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Downstream Frequency Spacing

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kamien
Downstream Frequency Spacing

We're having issues with our cable plant (spring and fall) when the temps swing over night. We noticed the losses are much more dramatic in the upper frequencies when the temp swings. We're rural so we have much longer distribution runs than we'd like, but we'll be splitting that up with new fiber nodes soon.

We've got 8 downstreams per node, from 531MHz to 573MHz. Since we're getting so much loss on the higher freqs, we thought we'd try moving some of our downstreams into the 93MHz to 117Mhz range. We were hoping that by moving 4 of the downstream channels into the mid band, when temp swings those losses might be manageable and the CMs would be able to continue service albeit with a loss of bandwidth. Our service is dominated by old-style CATV channels which take up almost our entire spectrum, so I don't have a lot of space to put my downstream channels.

cable downstream bonding-group 1 1/4/0 1/4/1 1/4/2 1/4/3 1/5/0 1/5/1 1/5/2 1/5/3
cable downstream bonding-group 2 1/4/0 1/4/1 1/4/2 1/4/3
cable downstream bonding-group 3 1/5/0 1/5/1 1/5/2 1/5/3

Anyways, I set the channels on port 1/4 in the 93 to 117 MHz range, and set port 1/5 to the 531 to 549 MHz range. The cable modems won't register with all 8 channels, but will pick either bonding-group 2 or 3. If I set port 1/4 back to the upper freqs (555 to 573 MHz in my case) the modems happily register with all 8 channels in bonding-group 1.

Is this a limitation of the CM radio or something? I guess they have to tune to a certain band that's wide enough to accommodate the downstream channels?

cmcaldas
rule of thumb

way back when... the rule was no more than 50Mhz difference from the 1st group to the 2nd group.
others can chime in, but your spread too far apart.
why can't you move some of the video channels? do the amps support agc? with cooler temps, would have less loss than hot summer months

mbowe
It is a limitation with the

It is a limitation with the tuner chip in 8x4 modems.
When doing 8CH DS bonding group, for maximum compatibility the 8 channels should be contiguous.
Next best option is 8 channels within a 60MHz window.
If you spread the channels any further apart, most 8 DS modems will have trouble.
You may be able to setup custom Receive Channel Profiles to get things working, but this is pretty messy and I would not recommend it.
(16/24/32 DS modems don't have this issue. They can tune to any combination of channels).

As previous poster said, do your amps support Automatic Level Control?
This is a common way to keep levels steady during change of temperature.
Not every amp needs to have it, but if at least some support this feature it will help keep the levels under control.

Also, on my plant I have noticed that DS frequencies below 200MHz are a bit more susceptible to FEC errors.
I reckon the 200/300/400 range is the sweetest spot if you have a choice.

cmcaldas
downstream freq

mbowe makes a very good points and I respect his comments as he's been in the group a very long time.
I just wanted to chime in on the downstream frequencies from 560Mhz to around 620Mhz (I don't recall the exact freq band, but believe it's around that spectrum) in regards to 5G used by T-Mobile and ingress around that part of the spectrum, to simply be mindful and check for errors. I've seen modems in partial service saying channels around 580-620Mhz having low mer or low levels in the some areas that are close to cell towers.
just my two cents

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