Need Rf Ground Breaker Design, CATV giving me 1.5v from coax

[gabito]

static protection resistor

Hi guys.. this is my first post here.

Sorry for my english. hope you can understand me.

BTW: I'm not a technician or anything.


I live in Argentina, where we still have analog CATV. The other day I measured the voltage that my cable company sends through the coax cable. To my suprise, it was 1.1v and sometimes fluctuating to 2.0v!

I guess I know now why a TV set of mine died on me last month.

Any how, I came up with a filter to get rid of the current and just let the RF signal through.

Here's my schematic:

**broken link removed**


Voltage is down to 0, of course due to the caps.

But something is wrong. We have approx 70 channels here, so:
1- channels from 2 to about 10 have very poor quality
2. channels from 20 to 30. poor.
3- 50 and up, poor.


I guess its the caps I'm using that is blocking or interfering with some of the frequencies.


Any suggestions guys?

Thanks!

 

[RF-OM]

why is a tv set not grounded

As I understood you need to use your circuit up to 500 MHz band. Try to use one cap 1000pF at the input and one the same cap at the output into hot wires only. Keep ground as it is. Inductor must be not more than 150 nH. These changes may help to keep insertion and return losses relatively low. The serious problem are the diodes. This type has 4 pF capacitance per diode, so you have 8pF total. It is too much for having good match. You need to reduce diodes' capacitance or remove them at all.

How did you measure voltage into cable? VOM is not a good meter for high frequency measurements. Also check you TV. If it has DC input, which means it have no DC blocking capacitor, connecting your circuit without output 1000 pF capacitor in hot wire will change the bias voltage for the first stage and TV will not work properly.

Best regards,
RF-OM

 

[GrandAlf]

coaxial noise ground breaker

Can you get isolated TV outlet sockets where you are. They are usually quite cheap. Saves a lot of playing around trying to find correct values. As an alternative, just try a cap in line with the centre core. May possibly help.

 

[Borber]

Re: Need Rf Ground Breaker Design, CATV giving me 1.5v from

More likely is that CATV cable is on half of the mains voltage (110VAC) against ground. This happen when cable on other side is not grounded or grounded to different potential. Here we use so called isolator which basically consists of two 1nF capacitors placed in series with cable connections. It is not easy to make it yourself because a low RF attenuation must be acheived.
Your circuit can be simplified and leave only C1 and C2 which should be 1nF. Other elements are redundant.

 

[RF-OM]

Re: Need Rf Ground Breaker Design, CATV giving me 1.5v from

I think that ground must be kept untouched for the sake of safety. 1nF cap in series with ground may be dangerous in case when cable is under half or any other part of line potential. This is what we have a safety ground for and this why you need to touch TV set with only one hand when you connect or disconnect the cable (I mean the very moment of connecting or disconnecting).

Inductor may be very helpful there because it is good anti-static protection. Resistor may be installed instead, but the best is the RF choke with so called progressive winding when the last third or quoter of winding is done with progressively increasing pitch. This almost eliminates capacitance between turns and such RF choke perform very good. I am not sure about diodes. They may be helpful if there is powerful radio transmitter in close proximity or some other big non-TV RF voltage in the cable. But they may dramatically increase RF noise, especially around the first channel. This is a kind of not so well known phenomenon.

Best regards,
RF-OM

 

[gabito]

Re: Need Rf Ground Breaker Design, CATV giving me 1.5v from

Thanks for the replies guys.


In the diagram I used a 1.5uH RF choke as isolator, but did not use this when I made it since my local shop didn't have them and I didn't want to make it myself with a coil.

Do you think that putting the isolator in, and eliminating the diodes would work?


By the way, we have 220VAC here, if that helps.

On the other hand, I want to get rid of that 1.1v and I just noticed some hum bars on one of my tv sets..

So maybe I could kill two birds with one stone if we can get this right..

Thanks again.

 

[RF-OM]

Re: Need Rf Ground Breaker Design, CATV giving me 1.5v from

I do not know about birds and one stone, but you are killing us with your terminology.

