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Electronics Repair (sci.electronics.repair) Discussion of repairing electronic equipment. Topics include requests for assistance, where to obtain servicing information and parts, techniques for diagnosis and repair, and annecdotes about success, failures and problems. |
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#1
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CATV RF Notch Filter
Hi,
I never made a NF (Notch Filter), but I am thinking about making a "stub" (I think that's the term) NF for CATV to "block" a single "channel" coming in on the digital TV cable system. Note: This "channel" consists of ANNOYING ads/promos that you can not avoid or "turn off" when you use the "On Demand" feature to look at a list of "on demand" available movies, TV shows, music videos, etc. I know the frequency, which by the way is between 200 and 300Mhz. If I have this correct, the "stub" is a 1/4 wave length of the offending frequency. The 75 ohm coax is cut to that length and is shorted on the far end. The other end is connected via a "T" connector. Also, I don't know if the notch will be "deep" (high Q) enough to make the digital data "un-usable" (can't decode). Another concern is the width of the notch. If it is too wide, it may affect the adjacent channel. Does anyone have any suggestions, or can you recommend a place where I can order a good CATV NF (I am willing to pay)? Thank You in advance, John |
#2
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CATV RF Notch Filter
In article ,
wrote: I never made a NF (Notch Filter), but I am thinking about making a "stub" (I think that's the term) NF for CATV to "block" a single "channel" coming in on the digital TV cable system. Note: This "channel" consists of ANNOYING ads/promos that you can not avoid or "turn off" when you use the "On Demand" feature to look at a list of "on demand" available movies, TV shows, music videos, etc. I know the frequency, which by the way is between 200 and 300Mhz. If I have this correct, the "stub" is a 1/4 wave length of the offending frequency. The 75 ohm coax is cut to that length and is shorted on the far end. The other end is connected via a "T" connector. If it's a quarter-wavelength long at frequency of interest, then shorting the far end will cause it to behave very much like an open circuit at the "T" at that frequency. A quarter-wavelength transmission line results in the maximum transformation of the impedance from one end to the other. If what you're attempting to do is "short out" the frequency of that one channel, then you'd want either a quarter-wavelength stub which is open at the far end (and these can be tricky due to parasitic capacitance, and could also radiate signals from other channels) or a half-wavelength stub which is shorted at the end. Be aware that any shorted stub is going to look like a short at DC, which might be an issue on some cable systems. Also, I don't know if the notch will be "deep" (high Q) enough to make the digital data "un-usable" (can't decode). Another concern is the width of the notch. If it is too wide, it may affect the adjacent channel. The latter is not at all unlikely. Single-channel filters, deep enough to be useful and shallow enough not to affect adjacent channels significantly, tend to be fairly big and heavy and complex affairs. http://www.tinlee.com/CATV-ChannelEl...e.php?active=1 is one source for them - these are the sort of "big guns" used by CATV companies if they want to knock out one channel, and insert their own content on that frequency. http://www.tinlee.com/NotchTraps.php...=CATV&active=1 might be more what you'd need. The CR7-(Fo)-HQ line seems appropriate for your need, if the notching is deep enough. I don't think these are designed to have enough attenuation to allow re-use of the same signal (e.g. injecting new content) without interference... they're just designed to attenuate an unwanted signal. Does anyone have any suggestions, or can you recommend a place where I can order a good CATV NF (I am willing to pay)? I bought one of TinLee's single-channel units some years ago, when I wanted to notch out Channel 11 from my main over-the-air antenna (pointed north) and insert a separate Channel 11 feed from a Yagi I built (pointed south). It worked very nicely and it survived outdoor weather for as long as I needed it (several years). |
#3
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CATV RF Notch Filter
On Thu, 19 Feb 2015 13:22:56 -0800, Dave Platt wrote:
