Bandstop (notch) RF Filter question.

Does anyone have a diagram for a basic notch filter that will block out a cable tv channel (3 or 4) so i can insert my own signal into the
line for local viewing? Channel 3 or 4 would be the target to get rid of, 60-70 MHz range. I've never made a filter and google searchs have been void from good infomation.
One thing to note, I also have digital cable and a cable modem. I'd like to not tick either of them off ;-). I found the following paragraph about the cable modem's freq's,
"To deliver data services over a cable network, one television channel (in the 50 - 750 MHz range) is typically allocated for downstream traffic to homes and another channel (in the 5 - 42 MHz band) is used to carry upstream signals."
The filter would have to leave <42Mhz alone. also, since i get a channel 3 and 4 i know they aren't using 60-70 MHz for the downstream so i think i'm safe.
Here is a freq list for channels 2 and 3's picture/sound. 61.2500 MHz 3 pic 65.7500 MHz 3 snd 67.2500 MHz 4 pic 71.7500 MHz 4 snd
I've read that some filters can cause problems on the system to other users close to the filter. That's another think i'd really love to avoid.
Thanks, -N
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
snipped-for-privacy@bleep.com says...

Do a google search on "Bridged-T filter". They can easily be made to separate parts of a video signal, or even interference between the audio and video carriers. ;-) It shouldn't be too hard to de-tune one to cover a whole channel.

A bridged-T will be plenty sharp enough for your needs.

Thirty years ago I made one for channel 5 that passed the audio and video and suppressed everything (40-50db down, IIRC) inbetween.
You might get a better response from sci.electronics.design than here though.
--
Keith

Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
On Sun, 9 May 2004 09:33:45 -0400, KR Williams put forth the notion that...

This brings up a question I've had for some time now. Is there a way to filter out whatever signal networks or local stations ad that inserts the station ID "bug" in the lower right hand corner? How about the scrolling marquis, pop-up banners and other assorted garbage that some stations think is so cute? TV's starting to get as annoying as a lot of web sites.

--
Checkmate

Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
snipped-for-privacy@The.Edge says...

Sure. Duct tape.
--
Keith

Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
On Sun, 9 May 2004 11:40:09 -0400, KR Williams put forth the notion that...

If I did that, my 36" TV would probably be closer to a 24" when I got done.
--
Checkmate

Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
snipped-for-privacy@The.Edge says...

Nah, measure it. The point is that a "filter" isn't going to get rid of that crap. He only thing that will is turning the damned thing off. Sooner or later they'll get the message. Until then, suck it up. They "control the horizontal" and they "control the vertical". ...get used to it, or turn the damned thing off.
--
Keith


Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

to
of
What the world really needs is a way of detecting the presence of the tumor on the bottom of the screen and using it to make a fool-proof commercial killer. Notice - no tumors during the ads!
Myself, I prefer to watch DVDs - some times I even put them in the player.
Bill
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

i found a few pdfs with the gritty of these things. they look like what i want. :D http://www.drp.fmph.uniba.sk/ESM/twin.pdf http://www.crbond.com/papers/ent22.pdf I'm a little lost when it comes to inductors thou. never did stuff with rf. I'll hack my way thru it. thanks.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
snipped-for-privacy@bleep.com says...

They may tricky to tune sharply (though you don't need what we were looking for), without some expensive tools. A friend used some rather expensive HP network analyzers and I did it in the time-domain with the help of an even more expensive Tek Signal- Processing-System. Cool stuff! ...but that was 25 years ago, so it's likely a $.10 deal now. ;-)
--
Keith


Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

Yea, Initial tests are not going well. It seems that i can only induce static across 15 channels with the center one almost blocked out (i'd guess around -15 to 20 db at center). i'm starting to tweak componets but not sure which is responsible for how wide the notch is. not having an o.scope or any of those exp. toys is a bit annoying, but i'll just tweak until i get it right.
Just for referance, correct me if i'm wrong (because i'm guessing), the resistor is what decided the -db level at center freq, the 2 caps are what decides the width of the blackout area and the inductor tunes it? :-/ i hate not having an o.scope.. then i could just see what is happening.
fixed pitch ascii art. --------L1------- In ____| |____Out | | -----C1----C2---- | R1 | ----- --- -
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
snipped-for-privacy@bleep.com says...

Well, you really have to be able to measure filter response to be able to tune such.

Good luck! Trap filters are tricky, since they're very selective, which is what you wanted.

It's been a *long* time since I did this stuff (30 years or so). The resistor is what makes it a "bridge". The resistor is used to balance out the 'Q' of the inductor. IIRC, the inductor we used was a couple of turns of 14GA buss-wire around a 3/8" drill bit form (high tech here, I know), the resistor a few hundred ohms, and a couple of 30pf mica caps, but it's been many years. The inductor is squozen to tune.

That's the one, but notice the series L1 parasitic in the equations.
Hell, I'm lucky I remembered the name of the filter (and others have told me I'm all wet)! ;-)
--
Keith

Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

Polytechforum.com is a website by engineers for engineers. It is not affiliated with any of manufacturers or vendors discussed here. All logos and trade names are the property of their respective owners.