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Many receivers watching same channel = reduced signal strength??


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#1 OFFLINE   slice1900

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Posted 25 February 2013 - 11:31 PM

This past weekend I was talking to a friend of mine who is pretty knowledgeable about satellite systems. He had an old C band dish in the 80s/90s, then had Directv for about a decade, until 4 or 5 years ago when he moved to a house surrounded by tall trees with FIOS availability :)

He told me a lot of interesting things, but one thing he said really surprised me, though he insisted it was true. He said if you have many/all receivers on a multiswitch tuned to the same channel (or different channels, so long as they're all using the same transponder) the signal strength is reduced because the signal is essentially split a bunch of times within the multiswitch. If you tuned a different channel (on a different transponder) on all receivers except one, that one would realize improved signal strength.

Can anyone confirm whether this is true or not? If true, would it ever matter in the real world, or is splitting the signal 16 ways in the largest multiswitches not dropping the signal by enough to matter unless you're already right on the edge as far as signal strength is concerned? I could see maybe that might matter during a rainstorm where you're close to seeing rain fade, but it shouldn't matter on a clear day assuming your dish isn't out of whack. Unless it dropped the signal near/below the receiver's tolerable S/N ratio, you wouldn't be able to see it on the signal strength screen, since that measures bit error rate not signal power in dbm, right?

If true on legacy multiswitches, would it still be true on SWM? I know the SWM multiswitches (at least the -16, guess I'm not positive about the -8) have gain control and automatically amplify the SWM outputs to about -30dbm. If the incoming signal was weak, due to a long run from the dish, misalignment or rain, would the internal "splitting" of the signal 16 ways be enough to degrade the signal too far to where the amplification back to -30dbm wouldn't help? The only way around that would be to amplify the signal before it reached the multiswitch, correct? Or maybe the SWM switches already amplify the incoming signal before the "splitting" and then do the gain control for fine tuning at the end? I really wish there was more information out there about how these things operate internally.

Sorry if these are stupid questions, I never really gave much thought to how a multiswitch works before but now I'm really curious. Does anyone know of any good in-depth documentation? I could describe in detail how ethernet switches and IP routers operate, and point others to that information, but I have no clue how a multiswitch works. I don't even know if they are solid state or there are some mechanical switches inside. I've heard there are, but maybe that's no longer true.

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#2 OFFLINE   P Smith

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Posted 25 February 2013 - 11:35 PM

"these are stupid questions" :D - tell him to begin study RF circuit electronics, right now !

#3 OFFLINE   litzdog911

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Posted 26 February 2013 - 01:28 AM

He said if you have many/all receivers on a multiswitch tuned to the same channel (or different channels, so long as they're all using the same transponder) the signal strength is reduced because the signal is essentially split a bunch of times within the multiswitch. If you tuned a different channel (on a different transponder) on all receivers except one, that one would realize improved signal strength.

Your friend doesn't know what he's talking about.
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#4 OFFLINE   HoTat2

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Posted 26 February 2013 - 04:07 AM

... I could describe in detail how ethernet switches and IP routers operate, and point others to that information, but I have no clue how a multiswitch works. I don't even know if they are solid state or there are some mechanical switches inside. I've heard there are, but maybe that's no longer true.


Briefly notice its called a multi-"switch," not a "splitter" and like an ethernet "switch," which as you well understand I gather, they operate on the same principle of allowing one input source port to supply multiple output ports on an as needed basis without power-dividing the source signal among the outputs the way an actual signal "splitter" would.

Also, the numbers given in SWiM-"8" or -"16" do not refer to internal splitting, but to the maximum number of RF satellite tuners it can support simultaneously.

And notice even here its referred to as a "SWiM" (or sometimes "SWM" for short) for Single Wire Multi-"switch" where the same principle of a "switch" applies again this time by allowing one input data stream from a particular satellite transponder to supply up to 8 or 16 satellite tuners on an as needed basis without power dividing the signal stream among them the way a SWiM "splitter" does.

P.S. --- All latest satellite multiswitches used today, whether the legacy muti-line ones or SWiM type, use all solid state circuitry.

