I'm looking for a signal combiner that will allow me to insert several modulated sat receivers into a CATV system. The output of the modulators vary from ch-85 up to 125 and are cascaded into each other, so I only need a single input to the CATV system. So far any thing I've tried has poor isolation.

Thanks,
GP

Just a quick heads up, very often the channel range you are describing is used for cable internet and digital features, I know it doe'snt answer you question but it may help none the less.

Proggieus

Thanks,

I forgot to mention that I put a trap on the head end. Our client is only interested in basic.

GP

I'm not sure I understand what the problem is.

You are using a trap on the CATV system as it comes into the house to filter out a range of channels You are then using a signal combiner to combine the modulated output from the sat receivers into the CATV cable spectrum.

You say that anything you've tried has "poor isolation". What does this mean. In other words, what is the symptom, rather than your diagnosis. Are the cable channels showing interference? The satellite channels? Both?

The problems could be many. The trap could be not attenuating the frequency range you are interested in adequately. One idea would be to remove the modulated input from the combiner, and put a termination resistor on the input to make sure the combiner is balanced. Now use the CATV receiver to see if the channel range in question does contain no signal. (Be sure to look at the CATV system on these channels, so you know if the filter is making things look different.) I would suggest only connecting the modulated input and terminating the input where the CATV enters, and make sure you have no signal on any of the CATV channels as well.

By definition the combiner has only 3dB of isolation between inputs. You may need a buffer amp both before and after your filter. It depends on the filter, and if the cable companys amp uses a feedback mechanism to boost output when it sees more load. If your filter does not use seperate cavities for the input and output, you may get a lot of bleed through of the filtered signal into the output. Depending on the input level, you may still be getting enough signal to tune into, or at least disturb your modulated signals. If this is the case, have to buy a better filter, or modify the one you've got to perform better.

If these fail, the only chance of really finding out what's wrong is to connect a spectrum analyzer to the system, and look at the system both before and after the filter and combiner. This will let you know for certain what isn't working, or working other than expected.

Use a channel plus splitter/combiner or a channel vision. If you put in a trap or LPF then it should work fine if you balance your cable input db with your modulated db

bcf1963 thanks

If I bupass the combiner each leg performs perfectly. When I reinstall the conbiner I get windshield interferance on channels 2 and 4.
When I remove the filter the modulated channels are full of beat from the data channels cable uses, but I still have the windshield effect on Channels 2 and 4.

GP

who's brands are you using? and model numbers?

Change the modulated channels, for example ch. 115 instead of ch. 125. I've found even with a trap you sometimes need to try a couple different channels to find the ones that combine best.

Lately, I've noticed interference problems with analog cable 2, 4, and 7 when amplifying or modulating/combining. I think with cable concentrating on digital there done with trying to correct the problems with the lower analog channels, not that they ever did.

Its possible that the condition of the incoming cable signal on those lower channels is the problem, even if it appears clean before the combining. If so, you may have to try custom filters or get the cable co involved to attempt to improve the incoming signal.

I just had exactly this symptom on Saturday, and I amped the signal which solved the problem. Cleaned up the 'windshield' problem on the first try.

Please reread my earlier thread with the following in mind. I did not want you to remove the combiner. Simply disconnect an input, and replace that cable with a terminating resistor. In case you have done something wrong with the combiner, or it is damaged, etc. , the issue will show up with only one input at a time connected.

From your comments channels 2 and 4 are not the modulated channels. Are these cable channels? If you are getting interference on the cable channels, the interference is coming from the modulator, or is leaking in from over the air broadcast. Most modulators work by modulating at some intermediate frequency, and then use a beat frequency to move to the desired channel. The side effect is that the modulator puts out quite a bit of signal in the low channels. I would try following the modulator with a filter at channels 2-4, to remove these from the modulator output.

Do you have local stations at channels 2 or 4? Perhaps they are being picked up somewhere in your setup. Are all your filters, combiner, enclosed in metal boxes?

What is windshield interference? I've never heard that term before.

I think he may be talking about a "ghosting" problem?
BCF1963 makes a good point of isolating where the problem is originating from.
I only use sat for my customers.
I am glad I do not have to deal with these problems. It is only going to get worse as the cable companies put more and more data behind the used or unused channels.

Windshield wiper effect is the movement of a second video frame behind the primary image. It is generally witnessed as a time delayed mirror of the primary.

After some trial and error, I finally found a channel plus combiner that does the job. Channel 4 is a bit weak, but for our cable system that's normal.

Thanks for all the help.

GP