Lineman Forum: 750 mhz plant channels go out to 800 mhz - Lineman Forum

Any body else have this issue, the plant here goes out to 750mhz but we have added more channels and had to take the channels out to 800 mhz. This has created some balancing a sweeping issues. The correct fix would be a plant uprade to 1ghz, until then !!!!!!!!!!!!!!!!!

Do you have 850MHZ eq's in your amplifiers???? If you have 750MHZ eq's in your cascades the roll off will happen. You can't go past the number of MHZ on your eq's...

This post has been edited by System Tech Senior: 23 December 2009 - 02:36 PM

I have heard of going to 771MHz with 750MHz SA line gear but not 795MHz or 801MHz is your highest qams QAM 64 or 256.

We have 862 mhz eq's and 750 mhz in spots, but it does no good when your headend has 15 year old 750 mhz lasers. Trying to balance this is tricky, so we use correction factors and pilot channels.

If you equipment is specked to 800 mhz then it is capable of being balanced make sure they add a channel for you new Hi end. I hadn't heard that a laser is 750mhz, 860mhz, or even 1ghz they are normally measured by light so it would be a 1300mw, 1310mw, or 1550mw...ect

If you are trying to use 750 gear at 800 then there will be a roll off and it will get worst the higher you go. But you have figured that out

olo131

It's the 750 gear with an over 800MHZ spectrum.. you're going to have to upgrade gear or go with all 850mhz and above eq's..

Adding bigger bandwidth EQ's in the amps really does no good if the diplex filters, plant passives, headend electronics, and amplifier circuitry is built for 750 MHz. Yes, you can to a degree add more channels, however the result is that in order to get those higher frequencies to a level that is acceptable to the end users, you have to over equalize the amplifiers. This results in a large upward bow at the frequencies below the added bandwidth. This in turn causes the levels to both be too high at the subscriber premise, and adds to the overall distortion numbers on the plant. This will most evident when the signals are applied to an in-house amplifier. It's the same thing that happens when you overcompensate for bad cable by raising the high end output of the amp to overcome the excess loss. The lower frequencies which are not affected by the rolloff are amplified too much.

You are just going to have to find a happy medium between bad response and customer satisfaction. I for see a lot of service calls in your future, depending on what services are being offered in the added bandwidth.

It's surprising that Memphis, being a Comcast system, is doing this kind of engineering. There must be a push to roll out more services in the short term, while the money is being provisioned for an upgrade. I GHz is in your future, but even that amount of bandwidth isn't enough anymore. Maybe you'll get a bigger upgrade.

Good Luck Memphis, you have a lot of challenges ahead.

Dak

While the lasers may have a large bandwidth, the RF portion of the transmitters and receivers have the same limitations as any RF device. They are made for certain bandwidths, just like amplifiers, taps, etc. Many of these have bandpass filters built in to limit distortions in the laser.

Dak

Makes sense... never heard someone say hand me that 750mhz laser usually it's hand me that 1310 6mw laser. I just never thought of it like that.

Thanks , and this is exactly whats happening, the midband hump is causing distortions in the amps and we are getting a lot of service calls. Yes Olo we have old 750 lasers that are still in service. So no matter what your eq in the node is you will start to notice a roll off around 780 mhz. That being said imagine what it looks like 6 actives deep. That test carrier thats at 800 mhz starts out with a 11db tilt eventually with roll off is a 4db tilt. ughhhhh!

We had SA Prisma I lasers in the headend but they replaced them I think and now were running to channel 120 on SA SYSTEM II & III 750 MHz gear with SA 6940 nodes. We use AGC on Ch 61 and been sweeping a CW carrier on Ch 120 for a few months before turning it over to 256QAM. Levels on 120 aren't that bad actually, and were using it because of the demand for HD and other services. We just made room for DOCSIS 3.0 (3 carriers on 112,113,114). Returns been cleaned up a huge deal and were looking to push D3 services to customers in Feb. When the system was first upgraded in 99' we ALWAYS swept to Ch 118 (CW Carrier). I think Ch 116 *should be* the highest channel, but we were always trying to squeeze a little headroom out of the system.

I've heard of a system running 121 channels on 750 MHz gear. Not sure if were ever going to need to surpass 120 channels, as March were pulling a bunch of analog, and doing it again in April, along with a major HD lineup expansion. Right now were pushing about 42 HD channels and the full digital simulcast, phone, D3, and the usual digitals. There's NO plans to go 1 GHz, yet alone 870 MHz, because they feel eliminating 70 channels of analog is plenty bandwidth for now. By the time that's used up, they are looking at RFoG. Lancaster, PA Comcast is slated to have an RFoG test system designed in the next 2 years. If it goes well, more will follow.

Sweet.. Post more when that's launched!!

Yes please post the outcome good or bad, IT might help everyone

olo131

Sure, but we all have a good 2 year wait!

They said Lancaster, PA was a good test area. It features a little bit of everything. Downtown Lancaster - so dense city. Affluent suburb outlying area's (both UG and Aerial), and then very rural long haul areas. So it's like a 'practice spot'.

Better to go in the rural country areas.. Most of the people don't care if the cable goes out lol..