See here for more ...

http://api.viglink.com/api/click?format=go&key=e652088f26975de9b83439c1dd935df0&loc=http%3A%2F%2Fwww.dbstalk.com%2Fshowthread.php%3Ft%3D200024&v=1&ref=http%3A%2F%2Fwww.dbstalk.com%2Fforumdisplay.php%3Ff%3D115&libid=1361872978935&out=http%3A%2F%2Fhr20.dbstalk.com%2Fdocs%2FDish%2C%2520Multiswitch%2520and%2520Cable%2520FAQ.pdf&title=Tips%20%26%20Resources%20FAQ's%20and%20Important%20Links%20-%20DBSTalk.Com&txt=%20%3Cfont%20size%3D%224%22%3E%3Cfont%20color%3D%22Blue%22%3EDish%2C%20MultiSwitch%20%26amp%3B%20SWiM%20FAQ%3C%2Ffont%3E%3C%2Ffont%3E&jsonp=vglnk_jsonp_13618731035005

Edited by HoTat2, 26 February 2013 - 04:16 AM.


#5 OFFLINE   veryoldschool

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Posted 26 February 2013 - 09:40 AM

If I use a 4-way splitter the RF power drops by 6 dB to each output.
If I use a 4-way switch to feed four outputs, the RF power is still going to drop by 6 dB to all four outputs. This is simply the laws of physics.

Why this isn't a problem with DirecTV switches is they also have buffer amps, so they're really only switching the input voltage.
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#6 OFFLINE   HoTat2

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Posted 26 February 2013 - 11:46 AM

If I use a 4-way splitter the RF power drops by 6 dB to each output.
If I use a 4-way switch to feed four outputs, the RF power is still going to drop by 6 dB to all four outputs. This is simply the laws of physics.

Why this isn't a problem with DirecTV switches is they also have buffer amps, so they're really only switching the input voltage.


Are you speaking "theoretically" about a 6 db loss to each output of the four-way splitter example VOS, as opposed to the real world 8.3 to 9.5 db estimate you posted here?;

http://www.dbstalk.c...ad.php?t=186663

#7 OFFLINE   veryoldschool

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Posted 26 February 2013 - 11:51 AM

Are you speaking "theoretically"

Yes.
I didn't want to go into all the losses, but to just point out how a splitter & switch would both have a drop in power.
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#8 OFFLINE   slice1900

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Posted 26 February 2013 - 04:22 PM

If I use a 4-way splitter the RF power drops by 6 dB to each output.
If I use a 4-way switch to feed four outputs, the RF power is still going to drop by 6 dB to all four outputs. This is simply the laws of physics.

Why this isn't a problem with DirecTV switches is they also have buffer amps, so they're really only switching the input voltage.



Thank you for the explanation, I didn't know about the buffer amps (in fact I hadn't ever heard of them before, had to google them) So my friend was either totally wrong, or what he said was only true back in the C-band days and he assumed it still held true for Directv. He was very insistent, I may have to call him out on this one next time I see him :)

This brings up another question if you don't mind. Consider a legacy multiswitch like a WB68 or WB616, if the buffer amps allow a signal to leave the multiswitch as strong as it was on the input, would using a legacy multiswitch be better than splitters for connecting multiple multiswitches? I would have assumed there was some signal loss between the input and output of a legacy multiswitch since I didn't think they amplified the signal at all (unlike the SWM-16) but it sounds like even if the signal isn't leaving them stronger, it isn't leaving any weaker, either. If so, would a WB616 or even a WB68 be preferable to using splitters to avoid that signal drop?

I imagine it wouldn't matter in good conditions, because even after a 4 way split, unless you have a long run from the dish, the signal is still strong enough for the SWM-16's AGC to work with. But what about poor conditions, when it is almost bad enough to cause rain fade but not quite? Is getting a stronger signal at the SWM-16 inputs going to make any difference for the onset of rain fade, or this purely a question not of signal power in dbm but signal quality? (S/N or bit error rate) Splitters will not diminish signal quality, and if the buffer amps are introducing noise, splitters might be better in this situation despite the db drop.

#9 OFFLINE   veryoldschool

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Posted 26 February 2013 - 04:52 PM

Thank you for the explanation, I didn't know about the buffer amps (in fact I hadn't ever heard of them before, had to google them) So my friend was either totally wrong, or what he said was only true back in the C-band days and he assumed it still held true for Directv. He was very insistent, I may have to call him out on this one next time I see him :)

This brings up another question if you don't mind. Consider a legacy multiswitch like a WB68 or WB616, if the buffer amps allow a signal to leave the multiswitch as strong as it was on the input, would using a legacy multiswitch be better than splitters for connecting multiple multiswitches? I would have assumed there was some signal loss between the input and output of a legacy multiswitch since I didn't think they amplified the signal at all (unlike the SWM-16) but it sounds like even if the signal isn't leaving them stronger, it isn't leaving any weaker, either. If so, would a WB616 or even a WB68 be preferable to using splitters to avoid that signal drop?

