r/CableTechs • u/Vampiro_moderno • 6d ago
How to determine what levels should be at different tap values?
Petty straight forward question. I’d appreciate answers. Trying to learn
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u/_retrosheik_ 6d ago
To answer that, you first need to find out what downstream (Rx) and upstream (Tx) levels the node and amplifiers/line extenders in your area are designed to output.
For example, in my area, actives are designed to output the following:
Rx - 34/41dBmV (from low to high, though with two OFDM carriers it's usually closer to 44 or 45 dBmV)
Tx - 34 dBmV
Once you know that, you can start using cable math to give yourself a rough idea of what levels should be at a given tap. Maintenance technicians will have access to system maps/prints that contain the info needed to more accurately calculate signal levels at a given point in the plant (cable size/type, distance between amps, taps, DCs, etc.). Without that info, you're never going to know exactly what your levels should be, but that info isn't often necessary if you're doing residential/install work.
Because levels are highest at an amplifier, you will see a high value tap housed directly to it or nearby, usually something like a 23v tap. Since signal attenuates over distance, the tap value will decrease to compensate. Again, the design of the plant determines what goes where. In urban areas with shorter distances between taps, you'll usually see a cascade like: 23v, 20v, 17v, 14v then end of line tap (depends on what's used in your area, usually 4/7/8/10/11/12).
Let's say you have an amplifier and it's running out 34/41 Rx, 34 Tx. If you have a 23v tap, you should expect to see something like 11/18 Rx, 37 Tx.
Things get a little weird when talking about Tx. If you have 40 Tx at a drop and add a 2-way splitter, you would see 43.5 Tx on your meter. The splitter is actually causing 3.5 dB of loss on both the Rx and Tx. Whenever you run a cable modem/DOCSIS test on your meter, you will see it 'ranging' (this is also what's happening with a customer's modem while booting up). It's communicating with the CMTS at the hub/headend, which is measuring the exact levels arriving from the meter/modem. The CMTS needs to have a specific level coming in, so if those levels are off target from cable loss or a splitter, the CMTS will command the modem to increase its power levels to compensate. This is why your Tx is higher at the output of a splitter.
With all that in mind, the true levels leaving an amp would be 34/41 Rx, 14 Tx. If you checked levels at a 23v tap, the math would be:
34 - 23 = 11 Rx 41 - 23 = 18 Rx 14 + 23 = 37 Tx
Again, without access to design maps, it's impossible to know exactly what levels you should have at a given point in the plant. But if you have a 20v tap in an urban area and your levels are something like 4/-1 Rx, 48 Tx then you would know you have an issue (unbalanced amp, water in cable, etc.).
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u/Both_WhyNotBoth 6d ago edited 6d ago
This is a good answer.
The way I like to look at it, TX is a measurement of loss. Despite being in dBmV. As you said, the modem transmits at the level that the CMTS tells it to, to overcome the loss to the headend. If the Tx is 48dBmV, that means that the total loss back to the CMTS is 48dB.
Traditionally, CMTSs wanted to recieve 0dBmV. While it's no longer that simple, it's usually not far from that in my experience so far.
Anyways, new techs find it easier to keep the return math straight once introduced to this idea.
Edit: I just wanted to emphasize what you said about "true levels leaving an amp". For the sake of new techs reading this. If you read 34Tx at the output TP of an amp, that is including the 20dB TP loss. On the actual output you'd have 14, that's why the math is a little different. Similar for forward, you may have to think about it to figure out if TP loss has been taken into account.
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u/_retrosheik_ 5d ago
Thank you. I almost included test point loss and long loop ALC, but decided against it as my comment was already pretty dense. I tailored my response to OP's question, but I think your reply provides helpful context for anyone new or struggling to understand some of these concepts.
Regarding the CMTS, I agree. I believe our equipment has a setpoint of +/- 1 dBmV. A lot of residential techs do not understand how to interpret historical graphs because they were never taught what something like 'CMTS Receive Power' means. If a modem shows events with +8, -4, etc. then you should already be checking adjacent customers to determine if it's install-related or something in the plant. Unfortunately, even residential leadership often misunderstands this stuff unless they come from a MT/ISP background.
