Cable Forum - View Single Post - 100M Power levels went up a notch and speed down?

Sephiroth · 07-12-2012, 19:13

Yes - but remember the cumulative power is calculated on the 3dB doubling principle.

It's not a case of just adding up the dBmv listed on your 4 or 8 channels. The (logarithmic) dBmv scale is relative to 0dBmv and a +3 dBmv increase in power level doubles the power arriving at the SH.

Let's say that the power level on a single channel is 0 dBmv. Now let's double that to 2 channels so you aggregate power in dBmv terms is +3 dBmv. Make that 4 channels and it's +6 dBmv; 8 channels and it's 9 dBmv. Your SH currently sees a spread of 16 DS channels (each 8 MHz wide) in your set from which you can actually use 4 or 8 (or whatever VM set) according to the valid BPI+ key associated with your lease.

But all that stuff hits your SH and is sifted by the tuners. 16 = 2^4 so the aggregate downstream power is at least 4 x 3 dBmv = 12 dBmv (when the reported power is 0 dBmv).

You will bust the EuroDOCSIS 3 limit of 33 dBmv if the average power across the downstream channels exceeds 22 dBmv.That's calculated by adding 3 dBmv to the 22 dBmv for each doubling of the number of channels. So, for 16 channels, that's 22+3+3+3+3 = 34 dBmv. Clear as mud, eh?

07-12-2012, 19:13	#7
Sephiroth Wisdom & truth Join Date: Jul 2009 Location: RG41 Services: RG41: 1Gig VOLT Rutland: Gigaclear 400/400 Posts: 12,507	Re: Power levels went up a notch and speed down? Yes - but remember the cumulative power is calculated on the 3dB doubling principle. It's not a case of just adding up the dBmv listed on your 4 or 8 channels. The (logarithmic) dBmv scale is relative to 0dBmv and a +3 dBmv increase in power level doubles the power arriving at the SH. Let's say that the power level on a single channel is 0 dBmv. Now let's double that to 2 channels so you aggregate power in dBmv terms is +3 dBmv. Make that 4 channels and it's +6 dBmv; 8 channels and it's 9 dBmv. Your SH currently sees a spread of 16 DS channels (each 8 MHz wide) in your set from which you can actually use 4 or 8 (or whatever VM set) according to the valid BPI+ key associated with your lease. But all that stuff hits your SH and is sifted by the tuners. 16 = 2^4 so the aggregate downstream power is at least 4 x 3 dBmv = 12 dBmv (when the reported power is 0 dBmv). You will bust the EuroDOCSIS 3 limit of 33 dBmv if the average power across the downstream channels exceeds 22 dBmv.That's calculated by adding 3 dBmv to the 22 dBmv for each doubling of the number of channels. So, for 16 channels, that's 22+3+3+3+3 = 34 dBmv. Clear as mud, eh? __________________ *Seph.* My advice is at your risk.