Splitters/combiners

[Psychobiker]

Hi,
Just curious-these things are meant to combine two signals (ie. Satellite feed and terrestrial) to one cable. Understood. Now- INSIDE them, what's afoot? I can't see any reason, that one can't simply combine the two without one, except for the 18v for the LNB which would overdrive/cause havoc with the UHF. Simple, decouple at the TV RF input with a tiny capacitor. Anyone care to shed some light?

 

[Robbo]

Impedance matching. The source expects to see a 75 ohm load. A splitter achieves this using the circuitry inside. Sticking on two 75 ohm loads on in parallel would cause a mismatch causing signal loss and possibly degradation on some frequencies.

In any case, for combining Terrestrial and satellite, you would not use combiner, you would use diplexer, as the diplexer multiplexes the two signals and is more efficient.

 

[Channel Hopper]

Plus, of course, you would need two of them, one at each end.

 

[Robbo]

I'm pretty sure that television RF inputs are AC coupled anyway, so thus would be unnaffected by having DC shoved up them.

 

[Psychobiker]

Understood, and I'd wager the diplexers are a notch-type arrangement to prevent bleed through from each input component. I'll snag a few online, cheers!
Why people still want terrestrial....:S

 

[Robbo]

I don't think that they would be notch filters as they are usually very sharp. Most likely a low-pass and a high-pass filter in combination, I would have thought.

 

[satelliteman]

yes band pass filter with some >40dB isolation.

_http://globalinvacom.com/products/pdfs/DplexersC2C.pdf

 

[Huevos]

I'd wager the diplexers are a notch-type arrangement to prevent bleed through from each input component.

No, they are not a notch. A notch is a filter to attenuate one single frequency (usually a 1/4 wave stub tee-ed into the feed line). For example let's say there is a strong radio signal in your vicinity and it is flooding your receiver and making it useless on all frequencies, that is when you would use a notch, which would block it out to some extent.

Anyway sharing the feed line for no good reason is madness. UHF TV runs between 475-850Mhz which means the 1st harmonic runs 950-1700Mhz. Since the LNB IF runs 950-2050Mhz this means any strong TV signal will be in direct competition with those from the LNB. And that is without exploring other problems caused by intermodulation products, etc.