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Required Attenuation Level

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Required Attenuation Level

We are working on a small headend that currently has both analog and digital video. This system has no fiber. The headend goes into a launch amp and then out to 3 LEs along the way. I assume we can input our CMTS signal into the combiner and provided the system is balanced correctly everything should fall into place from there out.

My question is how do we determine the attenuation required on our US and DS of the CMTS to come out of the combiner and into the launch amp at the right level?

We have a TPNA-1000 Meter.


DS should be around 50 dBmV

DS should be around 50 dBmV or 110 dBuV (depending on how your instrument shows them)
US should reach the CMTS at the preset level, usually around 0 to 10 dBmV.
Hope this helps.

okay, Let me make sure I

okay, Let me make sure I understand. We are already coming out of the CMTS at 50dBmV. If we hit the launch amp at 50dBmV we will end up with signal at the CM that is way too hot. How do we calculate the correct external attenuation (In line pads/attenuators) between the CMTS and the launch amp so we are at the correct level?

I appreciate the information. There doesn't seem to be a lot of info out there on how to work with CMTS's.

Assuming you want your

Assuming you want your signals flat going into the amp and the other services are set up correctly all you need to do is set the levels to be the same as the other digital services already present. If you have a test point on the input of the amp then you would just measure the level there, subtract the combiner/filter loss and that's your required level. That's assuming quite a bit though. You really need to know the recommended levels for the amps you are using.

Maybe that'll help a little.

Perfect. That does help a lot

Perfect. That does help a lot. I am trying to avoid having to balance the whole system if possible and just match what is already there.

Side question:
Could I theoretically place a modem at the first tap or any tap for that matter and balance my CMTS to that CM? Again this would rely on the system being half way balanced to begin with.

Yes, you can use the modem as

Yes, you can use the modem as a meter. Probably not as accurate but close enough. Keep in mind though, for the downstream you have to have a meter to compare the levels to the modem. The modem won't show you the levels of your video for comparison. Also, your expected levels at the tap won't be flat, there will be tilt so you have to determine the levels based on where in the spectrum you have the modem channels.

return signal

Your not planning to balance the return with a cable modem are you?
depending on the tap value and signal loss between the modem and the tap, levels could be all over the place. In general, the modem transmit level back to the cmts should be around 45dbmv, with 0 into the cmts receiver. this will give good signal to noise and room should the modems need to adjust levels due temp changes during the year. I recommend a signal generator at the end of the line or the 11/2 tap. figure the average loss from the home to the tap. example 150 foot drop, a two-way and cable from the splitter to the modem. loss from the modem to the first return amp should hit the first amp around 20 dbmv. output of that amp should be adjusted to hit the next amp with 20 dmbv and so on until you get back to the headend. assume it's just coax and no fiber.
general rule, modem receive level from the headend should be 0 +/- 10 (docsis spec is +/- 15). you can log into the modem's web page or to confirm it's levels.
transmit level should be around 45 dbmv, if you see modems around 58 dbmv now in the winter time, come July they will never stay online because the return loss goes up and the modem can't transmit enough level to hit the cmts with 0.
just my two cents


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