Noise, Noise Filters and Automatic Gain Control
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|-||1 Technically AGC either amplifies or attenuates the signal to keep the noise at a constant level. Either way though, the net result is the same.||+||<sup>1</sup> Technically AGC either amplifies or attenuates the signal to keep the noise at a constant level. Either way though, the net result is the same.|
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Revision as of 12:03, 7 April 2006
What is Powerline Noise?
After working with X10 for a while you will no doubt come across someone telling you that you have a noise problem. For many people, their first thought is, "Noise? I don't hear anything." What the person is talking about is electrical noise on the powerline (often called Powerline Noise). Although you can't hear this type of noise, it really isn't all that much different from the type of noise you can hear. At low levels the noise might not cause any problems, but if it is loud the noise might prevent you from hearing what you are listening for. Noise filters attempt to reduce the amount of noise making it easier for the receiver to hear the signal.
Common Sources of Powerline Noise
There are many different sources of powerline noise but two of the most common ones are Switching Power Supplies and Electric Motors.
Switching Power Supplies
A power supply is needed to convert the 120VAC that is in your house to the low voltages needed by electronic equipment. Traditionally a transformer was used to do this since they are easy to build. Transformers are very friendly when it comes to X10. Not only do they not generate any powerline noise, but they are inductive in nature meaning they provide a small load on the X10 signal and help filter some of the noise generated by the device from flowing back onto the powerline.
Transformers have some drawbacks though. If the device needs a lot of power, the transformer needed is big, heavy and expensive. Also they aren't very efficient (you will notice that a lot of power supplies will get warm and heat is the product of wasted electricity).
Because of these disadvantages, many devices use switching power supplies. They are smaller, lighter and more energy efficient. For small power supplies, there is little cost benefit for using switching power supplies, but larger switching power supplies are much cheaper when compared to a transformer. For this reason computers have used switching power supplies for a long time. With new environmental regulations, more and more devices will start to use switching power supplies.
From an X10 perspective, switching power supplies aren't as good. Not only are they not inductive in nature, but they will generate noise on the powerline or have filtering capacitors which suck up the X10 signal. Newer designs are better than some of the older ones and sometimes replacing the powersuply is a better option than using a noise filter (or do both).
Some devices that use switching power supplies include (but are not limited to): PCs, Flat Panel TVs and Compact Fluorescent Lights (CFLs)
Although electric motors are inductive in nature, sometimes the brushes that transfer the electricity to the rotor will briefly lose contact with the commentator and will generate a large high frequency spike on the powerline. This is typically more of a problem on high speed motors and a good quality motor will behave better than an inexpensive one. Sometimes too much noise is generated by the motor for a single noise filter and two (or more) noise filters would be necessary.
Types of Noise Filters
A noise trap will attract the noise to it and capture it. This is done by putting a capacitor across the hot and neutral wires. As you may remember from my article on Phase Coupling, capacitors attenuate low frequencies more than high frequencies. Thus very little of the 60Hz AC power will travel through the capacitor. However the higher frequency noise will see an easy return path and will be attracted and dissipated by it.
Many power bars, UPSes and electronic equipment have noise filters inside. Typically they use a noise trap for two reasons:
- It captures the noise so that it won't bother anyone else and
- Since the very little of the 60Hz AC power travels through the capacitor, it can handle any current load.
The problem is, a noise trap will capture the X10 signal along with the noise making the signal weaker.
A noise blocker will put up a wall to prevent the noise from getting in or out. This is done by putting an inductor in series with the device. You may also remember from my article on Phase Coupling, inductors attenuate high frequencies more than low frequencies. Thus the 60Hz AC power will barely even see the inductor as a bump in the road. However the higher frequency noise will see a big wall preventing it from passing. Thus the noise blocker however will keep the noise on one side of the filter and the X10 signal on the other side.
Not only will a noise blocker filter noise, but it will provide protection from devices which suck up the X10 signal (such as noise traps). Using a noise blocker in this situation will prevent the device from weakening the X10 signal.
X10 Noise Filters
You can buy noise blockers for use with X10. These tend to be called X10 noise filters and come in both plug-in and in-line devices. The plug-in type are useful when you have something that is plugged into the wall which causing you problems where as the in-line filters are useful when the thing that is causing problems does not have a plug or a circuit has several devices which produce noise and you don't need to use X10 modules/controllers on that circuit.
Two things to watch out for when using noise filters are:
- Make sure the filter is rated for a high enough current for what you have it connected to. The filter has a fuse to protect you if it tries to draw too much current, but in most cases it is not user replaceable.
- Since the X10 signal cannot pass through the noise filter, make sure that all of your X10 equipment is on the same side of the filter.
Whole Home Noise Filters
What if the source of the noise is outside your home? Or what if a neighbour is also using X10 and his powerline signals are coming into your home? In cases like this, you need a whole home noise filter. Most whole home noise filters also have a built in passive phase coupler.
This type of filter is installed between the power meter and your main panel. Since you can't easily turn off the power to this line, most people would prefer to leave instillation of this to a qualified electrician.
Automatic Gain Control
The X10 protocol says that receivers will detect signals above 100mV. That means a perfect receiver would require that the signal be at least 100mV and the noise must be less than 100mV. What if the signal was 600mV and the noise was 120mV? Similarly what if the signal was 80mV and the noise was 12mV? In both of these cases there is a definite distinction between the signal and noise, but in both cases a standard (cheap) receiver would not be able to distinguish one from the other.
For this reason, better quality (more expensive) receivers have a feature called Automatic Gain Control (AGC). A receiver with AGC will continuously measure the noise level is and will adjust the minimum signal level required accordingly1. That way if it sees 12mV of noise it might say anything above 20mV is considered to be a signal. Similarly if it sees 120mV of noise it might say anything above 200mV is considered to be a signal. However, if the noise is stronger than the signal, AGC won't help.
There are different ways of implementing AGC and some are better than others, but the net result is any receiver with AGC will be much more reliable than one without it.
1 Technically AGC either amplifies or attenuates the signal to keep the noise at a constant level. Either way though, the net result is the same.
Key Topics: on, off, noise, circuit, phase, coupler
Written by Roger H. 14:15, 6 April 2006 (PDT)