Rhino
Well-Known Member
- Joined
- Jul 20, 2009
- Messages
- 1,630
Quite a few forum threads and articles have at various times addressed ground loops, and their potential to cause noise or other interference in your avionics system. Simply put, a ground loop occurs when different paths to ground are used. The difference in resistance, or impedance, on those paths can cause oscillations that result in noise in your data or audio systems. As a result, considerable effort should be applied to ensure common grounds are used whenever possible, and you’ll see that mentioned in aviation forums with some regularity, especially those associated with homebuilt aircraft. But not all ground loops are created equal, and all situations and systems do not always require the same common ground. Not only are there different types of ground loops, but how they affect your systems, and how that effect manifests itself, can vary quite a bit. It can get very confusing.
One source of confusion is the oft repeated mantra that everything should use the same ground. That isn’t strictly true, but it is widely applicable. And it’s very important. The power system for your avionics will use a common ground on your avionics bus. Your audio system will use a common ground typically on your intercom or audio panel. Your data distribution systems, such as your serial bus, will usually use a different common ground. They aren’t all in the same place in most aircraft, because the path they may use to induce noise, and the manner in which they may do it, are different. But, while they don’t necessarily have to be grounded at the same location, they certainly can be. And if they are, it reduces the potential for ground loops
Another source of confusion is the different manner in which different systems cause noise. Power distribution systems typically cause noise simply by virtue of the ground potential difference, the ground loop. They’re also very susceptible to carrying engine electrical system noise, usually from alternators, into your other systems. Those are usually identifiable because they change with engine RPM.
Audio and data systems are also prone to ground loops, but they can also get induced noise caused by the wiring itself. That’s usually an issue with improper shielding, or improperly grounded shielding. Both ground loops and induced noise are affected by grounding and shielding. Grounding and shielding are actually two different things, but are sometimes shared by the same wires, depending on the grounding scheme used. There are differing grounding and audio routing standards and practices in the aviation world, and different equipment manufacturers will often advocate different practices.
In the early days of using electricity in aircraft, simple straight wires were used. When audio was introduced, noise was rampant, and generally considered a necessary evil by those who just didn’t know any better. It just went with the territory. But as avionics technology advanced, noise become more than just a simple irritant. It often made installed equipment unusable. The concept of ground loops had been understood early on in the electrical power systems of aircraft, so the first mitigation efforts using common grounds were often limited to the main electrical bus. It wasn’t until later than noise reduction efforts within the avionics systems themselves began in earnest. That’s how the common ground schemes mentioned above came about. There wasn’t always complete agreement on where the common ground should be, but it was agreed there should be one, or at least one for each subsystem. Unfortunately, efforts to reduce induced noise in wiring weren’t so easily accomplished, and often did not yield the desired results.
Induced noise in wiring is simply a signal in one wire ‘coupling’ into an adjacent wire or wires, sometimes referred to as crosstalk. It’s the same principle as an antenna receiving radio signals. But obviously we don’t want our avionics wiring acting as antennas. Efforts to mitigate this problem have varied over the years. Some simply twisted the audio and data wiring to reduce or eliminate the potential for induction. Ethernet data cables still use that method today. But that method has more limited ability when it comes to reducing audio noise. That has to do with frequencies involved and the differences between analog and digital signals, but I’m not going to delve deep into that here.
The next step in the noise reduction process was using shielded cable. The audio or data transmission wire was wrapped in a metallic shield that protected it from outside signals. The first iterations of this method usually involved one wire inside, audio high, being wrapped in a metal mesh that was used as the audio low path. This is what’s known as unbalanced wiring or cables. The most commonly seen example of that is the RCA cables used to connect home audio equipment for many years, and still used today. The problem with unbalanced cables is the audio lo in the shielding, although less susceptible than audio hi, can still pick up induced noise. If the audio shielding was also connected to ground at multiple points, ground loops could come into play, sometimes even if you use a common ground scheme. The best method generally available today is balanced cabling, where both the audio hi and audio lo wires are contained within a separate shield. This also applies to data lines. But we see more problems with audio noise in aircraft, so I’m addressing it in that context.
While different manufacturers may have different philosophies on types of audio cables and how to route audio lo lines, the shielding in your audio system should always ground at the same location. That’s an audio panel if you have one, and your intercom if you don’t have an audio panel. If you have neither, pick a common location within your audio distribution setup. I’d typically opt for a radio, or a mixer if you use one. I highly recommend you also use balanced cables, even if one of your equipment manufacturers doesn’t advocate it.
