by making them as small and multi-functional as possible.

 

For these more complex PCB designs, extreme care has to be taken and routing is not quite as simple. We want to design routes that match the signals they have to carry. Consult the following table for general considerations:  

 

Routing Considerations

Potential Problems

Routing Solutions

Trace length

Signal strength loss,

Minimize

Distance between traces

Interference

Route perpendicular

Trace angles

Crosstalk, radiation

Round, Use 45° instead of 90°

Via usage

Reflections

Minimize

 

Employing the above-suggested solutions reduces losses, space, and costs. However, to properly incorporate these requires using the best PCB design tools.

Make Sure to Mute

Noise in older TV signal transmissions left you trying to view your favorite program through a layer of snow. Today, noisy transmissions either give you a disjointed puzzle or no signal at all. The source of noise in PCB design can be from a number of sources. For example, perturbations from switching IC pin states, interference from radiating devices such as oscillators into nearby traces, multiple frequency signals on the same trace, and other sources.

 

It is virtually impossible to completely eliminate noise from most complex PCB designs. However, there are ways to minimize noise. Your best asset in eliminating noise is the placement of components. All component radiation is frequency dependent and the higher the frequency (signal change) the shorter the distance from the source. Therefore, you should place components that are processing the same signal away from other components. Place components where signals originate and terminate close to each other.

 

You could also try implementing bypass capacitors. These should be tied directly to the ground, if possible, and sized appropriately. Note that any inductance will combine with the capacitor to set up a filter circuit.

 

TV with antenna getting no signal

Noisy digital TV signal

 

 

Twist the Ears Together: Differential Transmissions

Signal integrity can be improved by using differential circuits, whenever possible. Most IC designers try to implement this; however, at times differential pairs may be routed to pins that are not adjacent to each other. From a PCB design perspective, we want to do the following:

 

  1. Route differential pairs together.

  2. Use the same trace width and length (as much as possible).

  3. Route on the same signal plane or layer.

Ground Planes Can Make Signal Integrity Easy

Ground planes can provide a central layer reference for multiple circuits, which can significantly reduce your pin connection and trace numbers, not to mention simplifying the visual layout of a complex PCB. When designing your PCB, the following tips can minimize problems associated with the ground planes:

 

  1. Ensure there are no direct paths from a power plane to a ground plane (this may seem like a no-brainer, but for complicated designs, it can be overlooked and not realized until parts start frying).

  2. Do not insert gaps in ground planes.

  3. Run clock signals (or other critical signals) between ground/power planes.

  4. Route signals that use the same return (ground plane) on the same layer.

  5. For boards with multiple ground planes, connect the planes together at the same single point on each plane.

  6. For boards with chassis connections, make sure all ground planes are connected together.

 

Ground plane with and without slot

Ground plane do’s and don’t’s

 

 

While it always felt like I was winging how I’d move my rabbit-ears around to get the channel I wanted, there was always some method to the madness.: swing far left, then swing far right and slowly work back-and-forth until I found the exact spot with optimum clarity. Thankfully, there are significantly more concrete ways to achieve the best signal integrity for your PCB design. By making the best routing decisions, employing noise reduction strategies, using differential pairs whenever possible, and applying good ground plane usage you will significantly improve your chances of maintaining signal integrity.

 

For most PCB designers, myself included, it takes quite a while to gain a good level of proficiency in these areas. However, Altium provides tools; including Circuit Studio that can have you designing PCBs that maximize signal integrity like a pro in no time.

 

For more information on signal integrity issues in PCB design, contact an Altium PCB design expert.

 

About the Author

Altium Designer

PCB Design Tools for Electronics Design and DFM. Information for EDA Leaders.

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