wrote: If I have this correct, the "stub" is a 1/4 wave length of the offending frequency. The 75 ohm coax is cut to that length and is shorted on the far end. The other end is connected via a "T" connector. If it's a quarter-wavelength long at frequency of interest, then shorting the far end will cause it to behave very much like an open circuit at the "T" at that frequency. A quarter-wavelength transmission line results in the maximum transformation of the impedance from one end to the other. If what you're attempting to do is "short out" the frequency of that one channel, then you'd want either a quarter-wavelength stub which is open at the far end (and these can be tricky due to parasitic capacitance, and could also radiate signals from other channels) or a half-wavelength stub which is shorted at the end. And, be aware that the _open_ quarter wave stub will also demonstrate 'zero' impeadance at all the odd 1/4 wave frequencies -- IOW, at 3X your "200 to 300 MHz" target, and at 5X, etc. Also, I don't know if the notch will be "deep" (high Q) enough to make the digital data "un-usable" (can't decode). Another concern is the width of the notch. If it is too wide, it may affect the adjacent channel. The latter is not at all unlikely. I have made 1/4 wave stubs of High Q out of 1/2" and 3/4" cable TV hardline to eliminate harmonic interference in TVs from nearby (non-TV) transmitters. WFM. HTH Jonesy -- Marvin L Jones | W3DHJ | W3DHJ | http://W3DHJ.net/ Pueblo, Colorado | @ | Jonesy | __ 38.238N 104.547W | jonz.net | DM78rf | 73 SK |
#4
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CATV RF Notch Filter
SNIP
Single-channel filters, deep enough to be useful and shallow enough not to affect adjacent channels significantly, tend to be fairly big and heavy and complex affairs. http://www.tinlee.com/CATV-ChannelEl...e.php?active=1 is one source for them - these are the sort of "big guns" used by CATV companies if they want to knock out one channel, and insert their own content on that frequency. Hi Dave, Thank you for this very useful information. I decided to abandon the idea of making my own NF. I sent an email to with the frequency of the channel I want to "block". I am waiting for a response. Again Thanks, John |
#5
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CATV RF Notch Filter
On some more traditional cable services the channels are transmitted in fixed frequency channels (MUX) usually 8Mhz wide just like air DTV. If the OP has such service it makes sense to add a filter to block that channel (but it may block other data that may cause the selection list to not work). It is not an ethernet like system but a DVB-C system.
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#7
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CATV RF Notch Filter
I used to mess around with cable notch filters back when the systems were
all analog and I can't see where you came up with this idea to work on digital systems. It's just not the same thing. The analog systems used the same 6 mhz channel bandwidth as the terrestrial broadcast systems so you narrowing down to the 200 to 300mhz range isn't much better than saying "it's in the cable somewhere". On digital systems the channels simply don't exist, unless they are analog. It's data like ethernet. Your cable box is more akin to a computer than a cable converter. What's worst is, some systems like fios and uverse, the "cable" ends at that box out front or in the alley and your converter is just telling what to send down the copper from the box to your house. Is how they feed the 500 channel universe into a pair of 50 year old abestos covered wires. Hi Bruce, Digital broadcasts over the air uses specific frequencies. However, cable uses coax instead of "air". Those digital "packets" require RF to travel over the air or on cable. I injected a 276Mhz signal into the cable and it "knocked out" those ANNOYING ads/promos you see when you use the "On Demand" feature to see a list of movies, TV shows, music videos, etc. With the ads "blocked", I was able to select and view (on demand) a music video because it uses a different frequency. John |
#8
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CATV RF Notch Filter
Jeff Liebermann wrote:
I know the frequency, which by the way is between 200 and 300Mhz. Do you know the bandwidth needed? If it's fairly wide band, you'll probably end up with a hipass and lopass filters in series. ** Not likely. It's not an analogue signal so you do not need to remove it all - just a part of it will do, enough to prevent decoding. ..... Phil |
#9
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CATV RF Notch Filter
Hi Dave,
Thank you for this very useful information. I decided to abandon the idea of making my own NF. I sent an email to with the frequency of the channel I want to "block". I am waiting for a response. Again Thanks, John Hi, Here's an update on the email results. As I said a while ago (02-20-15), I sent an email to . Later that same day, I received a reply!!! In the reply I was asked for more details regarding what I want (specs) in the filter. I told Sneva the frequency (276Mhz), etc. I have not received another reply. I sent another email to Sneva, but still no reply. I asked Sneva if he (assumed) can't make the filter, please let me know so I can look for another source. Still no reply. Can someone recommend another CATV RF notch filter source? Thank You in advance, John |
#10
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CATV RF Notch Filter
Here's an update on the email results.