1. What do you isolate with 1500nH choke?

2. I think you need to answer my question how did you measured the voltage.

3. If you really need RF filter you may use what I told you. It will work.

 

[Borber]

Re: Need Rf Ground Breaker Design, CATV giving me 1.5v from

TV set is not grounded it is only isolated for protection. And when antenna cable gets high potential because of lightning TV set goes dead. Grounding the antenna cable on receiver side is not allways the solution because of high current that can flow to ground and protection may switch off the mains voltage.
Some more info:
https://www.epanorama.net/documents/groundloop/antenna_isolator_building.html

 

[RF-OM]

Re: Need Rf Ground Breaker Design, CATV giving me 1.5v from

You are right, TV set have electrical cord without ground wire. I mean that including cap into ground connection may be dangerous. Safety rule about using one hand in the moment of connecting or disconnecting antenna cable is right for any case.

By the way, some old radios many years ago (around 30 - 40-ies) was built without transformers and have capacitor into ground path. These radios were very dangerous. There was another mechanism that creates danger, but it can be illustration of capacitor usage into ground circuit.

For this asked situation in Argentina a lot depends on how the cable distribution system is built. I recently was there but did not see anything strange. In contrast, in neighboring Chile wiring and cabling are nightmare, may be there are no rules at all. If cable ground (return path) is inadvertently connected to electrical wire there may be whatever it may be. This is one of the reason why I asked how the voltage was measured.

 

[Borber]

Re: Need Rf Ground Breaker Design, CATV giving me 1.5v from

Actually TV set has it' chassis potential elevated to half of mains voltage through mains RFI filter capacitors. They can be up to 100nF.
This old radios are called universals because AC or DC supplv voltage were possible and U series tubes were used. Their chassis had direct contact with mains.

 

[paulek]

Re: Need Rf Ground Breaker Design, CATV giving me 1.5v from

sup guys , well I'm in argentina as well and I built the circuit from epanorama using 2 10nf ceramic caps 250volts but image on tv channels was degraded on frequencies around channel 20 up to around 30.
I wanted to build up this isolator cause I measured voltage from my pc chassis and cable ground connector and on AC 200 volts scale I got 01.5 volts It might be nothing to be worry about but ... To make you have a better view of my problem whenever I plugged say the usb plug of my external sata case to my puter (which was plugged to my LCD and the LCD was plugged with CATV from my provider) you could see little sparks on the female usb connector , by the way this same scenario didn't happen if I unplugged the CATV cable from my LCD . Everything is going through ground to my pc chassis (I might suspect some sort of ground potential difference coming from the CATV installation and my devices).
So basically I wanted to ask borber what could actually happen if I replace C1 and C2 caps with 1Nf values will that work as a band pass filter for all frequencies without degrading quality ?. Any other ideas or suggestions highly appreciatted . Thanks so much

 

[Borber]

Re: Need Rf Ground Breaker Design, CATV giving me 1.5v from

You just prooved that potential difference exists between cable and PC ground (PC is allways grounded).
This C decoupler is dificult to build without experience in RF. It is better to use 1nF capacitor because of it's smaller dimensions and parasitics. You must make shortest possible lead wires.
It is easier to built transformer if you have propriate ferrite rod.

 

[RF-OM]

Re: Need Rf Ground Breaker Design, CATV giving me 1.5v from

Now I think I understood what you need to isolate. Borber is right: transformer may be a good choice for you. It is easy to do TV 1:1 transformer for frequencies up to 2.2GHz (the whole TV range, including satellite band). You need to find ferrite bead (single-hole core) or two-hole core (these are used for baluns). Both will work well. Material type is important. You need to use NiZn ferrite. You may find it on web sites of ferrite vendors like Fair-Rite (https://www.fair-rite.com), Steward (www.steward.com, Ceramic Magnetis, ferroxcube and other. I included sample data sheet for such a ferrite bead from Steward. From Fair-Rite the best material for you is 61 (or 67 may be used too), from Steward it is material 28 (it is in datasheet). Size of the core is not so important up to about a half of inch. Good size may be 2 to 3 mm internal diameter of the hole and 3 to 10 mm length of the core. Take two wires with good enough isolation and make two or three turns with both wires simultaneously. One turn is one wires passage through the hole, so there may be only integer number of turns. You will get two winding any one is primary and the other is secondary. Keep the length of the wires that out of the core as short as possible. These wires parts are out of magnetic flux and will create so called leakage inductance which is parasitic and degrade transformer performance on higher frequencies. You also need to keep in mind to keep all unused TV receptacles (when cable is not long) and splitters connectors terminated with 75 Ohm. When people use splitters they usually do not terminate unused F-connectors and this may lead to signal degradation due to mismatch reflections. Splitter performance depends heavily on termination. Ends of long coaxial cables may be left unterminated because long cable has enough attenuation for both paths.