I imagine it wouldn't matter in good conditions, because even after a 4 way split, unless you have a long run from the dish, the signal is still strong enough for the SWM-16's AGC to work with. But what about poor conditions, when it is almost bad enough to cause rain fade but not quite? Is getting a stronger signal at the SWM-16 inputs going to make any difference for the onset of rain fade, or this purely a question not of signal power in dbm but signal quality? (S/N or bit error rate) Splitters will not diminish signal quality, and if the buffer amps are introducing noise, splitters might be better in this situation despite the db drop.

Sort of why I posted was because "your friend" was sort of right, but it doesn't apply to DirecTV.

Now for your other question, it isn't as clear/cut and dry as you might think.
Yes a switch with a buffer amp is not going to have the same signal drop as a splitter does, but whether it matters or not is what isn't always cut & dry.

The SWiM has an AGC so input levels above -30 dBm get attenuated and signals down to -45 dBm get amplified.

Since the LNB will lose lock at around -40 dBm, you can have about 5 dB of loss to the SWiM and it will still output -30 dBm.

"Now" what if you have say 10 dB of loss to the SWiM? The output will drop to -35 dBm just before the LNB loses lock [rainfade]. With -35 dBm out of the SWiM, the next thing is how much loss to the receiver? The receivers work down to about -60 dBm, so if the SWiM output loss isn't maxed out, the input loss can be more than 5 dB.

"Basically" you need to look at the whole system and its losses before you can say a switch would be better than a splitter.
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#10 OFFLINE   P Smith

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Posted 26 February 2013 - 05:23 PM

Actually LNBF doesn't have such "lock", it's a term of demod chip inside of STB or sat tuner card like TH1020A or SkyWalker.
LNB is more likely linear device.

#11 OFFLINE   slice1900

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Posted 26 February 2013 - 05:51 PM

So assuming a relatively short run from dish to splitters (~30 feet for me) this would manifest as seeing the onset of rain fade showing up first on receivers that are near the limit of run length (receiving a signal not far from -60dbm even in clear conditions) followed by all receivers once things get bad enough that the LNB loses lock.

It sounds like if you have a short run from the dish it would potentially be worth using a multiswitch as a 'splitter' if you had a big enough system to be looking at >2 way splitters, but with the more common two way it would only make a difference if you have a fairly long run from the dish. And in both cases you'd have to have a receiver far enough from the SWM that it is near that -60dbm limit.

#12 OFFLINE   veryoldschool

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Posted 26 February 2013 - 05:55 PM

Actually LNBF doesn't have such "lock", it's a term of demod chip inside of STB or sat tuner card like TH1020A or SkyWalker.
LNB is more likely linear device.

I guess you haven't used the AIM then. The LO breaks lock if the input is too low, and the AIM measures the offset of the LO when it is locked.
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#13 OFFLINE   veryoldschool

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Posted 26 February 2013 - 05:59 PM

So assuming a relatively short run from dish to splitters (~30 feet for me) this would manifest as seeing the onset of rain fade showing up first on receivers that are near the limit of run length (receiving a signal not far from -60dbm even in clear conditions) followed by all receivers once things get bad enough that the LNB loses lock.

It sounds like if you have a short run from the dish it would potentially be worth using a multiswitch as a 'splitter' if you had a big enough system to be looking at >2 way splitters, but with the more common two way it would only make a difference if you have a fairly long run from the dish. And in both cases you'd have to have a receiver far enough from the SWM that it is near that -60dbm limit.

:confused:
You haven't made it clear if this is a SWiM LNB or a SWM8/-16.
You've got about 30 dB loss from wherever the SWiM is to the farthest receiver.
You can't use a switch between the SWiM & the receivers.
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#14 OFFLINE   P Smith

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Posted 26 February 2013 - 06:03 PM

AIM has own vocabulary, wait a sec - LOF is defined by mechanical parameters and usualy stable regardless input signal; I can play with its value at my bench ... Are you talking about specific SWiM LNBF ? Is it designed with PLL ?

#15 OFFLINE   veryoldschool

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Posted 26 February 2013 - 06:11 PM

AIM has own vocabulary, wait a sec - LOF is defined by mechanical parameters and usualy stable regardless input signal; I can play with its value at my bench ... Are you talking about specific SWiM LNBF ? Is it designed with PLL ?