So ideally, we use balanced cabling, we have a common ground location for power, a common ground location for audio, and a common ground location for data signal wiring. We may even have a single common grounding point for all three. What about something that doesn’t fit into those categories? There are numerous wires in your aircraft that are not part of those systems, or that don’t fit into those categories. In an ideal world, you would use different wiring locations or bundles for those wires, to keep them apart and reduce the potential for inducing or coupling noise. That’s especially true for anything involving electric motors such as autopilot and trim systems. But a separate wiring location isn’t always possible. In that case, use shielded wire wherever possible, even for wiring that has nothing to do with audio or data, and ground that shield to the common point your audio or data wiring uses (preferably audio). Where cables cross, try to have them to do that at right angles rather than running parallel to each other. None of these practices are mandatory, hard and fast rules. Different situations and circuits present different potential for inducing noise into your avionics system. But you’ll likely save yourself a lot of grief if you follow these practices as if they are mandatory.
There will be some aspects of your system that don’t fit into a readily defined category, or that fit into more than one, and seem to present grey areas. Examples of this are autopilot disconnect or PTT switches. Although they may be closely related to power, audio or data systems, they don’t actually carry any of those signals. They are usually sense lines that connect to ground. You will commonly see manufacturers depict them as going straight to chassis ground. Since they carry no signals, you’ll probably be perfectly safe doing that. But one thing you’ll learn from working aircraft, especially homebuilt aircraft, is that noise can come from the strangest places, and in the strangest ways. If any wiring for these circuits travels within or along your data or audio wiring, you should follow the above shielding and common grounding practices just to be safe. PTT is a prime example of that, since it often routes with audio wiring. Even if none of this wiring routes alongside your audio or data wiring, you can still follow those practices out of an abundance of caution. In fact, I highly recommend it. And you’ll find that many others will insist on it.
Lastly, I’ll point out that none of this guarantees you won’t have noise, or that noise will be simple to troubleshoot, should you experience it. Noise can come from places completely unrelated to the wiring, even when individual pieces of equipment are operating properly. Impedance mismatches, line level outputs going to audio level inputs, units configured improperly, settings incorrect, bad antennas or antenna cables, etc., etc. The possibilities for problems are almost endless. You may also discover a solution that works perfectly on one aircraft doesn’t work at all on another aircraft, even one that’s seemingly identical. Audio is incredibly fickle that way. And if you encounter this situation on a homebuilt aircraft, welcome to experimental aviation!
Thanks to Marc Zeitlin for doing an editorial review on this.
One source of confusion is the oft repeated mantra that everything should use the same ground. That isn’t strictly true, but it is widely applicable. And it’s very important. The power system for your avionics will use a common ground on your avionics bus. Your audio system will use a common ground typically on your intercom or audio panel. Your data distribution systems, such as your serial bus, will usually use a different common ground. They aren’t all in the same place in most aircraft, because the path they may use to induce noise, and the manner in which they may do it, are different. But, while they don’t necessarily have to be grounded at the same location, they certainly can be. And if they are, it reduces the potential for ground loops
Another source of confusion is the different manner in which different systems cause noise. Power distribution systems typically cause noise simply by virtue of the ground potential difference, the ground loop. They’re also very susceptible to carrying engine electrical system noise, usually from alternators, into your other systems. Those are usually identifiable because they change with engine RPM.
Audio and data systems are also prone to ground loops, but they can also get induced noise caused by the wiring itself. That’s usually an issue with improper shielding, or improperly grounded shielding. Both ground loops and induced noise are affected by grounding and shielding. Grounding and shielding are actually two different things, but are sometimes shared by the same wires, depending on the grounding scheme used. There are differing grounding and audio routing standards and practices in the aviation world, and different equipment manufacturers will often advocate different practices.
In the early days of using electricity in aircraft, simple straight wires were used. When audio was introduced, noise was rampant, and generally considered a necessary evil by those who just didn’t know any better. It just went with the territory. But as avionics technology advanced, noise become more than just a simple irritant. It often made installed equipment unusable. The concept of ground loops had been understood early on in the electrical power systems of aircraft, so the first mitigation efforts using common grounds were often limited to the main electrical bus. It wasn’t until later than noise reduction efforts within the avionics systems themselves began in earnest. That’s how the common ground schemes mentioned above came about. There wasn’t always complete agreement on where the common ground should be, but it was agreed there should be one, or at least one for each subsystem. Unfortunately, efforts to reduce induced noise in wiring weren’t so easily accomplished, and often did not yield the desired results.