I simulated filter schematic with 1nF caps at input and output and 150 nH inductor in parallel branch between caps. Such filter will have acceptable performance up to at least 500MHz, but transformer may be better if it done right.

 

[paulek]

Re: Need Rf Ground Breaker Design, CATV giving me 1.5v from

Hi borber ,Rf-om something I really need to know , the LCD I got plugged my CATV cable , started to make some high pitched noise and I'm almost quite sure it didn't when I bought it. Could the voltage that kept flowing through the catv might have fucked something up on the lcd psu ?. As soon as I leave the lcd on the noise starts to go away , and whenever I change channels, due to the backlight on the LCD is makes some different whine until it normalizes , I don't think I got a defective unit, could this might have screwed up something on the lcd psu?. If thats not the case, what could be related to it, as long as it starts to heat up the noise goes away, quite odd. In relation to the catv filter I'm going to try what borber said with 1nf caps I mean the simple circuit from epanorama with 2 , 1Nf caps and thats it , or should I add an inductor? (what role will the inductor play? ) Thanks a bunch guys you're really helping me out on this ! we'll keep on touch

 

[Borber]

Re: Need Rf Ground Breaker Design, CATV giving me 1.5v from

LCD has two switcher psu units one general and other for backlit HT voltage. Either can produce noise because loose ferrite transformer. Probably the second. Is it defective I can only guess. It happens sometimes with no reason.
Inductor is not needed at all. It's only purpouse would be the attenuation of signals with frequencies lower than 10MHz. Capacitors and this inductor forms a so called high-pass filter. Sich a filter is allready a part of circitry behind the antenna connector.

 

[paulek]

Re: Need Rf Ground Breaker Design, CATV giving me 1.5v from

Hi borber thanks so much for your reply, well could this eventually screw up the backlight psu HT circuit. I'll tell you exactly how it goes through.. whenever I turn off the lcd for a long time.. and then turn it on it starts doing this high pitched sound, which sounds exactly the same way as an Asus motherboard I had and it happened to match precisely what you said. In the case of this mobo it was like a ferrite donut shaped I think that's and inductor doing that high pitched sound. Borber what's your advise should I take the lcd for servicing? I don't really trust people over here doing lcd servicing.. so I think I can live with this, this sound goes away.. when the unit starts heating itself lasts let me see.. around 10 minutes till it fully goes away. Now is this something simple to fix? . And don't tell me why but I'm suspecting the CATV voltage had something to do with it .. am I mistaken, correct me if I'm wrong.. yes 01.5 volts maybe nothing to a circuit of this nature but.. . Once more thanks for helping out

 

[Borber]

Re: Need Rf Ground Breaker Design, CATV giving me 1.5v from

CATV is not causing problems with noise. To fix it is sometimes impossible and only replacing the transformer can solve the problem. All depends on your knowledge and experience.

 

[paulek]

HI borber well I'm not an in depth technician on this matter, I would have to send it and service it. What worries me is if this could eventually lead to the transformer stop working. What I said about CATV was that maybe the AC voltage flowing could have affected the HT backlight transformer, cause I had it without the ground breaker filter. Thanks

 

[RF-OM]

Re: Need Rf Ground Breaker Design, CATV giving me 1.5v from

To paulek,

Could you please use normal language when you are talking to the engineers. I do not want to decipher your sup and psu and so on.