What is a notch filter?

Not one of those things you’d tend to need for your average home theater system, you still might wonder from time to time what a notch filter is. Very simply, a notch filter very effectively blocks a very narrow range of frequencies and it does so very well. In other words, it’s like it creates a little “notch” in the signal where nothing gets through.

When should you use a notch filter?

Notch filters are used when two or more signals are combined to make sure that you don’t create interference. A perfect example is in an apartment where cable television or satellite TV comes in on the same line as internet service. If the two services come from different providers, a notch filter is used to isolate out a small range of frequencies where the internet signal can operate. The notch filter is used on the cable or satellite incoming signal before it goes into a combiner. If the notch filter is built into the combiner, it’s referred to as a diplexer.

What’s the difference between a band stop filter and a notch filter?

The term “band stop filter” this is sometimes used interchangeably with the term “notch filter” in the DIRECTV world. In the greater world “notch filter” really tends to refer to a very narrow band of frequencies with very high isolation.  However, “band stop filter” could talk about a large range of frequencies, such as hundreds of megahertz.

For DIRECTV users, the terms are pretty interchangeable because the only frequencies you’d want to block are the ones used by antennas or connected home filter. You use a band stop filter to block those frequencies.

And then there are band pass filters

The opposite of a notch filter (or band stop filter for that matter) is a band pass filter. Where a notch filter lets through every frequency except a small range, a band pass filter lets in only a small group of frequencies. If you were really going to be super careful to avoid interference, you would put a notch filter on one cable and a band pass filter on the other before they were combined. In reality, that’s more than you need to do. Even though the parts are relatively inexpensive, you don’t need them. Installers and system designers use band pass filters in areas where signals are really critical and there’s a potential for a lot of interference.

Where should you use a notch filter?

You can use a notch filter to filter out extremely narrow ranges. It just depends if you are trying to isolate interference from a very strong local source. You can use a notch filter to isolate out strong cellular signals that could interfere with weak antenna signals.  One common use of notch filters is in TV antennas. Until the mid-2000s, TV broadcasting used the range from 600-800MHz. Antennas tend to receive those frequencies very well. That’s part of their design. However, cellular data now uses those frequencies. Those cell signals can be a lot stronger than distant antenna signals. A lot of today’s antennas do already block these signals and it’s becoming more common to find those filters.

If you’re looking for a notch filter or anything else to help your antenna distribution system work better, shop the great selection at Solid Signal.

About the Author

Stuart Sweet
Stuart Sweet is the editor-in-chief of The Solid Signal Blog and a "master plumber" at Signal Group, LLC. He is the author of over 8,000 articles and longform tutorials including many posted here. Reach him by clicking on "Contact the Editor" at the bottom of this page.