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Splitter with highest port to port isolation


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7 replies to this topic

#1 OFFLINE   slice1900

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Posted 17 October 2013 - 10:47 AM

I'm trying to find splitters with the highest possible port to port isolation. Unfortunately this isn't a query Google is well suited for, but I'm hoping maybe one of you guys might have some experience here. Not looking for some sort of specialty/custom product costing $50, just regular but high end splitters.

 

The reason I need this is because I'm running bidirectional RF over coax, and trying to minimize the crosstalk between the two 'outputs' of the splitter where I have signals travelling in opposite directions. When I first used a random cheap cable splitter I had laying around there was noticeable interference. I switched to a Pico Macom WS3G-2 satellite splitter and it cleared up quite well, so if this is the best I can do it is quite acceptable. However, since I can still see a snowy version of the (analog) signal when I disconnect the intended video path and rely solely on crosstalk, there's clearly still more to do - if possible.

 

The Pico Macom splitter is rated for a port to port isolation of 20 db, which is the same as the cheap GC splitter I used at first. I assume Pico Macom lists the QC minimum for isolation and the actual value is higher. This is a 3GHz satellite splitter so it is likely better built than a regular cable splitter.

 

The upstream RF in this case is in the range of 750-800 MHz, so the ones advertising high isolation in the return band (I assume intended for cable modems) may or may not be what I'm looking for - not sure how easy it would be to build one with much higher isolation in the <50 MHz range of a cable modem return band than at higher bandwidths.

 

I found a Steren 201-222 listing 30 db, and a Signal Vision SV-2G listing 40 db 'in the return band'. Anyone know of anything higher?

 


SL5, PI-6S, SA-6AL 3xSWM16, 21 H20-100, 1 H20-600, 7 H24-700/AM21


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#2 OFFLINE   gov

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Posted 17 October 2013 - 11:11 AM

I have used the 2 way Steren 2.4GHz models and found much less signal going the wrong way compared to any of the 900 MHz or 1000 Mhz models.

 

I never use a 'regular' splitter for combining signals, always use the high frequency ones.

 

ALWAYS.

 

 

(sorry I don't have actual dB numbers for you, just the old voice of experience)



#3 OFFLINE   harsh

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Posted 17 October 2013 - 11:35 AM

I never use a 'regular' splitter for combining signals, always use the high frequency ones.

Perhaps this is because the high frequency splitters tend to be more lossy.

Too often we enjoy the comfort of opinion without the discomfort of thought. -- JFK


#4 OFFLINE   slice1900

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Posted 17 October 2013 - 12:10 PM

Perhaps this is because the high frequency splitters tend to be more lossy.

 

More lossy? In what way? If you look at a single number rating the insertion loss it would look that way, but that's because the 900 MHz models are rated for the loss at 900 MHz, and the 2.4 GHz or 3 GHz models are rated for the loss at those frequencies. If you poke around, you can find some that spec the insertion loss at various frequencies, and you'll see they have pretty much the same loss at cheap splitters over the lower frequencies, but the higher the frequency the more loss.

 

i.e. a satellite splitter will spec a 5 db insertion loss, rather than the ~3.5 db you'd see from a cheap splitter, but if you look at the specs the loss at 900 MHz will be right around 3.5 db.


SL5, PI-6S, SA-6AL 3xSWM16, 21 H20-100, 1 H20-600, 7 H24-700/AM21


#5 OFFLINE   harsh

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Posted 17 October 2013 - 03:21 PM

More lossy? In what way? If you look at a single number rating the insertion loss it would look that way, but that's because the 900 MHz models are rated for the loss at 900 MHz, and the 2.4 GHz or 3 GHz models are rated for the loss at those frequencies. If you poke around, you can find some that spec the insertion loss at various frequencies, and you'll see they have pretty much the same loss at cheap splitters over the lower frequencies, but the higher the frequency the more loss.
 
i.e. a satellite splitter will spec a 5 db insertion loss, rather than the ~3.5 db you'd see from a cheap splitter, but if you look at the specs the loss at 900 MHz will be right around 3.5 db.

I picked a couple of competitive splitter brands (Holland and Perfect Vision) and compared their loss at 500MHz. The much cheaper (30%+) 950MHz models were at least 10% less lossy (in terms of dB) than their 2.xGHz counterparts. Other than providing bandwidth in frequencies that aren't used, the wideband splitters I looked at offer higher loss across the OTA range.

To see the Holland numbers: http://www.dbstalk.c...nd#entry3187234

I'm interested in why you think that wideband splitters should have less loss. I liken it to asserting that a large dipole antenna is inherently better than a Yagi-Uda at covering a particular frequency band.

Too often we enjoy the comfort of opinion without the discomfort of thought. -- JFK


#6 OFFLINE   slice1900

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Posted 17 October 2013 - 04:32 PM

I didn't say I thought wideband splitters would have less loss, just a similar amount. I checked the WS3G-2 I'm using and found it has higher losses of over 4 db on lower frequencies. I guess it is probably as you're implying, that the changes required to make it effectively split a wider frequency band (3 GHz in the case of this splitter) make it more lossy. But for my purposes I don't care if it has a 3.5 db loss or twice that, so long as I get good isolation. Signal strength isn't an issue, since the modulator outputs a fairly strong signal, and it gets looped back through a 25db amp before the 32 way split out to all the TVs. It actually works pretty well without it hitting the amp, but not quite as good.

