notch rf filter

[lev]

how can I make a filter that notches out 14.7 K HERTZ

 

[john monks]

Yes. Did you consider a capacitor and an inductor in series?

 

[lev]

Thank you for your reply but the problem is I don't know what value Inductor or capacitor to use in the circuit to notch 14.7k hertz

 

[john monks]

If you want the values there are plenty of sources for this exact formula in books. But if you want to understand the circuit I recommend that you look up the characteristics of inductors and capacitors and study them until it becomes second nature. But I will give you a clue. If you place 1 ampere of direct current on a 1 farad capacitor you will get a changing voltage of one volt per second. And if you place 1 volt on a 1 Henry ideal inductor you will get a changing current of 1 amper per second. These are linear relationships. Now with some trigonometry and algebra you should be able to figure it out. Sorry, but I already got busted for doing people's homework.

 

[CocaCola]

Thank you for your reply but the problem is I don't know what value Inductor or capacitor to use in the circuit to notch 14.7k hertz

You live in the 21st century, a simple Google search will reveal multiple online free notch filter calculators that will spit out the values with little effort...

 

[(*steve*)]

It would also help to know *why* you want this. Is it homework (that requires a theoretical solution) or some other reason that requires a practical solution (possibly with other unmentioned constraints)?

 

[lev]

I thank you all for an interest in my problem
To answer some questions possed
No it is not homework
I am a man of 65 and my homwork days are behind me
The reason for the search is I have a private video feed that has 14.5 k hertz noise induced into it by a near by AM radio station that is effecting the quality of the picture Once more I thank you all for your interest with my problem

 

[(*steve*)]

OK, that's important information.

If you're wanting to filter the 14.5khz whistle from the audio, then we have some particular requirements that we can work with. Is this what you're after?

 

[lev]

Stev
My unit has only video there is no sound, what this signal is doing is causing a herring bone type of noise throughout the visual picture. I think if I had audio, it might very well induce a whistle also

 

[(*steve*)]

OK, then the problem is probably that the other signal is 14.5kHz from the signal of interest. It probably means you want to attenuate that signal rather than the resulting difference signal.

But if Davenn is about, he will probably be better able to tackle this one.

 

[davenn]

Hi Lev
a belated welcome to the forums

just one question before I ponder a response, is this a composite video signal or a RGB signal that you are using for your video feed ?

cheers
Dave

just had this thought ..... are you REALLY SURE that the radio transmitter is 14kHz ?
that isnt broadcast ..... its way down in the ELF range used by Omega stations etc

 

[lev]

Hi Davenn
Thank for your reply
I am absolutely sure that the interferance is coming from a AM radio station broadcasting at 14.7k hertz (that information was the subject of another search that I made, which was step one in solving my problem )
my signal is a composite video
Once more thank you for your input

 

[(*steve*)]

Just out of curiosity, what's the name of the station?

 

[CDRIVE]

This is very odd because 14.7kHz is audio frequency which does not lend itself well to radiation via conventional antennas. The IF stage of an AM radio is 455kHz, which is nearly 31 times greater. In the US, the AM broadcast band spans from 540kHz to 1600kHz, which puts the lower end of the band at nearly 37 times greater.

Is your system 75 Ohm coax, CAT5, Wireless or a combination of any of them?

 

[BobK]

Are you sure you do not mean 1470 KHz, that would be in the AM broadcast band.

Bob

 

[CDRIVE]

Are you sure you do not mean 1470 KHz, that would be in the AM broadcast band.

Bob

Ah, those darn decimal points!

 

[john monks]

The best solution is probably to use a 0.5 henry coil in series with a 350pF tuning capacitor and tune your 14.5kHz signal out.
Sorry about the confusion.

 

[john monks]

Your best bet is probably to use a 0.5 henry coil in series with a 365pF tuning condenser.
Sorry about the confusion.

 

[CDRIVE]

That might be jumping the gun a bit. If his system is a typical CCTV 75 Ohm coax, they have extremely high immunity when properly terminated. Many CCTV monitors, actually most, have an integral switch and 75 Ohm load that's used to terminate the line. If there's more than one monitor on the line then only the monitor at the end of the line should have the termination switched in. I mention this because I've installed and run thousands of feet of camera cable in the ceilings of a hospital. These cables are subjected to multiple sources of noise. I've never had an issue with properly terminated cable.

For sure, improper terminations of any sort turns a coax cable into a series of lumped values of capacitance and inductance, along with resistive and dielectric loss thrown into the mix. An improperly terminated coax losses all of its immunity properties and gains massive signal loss or peaks along the line.

 

[lev]

Hi all
To answer a few inputs and questions
MY cable is a 75 ohm coax
It runs about 150 ft
The freq thats causing the problem is 14.7k it is not a decimal point error.
Once again, I thank each of you for your input and thoughtfulness
Im checking all input given to me