How to make a simple notch filter?

Question

I hate this "beeb-beeb" sound in some channels of my TV when there is radio near me or my PC is on. I've search the net for a solution and I saw this TV Notch Filter. Can someone teach me how to make a simple notch filter, pls.

Answer 1

It's just an LC circuit.

You need to know the frequency of the interference, and then you can make an LC circuit to notch out that signal. If you take a variable capacitor, and a coil, and hook them up in series you will create an LC circuit. The Circuit will be a dead short for a specific frequency, the trick is making one for the frequency you want. You put the LC circuit in parallel with the incoming signal. What it does is creates a short at a specific frequency, notching out the interference.

A single LC circuit will only give you -30 dB. To get more of a notch, you parallel more of them together.

Since you said a TV, I won't bring up the series option, as you can setup a single LC circuit to pass a specific frequency. This is called a band-pass filter, and is simply an LC circuit in series with the incoming signal.

Most TV frequencies are VHF and UHF. You need to find the channel with the interference, and then figure out what frequency the channel is. Then you need to figure out what the frequency of the interefence is.

You should be able to get a variable mica capacitor, and make a coil out of wire, maybe 3 or 4 turns, at pencil diameter. This would be in the VHF end of the TV range. You need less capacitance and less inductance to go higher in frequency. Also, the hookup wire to the devices become more significant as you go higher in frequency. That why it's best to have real short leads.

Also, you can switch the adjustment around, and use a variable inductor, and a fixed capacitor.