 

I read the post you linked there, sounds like the guys in that thread are making this way too complex with all the UVSJ type stuff they're using. I'm just using splitters. Even though the signal I'm feeding back is then amplified and sent back out down the same wire I'm able to view it on the TV it originates from and it looks pretty good. Since I needed a composite source to test with and had a Directv receiver handy that's what I used for these tests. The modulated picture sent out and back was almost as good as plugging the composite into the TV. When I was using the cheap splitters I was seeing a double image on (just) that TV, presumably from the port to port crosstalk getting looped back, amplified and so on a few (or few dozen) times.

 

I would imagine that diplexers such as UVSJs offer better isolation between ports than splitters typically do. I could only find information for one (a Sonora diplexer) that was 35db on one side and 40 db on the other, but I can't use a diplexer or UVSJ-type device for what I'm doing. Well, I guess I could, if they made a diplexer with a 750 MHz cutoff... You can buy surplus cable company single channel traps and negative traps on Ebay pretty cheap - if you can decade their crazy alphabet soup of part numbers to figure out what channel they deal with. They probably have better isolation, but since I've got it working pretty well with just splitters I'll see if I can tweak it a bit better with highly isolating splitters and call it good.


SL5, PI-6S, SA-6AL 3xSWM16, 21 H20-100, 1 H20-600, 7 H24-700/AM21


#7 OFFLINE   gov

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Posted 17 October 2013 - 04:47 PM

And note I was careful to say 'signal going the wrong way'.

 

When I combine OTA and a modulator with a 900/1000 MHz splitter and lose a few weaker OTA channels, and then get them back when I put in a 2.4 GHz splitter, that is pretty much all I need to know.

 

I work in a deep rural area, 40 miles out in one DMA and 65 from another, I need every dB of signal my antennas catch delivered to the TV tuner.

 

I could see an installer in the heart of a DMA near the tower farm not having to worry about this at all, out here I have to watch signal levels carefully.  

 

Frequently on a complex install, I directly connect the antenna to a TV first and note the channels I get at that address, then 'build up' the system and monitor results to make sure I am not losing anything as I add RF amps, splitters, and modulators.



#8 OFFLINE   slice1900

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Posted 18 October 2013 - 01:31 AM

Its interesting that you see a difference there, but it may be because of the slightly higher loss that harsh is talking about. I assume that the modulator you are using in such an instance is using a channel far from any broadcast channel you're attempting to receive. Maybe a channel 3/4 modulator unless you're in one of the few areas with a low VHF station, or maybe a high UHF channel since channel 51 is the highest currently (and the FCC is trying to move them off 51 because of interference in the lower LTE bands)

 

If your modulator was an ideal transmitter it would generate only the frequency it is supposed to, and nothing else. Given that even channel 51 transmitters at TV stations aren't ideal transmitters (hence the interference with LTE) then obviously yours isn't either. Unless it is very high end, it probably puts out a lot of stray RF across a lot of frequencies - certainly at least the harmonics of the desired frequency. Normally this wouldn't matter, but when you mix the signal with OTA where any added noise can kill fringe reception it could be a problem. In such a case you're better off if the splitter has a bit more loss, as it'll attenuate those stray frequencies better. If you're working with fringe OTA reception, even a single db of additional noise could make or break you.

 

Port to port isolation shouldn't be too much a factor in the performance of a splitter when used as a simple combiner. If you leak signal from the OTA leg it'll go into the modulator, which doesn't matter. If you leak signal from the modulator it'll be broadcast from your antenna (only a problem if it is really bad and the FCC shows up at your doorstep ;)) Now obviously any signal that leaks from the OTA leg is signal that isn't making it into the combined signal, but the difference between a port to port isolation of 20db and 40db leaves much less than 1 db of signal that's lost. I would guess you're better off with a splitter that has high insertion loss than one that has high port to port isolation. That assumes that a splitter with high insertion loss when used in the normal direction is also losing more signal when used as a combiner. That assumption might be wrong.

 

One thing you could try to test if I'm right is to attenuate the signal coming from your modulator before combining it with the OTA signal. Most modulators output pretty powerful signals, so the noise will be powerful as well. If you damp that signal down to a lower but still usable level you'd also damp down the noise. You could do with that an attenuator or a splitter or two inline. However much you can drop the signal and not hurt the quality of the picture from the modulator. Give that a try sometime, I'd be interested to hear if it helps as much as using a 2.4 GHz splitter when combining signals.

 

 

For me, the big difference is that I'm not using splitters as a combiner (well, am I in one spot, but that's not where I noticed the type of splitter having any visible effect on my results) Instead I'm using a splitter as both a splitter and a combiner simultaneously. The signal coming out of the modulator into the "in" leg of a splitter caused a double image on the screen when enough of it leaked out the other leg of the splitter, which is connected to the TV's tuner.

 

Likewise, at my "head end" I have another splitter that's both a splitter and combiner simultaneously. A splitter for the modulator signal coming from the coax that originates at that TV (i.e. from that other splitter/combiner) with one leg connected to another splitter that acts as a combiner only to mix the modulated signal with the CATV signal which then goes into the 25db amp. The other leg of that splitter/combiner is connected to one of the outputs of a 16 way splitter. Obviously that leg is intended as an input, to feed the CATV signal to the TV on the other end of that coax, but I do have half my modulated signal traveling out that leg into the 16 way splitter.

 

I don't know what the port to port isolation numbers on such a big splitter are, but I doubt it is too much of a problem. Most of the signal would travel back through that splitter's input and hit the output of another two way splitter that serves the two 16 ways. Any signal that makes it through there will stop cold when it hits the amplifier's output, so I'm not too concerned about interference from that half of the modulator's signal that's "lost".

 

 


SL5, PI-6S, SA-6AL 3xSWM16, 21 H20-100, 1 H20-600, 7 H24-700/AM21





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