I tested both a SWiM & legacy LNBs with the AIM.
The offset [lagacy] varied. One was about 3 MHz, while the other was less than 1 MHz.
Attenuating the input [in steps] of the LNB would cause the CNR to drop and finally the meter reports unlocked.

http://www.dbstalk.c...=1&d=1336233190


I'd need to check through my notes from the testing last summer, but:

The three things I was able to characterize between two legacy LNBs were the LO offest, the gain of the LNA, and the gain of the output amp.

"Output power" turned out to not be as important as I'd though. One LNB pointed at the ceiling of my dining room outputted -30 dBm and had a lower offset, while the other LNB had a lower output, but a better CNR, and the 3 MHz offset. This last one is what I selected to use with my SWiM-16.

Edited by veryoldschool, 26 February 2013 - 06:27 PM.

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#16 OFFLINE   HoTat2

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Posted 26 February 2013 - 07:14 PM

I tested both a SWiM & legacy LNBs with the AIM.
The offset [lagacy] varied. One was about 3 MHz, while the other was less than 1 MHz.

... I'd need to check through my notes from the testing last summer, but:

The three things I was able to characterize between two legacy LNBs were the LO offest, the gain of the LNA, and the gain of the output amp


Just to be clear, doesn't a Ka/Ku LNB actually have three L.O.s?

Ku -- 11.250 GHz for Sat A and B
Ku 110 -- 11.542 GHz for Sat C
Ka -- 18.050 GHz

So which do you mean with the single figure quotes of 1 and 3 MHz offset for each LNB respectively?

#17 OFFLINE   P Smith

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Posted 26 February 2013 - 08:00 PM

Definitely, here is misinterpretation of AIM capability to lock signal and LNBF parameters.

#18 OFFLINE   veryoldschool

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Posted 26 February 2013 - 08:25 PM

So which do you mean with the single figure quotes of 1 and 3 MHz offset for each LNB respectively?

I'd need to dig into the notes, but checking a TP & SAT gives the offset.

Pg 66 of the AIM manual:

Capture.PNG

– Frequency offset of the transponder signal from its expected frequency (in megahertz).
– Indication as to whether the transponder signal is above the power lock threshold.

Edited by veryoldschool, 26 February 2013 - 08:47 PM.

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#19 OFFLINE   veryoldschool

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Posted 26 February 2013 - 08:26 PM

Definitely, here is misinterpretation of AIM capability to lock signal and LNBF parameters.

But I'm afraid it may be on your end.
A.K.A VOS

#20 OFFLINE   slice1900

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Posted 26 February 2013 - 08:33 PM

:confused:
You haven't made it clear if this is a SWiM LNB or a SWM8/-16.
You've got about 30 dB loss from wherever the SWiM is to the farthest receiver.
You can't use a switch between the SWiM & the receivers.



Sorry, I guess I didn't make it clear I was still talking about the same situation - a legacy LNB, and whether it would help to use a legacy multiswitch as a 'splitter' between SWM-16 multiswitches versus the standard solution of using actual splitters. I had assumed this was a silly idea because I wasn't aware that the buffer amps would allow a legacy multiswitch to maintain the same signal strength at the output that was received at the input. I'm just curious if/when using a WB616 in lieu of splitters to feed multiple SWM-16s might make sense.

If the LNB loses lock (stops sending usable signal, whatever one wishes to call it) at around -40 dbm, then you can do some simple calculations to answer this question. If you assume ~2db of loss from 30 feet of cable, plus ~4.5db of loss from a two way splitter, you would have signal entering the SWM multiswitch at about -46.5db worst case. Any worse and you wouldn't have a usable signal at the output so the question is moot below that level. So taking that worst case figure of -46.5dbm, if the SWM multiswitch can add a maximum of 15db, signal strength from the SWM outputs is -31.5dbm, instead of -30dbm as it normally would be. Probably not enough difference to notice unless you're so far on the edge of SWM to receiver length that you're lucky it ever works at all (or have other problems to address like bad cable/connectors, too many splitters, etc.)

If you have more than two SWM multiswitches, you take out another ~4.5db of input signal due to the larger splitter, and the signal from the SWM outputs is -36dbm instead of -31.5dbm. That would be enough to easily notice on a longer run - that's equal to around 75 feet of coax. In that case using a WB616 as your four way 'splitter' may be a good idea, since you don't have the 9db loss you'd have from a four way (or three way, unless can you find one that's balanced) splitter.

Obviously if one has much longer runs from the dish, using a legacy multiswitch as a 'splitter' may make sense even in the two way scenario, IF you have some pretty long SWM to receiver runs where you'd risk signal falling below -60dbm at the receiver.




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