Induced noise in wiring is simply a signal in one wire ‘coupling’ into an adjacent wire or wires, sometimes referred to as crosstalk. It’s the same principle as an antenna receiving radio signals. But obviously we don’t want our avionics wiring acting as antennas. Efforts to mitigate this problem have varied over the years. Some simply twisted the audio and data wiring to reduce or eliminate the potential for induction. Ethernet data cables still use that method today. But that method has more limited ability when it comes to reducing audio noise. That has to do with frequencies involved and the differences between analog and digital signals, but I’m not going to delve deep into that here.
The next step in the noise reduction process was using shielded cable. The audio or data transmission wire was wrapped in a metallic shield that protected it from outside signals. The first iterations of this method usually involved one wire inside, audio high, being wrapped in a metal mesh that was used as the audio low path. This is what’s known as unbalanced wiring or cables. The most commonly seen example of that is the RCA cables used to connect home audio equipment for many years, and still used today. The problem with unbalanced cables is the audio lo in the shielding, although less susceptible than audio hi, can still pick up induced noise. If the audio shielding was also connected to ground at multiple points, ground loops could come into play, sometimes even if you use a common ground scheme. The best method generally available today is balanced cabling, where both the audio hi and audio lo wires are contained within a separate shield. This also applies to data lines. But we see more problems with audio noise in aircraft, so I’m addressing it in that context.
While different manufacturers may have different philosophies on types of audio cables and how to route audio lo lines, the shielding in your audio system should always ground at the same location. That’s an audio panel if you have one, and your intercom if you don’t have an audio panel. If you have neither, pick a common location within your audio distribution setup. I’d typically opt for a radio, or a mixer if you use one. I highly recommend you also use balanced cables, even if one of your equipment manufacturers doesn’t advocate it.
So ideally, we use balanced cabling, we have a common ground location for power, a common ground location for audio, and a common ground location for data signal wiring. We may even have a single common grounding point for all three. What about something that doesn’t fit into those categories? There are numerous wires in your aircraft that are not part of those systems, or that don’t fit into those categories. In an ideal world, you would use different wiring locations or bundles for those wires, to keep them apart and reduce the potential for inducing or coupling noise. That’s especially true for anything involving electric motors such as autopilot and trim systems. But a separate wiring location isn’t always possible. In that case, use shielded wire wherever possible, even for wiring that has nothing to do with audio or data, and ground that shield to the common point your audio or data wiring uses (preferably audio). Where cables cross, try to have them to do that at right angles rather than running parallel to each other. None of these practices are mandatory, hard and fast rules. Different situations and circuits present different potential for inducing noise into your avionics system. But you’ll likely save yourself a lot of grief if you follow these practices as if they are mandatory.
There will be some aspects of your system that don’t fit into a readily defined category, or that fit into more than one, and seem to present grey areas. Examples of this are autopilot disconnect or PTT switches. Although they may be closely related to power, audio or data systems, they don’t actually carry any of those signals. They are usually sense lines that connect to ground. You will commonly see manufacturers depict them as going straight to chassis ground. Since they carry no signals, you’ll probably be perfectly safe doing that. But one thing you’ll learn from working aircraft, especially homebuilt aircraft, is that noise can come from the strangest places, and in the strangest ways. If any wiring for these circuits travels within or along your data or audio wiring, you should follow the above shielding and common grounding practices just to be safe. PTT is a prime example of that, since it often routes with audio wiring. Even if none of this wiring routes alongside your audio or data wiring, you can still follow those practices out of an abundance of caution. In fact, I highly recommend it. And you’ll find that many others will insist on it.
Lastly, I’ll point out that none of this guarantees you won’t have noise, or that noise will be simple to troubleshoot, should you experience it. Noise can come from places completely unrelated to the wiring, even when individual pieces of equipment are operating properly. Impedance mismatches, line level outputs going to audio level inputs, units configured improperly, settings incorrect, bad antennas or antenna cables, etc., etc. The possibilities for problems are almost endless. You may also discover a solution that works perfectly on one aircraft doesn’t work at all on another aircraft, even one that’s seemingly identical. Audio is incredibly fickle that way. And if you encounter this situation on a homebuilt aircraft, welcome to experimental aviation!
Thanks to Marc Zeitlin for doing an editorial review on this.
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