 xymox1Premium
join:2008-05-20
Phoenix, AZ
2 edits | reply to xymox1
Re: High freq attentuator My understanding of a CMTS is that the handshake that occurs with the modem looks at the output level seen by the CMTS and then adjusts the modem output power to a "0" db level compared with a reference level. This keeps all the modems at the same level at the CMTS. This is a critical function.
You did however make sense in another way. Using a attenuator for the upstream and forcing the modem to generate a higher level of output would bring its output higher and could increase S/N on the output of the modem. Assuming distortion and things all stay the same. The attenuator then knocks down the level as seen by the CMTS. Its the same level at the CMTS but the modem's output is much higher.
Now is that a good thing ? Without really sophisticated test equipment its impossible to know. As that power output increases lots of things happen. Hard to even guess what would happen. Its possible it might produce a better response.
There is one thing i have seen is that as the modem needs to produce more output power less channels bond. This makes sense as a wider bandwidth is harder to do at high power so it may fall back in bonded channels in order to provide a higher output.
Cisco and Motorola ( broadcom design ) are pretty smart guys/gals and the CMTS / Modem link is most likely very well thought out.
My overall understanding of how it all works is the lower the CMTS sets your modem output power during the handshake the better your connection must be.
I have a FRSA50-15 and a 6 which are low freq attenuators which work below 50Mhz and I will play with and see if having a higher output power helps. »www.arcomlabs.com/33ReturnStepAt···ors.html
Why do I want to hit 0db in downstream ? Its the RF stage 0 point. It would make sense its designed to work best at 0db. 6db is a lot in voltage terms. Hitting zero will also give me the maximum range to accommodate any transient issues giving me +- 10db. Actually I cant think of a reason not to hit 0db as long as the trade off in the attenuation is not costly in other areas as you mention using a diplexer. This is why im searching for a attentuator that only effects the 800-900Mhz band.
There are companies that make a high freq attenuator. The ACS-870/6 is a "cable equivalent" filter that would work for me but you cant buy them online anywhere. »www.arcomlabs.com/34LineConditioning.html
Arcom makes lots of filters »www.arcomlabs.com/31CableTelevis···ers.html |
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xymox1Premium
join:2008-05-20
Phoenix, AZ | Another example of a cable simulator filter that you just cant buy online anywhere... »www.eaglecomtronics.com/pdfs/Equalizers.pdf |
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 DrDrewSo that others may surf.
join:2009-01-28
SoCal
kudos:8 | If you really want to mess with levels on the high end of the spectrum affecting the low end as little as possible.... a 100' coil of RG6 has about 6 dB of loss at 800 Mhz and 1 dB of loss at 30 Mhz.
Drop that between the wall plate and modem in place of a 6' jumper.
In reality though, your signals are fine.
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If it's important, back it up... twice. Even 99.999% availability isn't enough sometimes. |
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xymox1Premium
join:2008-05-20
Phoenix, AZ | quote:
This is one point on which we agree. The fewer connections and parts you can have in an RF network the better
In this we all agree 
I do still want to be able to hit 0db and play with some measurements in the RF section and look at some levels after the LNA.
I was thinking tho. Cable techs routinely use a splitter to drop levels when they really are too hot. If attenuation is really needed why not use one of these selective attenuators and just drop rcv levels ? Using a attenuator that also effects TX seems to make no sense. |
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DrDrewSo that others may surf.
join:2009-01-28
SoCal
kudos:8 | said by xymox1: Cable techs routinely use a splitter to drop levels when they really are too hot. If attenuation is really needed why not use one of these selective attenuators and just drop rcv levels ? Using a attenuator that also effects TX seems to make no sense.
Cost. Selective attenuators are more expensive and less commonly used then splitters. Plus it's more inventory to stock, track, and maintain.
In addition, it's very rare the forward levels are too high without the upstream levels being too low. In most cases, if they are, it's a distribution system balancing issue.
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If it's important, back it up... twice. Even 99.999% availability isn't enough sometimes. |
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