As I said a while ago (02-20-15), I sent an email to . Later that same day, I received a reply!!! In the reply I was asked for more details regarding what I want (specs) in the filter. I told Sneva the frequency (276Mhz), etc. I have not received another reply. I sent another email to Sneva, but still no reply. I asked Sneva if he (assumed) can't make the filter, please let me know so I can look for another source. Still no reply. Hi, UPDATE: I finally received a quote, $185.00 plus $20.00 shipping. Since this filter has to be custom made, the price is high. John |
#11
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CATV RF Notch Filter
wrote:
I finally received a quote, $185.00 plus $20.00 shipping. Since this filter has to be custom made, the price is high. If you have more time than money, probably your only alternative is to look under rocks at surplus sites and see if anyone has a cable notch filter for HRC channel 33 (channel T). Those have a cf of 276 mhz. But from memory, most common single channel notch filters only went up to channel J. The rest above that usually blocked an entire band. -bruce |
#12
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CATV RF Notch Filter
On Friday, February 20, 2015 at 8:26:01 AM UTC-5, Bruce Esquibel wrote:
wrote: I never made a NF (Notch Filter), but I am thinking about making a "stub" (I think that's the term) NF for CATV to "block" a single "channel" coming in on the digital TV cable system. Note: This "channel" consists of ANNOYING ads/promos that you can not avoid or "turn off" when you use the "On Demand" feature to look at a list of "on demand" available movies, TV shows, music videos, etc. I know the frequency, which by the way is between 200 and 300Mhz. I think you need to look up building a better tin-foil hat, the one you are using isn't working. I used to mess around with cable notch filters back when the systems were all analog and I can't see where you came up with this idea to work on digital systems. It's just not the same thing. In other words hardwire or air broadcasts visible on today's High Definition Cable/Dish TV are in digital broadcasts only. And that is in: A.) High speed online or B.) Satellite Dish. Even that may be wrong, because you can hook your phone up to TV or computer and get internet that way. Internet now gives you radio, TV, etc. So only a software program (now called an app) can code, decode or recode these broadcasts. Their app vs yours. Only the providers' help desks can help you - and that's if they're allowed to. |
#13
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CATV RF Notch Filter
Here's an update on the email results.
As I said a while ago (02-20-15), I sent an email to . Later that same day, I received a reply!!! In the reply I was asked for more details regarding what I want (specs) in the filter. I told Sneva the frequency (276Mhz), etc. I have not received another reply. I sent another email to Sneva, but still no reply. I asked Sneva if he (assumed) can't make the filter, please let me know so I can look for another source. Still no reply. Hi, UPDATE: I finally received a quote, $185.00 plus $20.00 shipping. Since this filter has to be custom made, the price is high. Hi, FINAL UPDATE: I received the filter from Tin Lee Electronics. This filter removed the streaming promos that ANNOY me. The results, with the promos "removed", when I go to "On Demand" (a small delay occurs), now I see whatever channel I was viewing, instead of promos. This can be handy when commercials come on, and I can check what is "on demand" during commercials of a program I am watching. With my signal generator, I corrupted the streaming video to the degree it could not be decoded. The filter removed the signal. John |
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