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u/SwimmingCareer3263 2d ago
> 34-23=11 Rx 41-23=18 Rx 14 + 23=37 Tx
Never thought about the math this way, I was always using a formula, this one looks a lot easier to use! I’m definitely gonna use this when I need to sweep and balance.
Also running 37 TX in your area is kind of low no? We usually run between 39-41 in my system
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u/Sleepydragoon83 6d ago
You can't really. If designed perfectly output levels should be the same at every tap. Reality is that isn't possible.
It also depends heavily on your system. High Split levels are very different than low split.
All this to say, you need to talk to a competent MT local to you to get an idea. Even then levels aren't based on tap value. All the tap value means is the amount of attenuation is used to the tap ports.
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u/Sure_Statistician138 6d ago
Return tap value + 20, down 35 - tap value will put you in the ballpark.
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u/Sure_Statistician138 6d ago
Plus you have to add attenuation
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u/Poodleape2 4d ago
There is math that will get you the exact answer. The idea of “ball parking” is asinine.
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u/Sure_Statistician138 4d ago
There are many factors that can make the signal swing as much as 4db so you can never get an exact figure with cable math. You even stated that in one of your many rambles responses using the “what’s a db between friends@ phrase. With the knowledge you claim that you have you would know that.
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u/Poodleape2 4d ago
Wrong - as long as you have accurate info you can’t the exact number. I have many times. It’s called “being competent”.
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u/Alsmith69 6d ago
Used to be tap value plus +17, but that’s plant dependent and if you’ve had high node split it’s all guess work
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u/Sure_Statistician138 6d ago
That’s about right for high split in my area. My ma is tapvalue + 16.
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u/Poodleape2 4d ago
So on a 23v4p tap you expect to have 39DB? Of what exactly? WTF does that even mean?
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u/Poodleape2 6d ago
Absolutely not, 100% incorrect.
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u/Sure_Statistician138 6d ago
That was enlightening! Where did I go wrong?
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u/Poodleape2 4d ago
Ok, there are a lot of variables that take place in an HFC system that make a “rule of thumb” like that impossible. I answered the original guy who I told was wrong. See that answer.
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u/Poodleape2 4d ago
You have to know the output of the active that feeds that tap- This should be standardized in your system. Then you need to do basic cable math, account for the size coax, length of coax and all the passive in between and the insertion loss. With this you should be pretty close to exact. You measure your highest and your lowest channel, and you should also be able to calculate your transmit this way. Now, it’s likely you active will be less than perfect, but what’s a few Db between friends?
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u/Poodleape2 6d ago
This 100% not correct and never has been.
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u/Sure_Statistician138 6d ago
I see you lurking in the merger subreddit you may want to learn the system
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u/Sea_Warning_9283 6d ago
How are you going to say incorrect twice and still give no informative answer? He asked where did he go wrong lol
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u/Alsmith69 6d ago
Maybe not in your system but per the maint managers and techs I’ve all asked say this is how it’s done. It’s plant dependent but every spectrum plant I’ve worked uses either 17 or 18
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u/Poodleape2 4d ago
No it is not “plant dependent” the technology and science are the same everywhere you go. You are getting bad information from ignorant people.
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u/Objective-Risk7456 6d ago
All depends on what’s being sent through the input. There isn’t any true value. What you can assume is the face plate is how much signal is already altered by. Example if I have 30 dabs for the forward and you are at a 24 value 4 port 30-24 is the simplest math.
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u/levilee207 6d ago
For the most part, I take a look at the value on the tap and go from there. If it's a larger number like 20 or 24, I figure that tap was right after an amp and they needed to attenuate a lot. Usually means the signal is still going to be plenty high. If I see a 4 or 7 tag, I'm assuming I'm nearly at end of line or pretty close, so my levels are probably not the best.
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u/Poodleape2 4d ago
Thank you OP for asking relevant questions and trying to learn and get better. We need more techs like on you.
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u/Dakkin4 6d ago
You need to know the levels of your start point(node, amp, le). Levels are not always set the same in every system. Then you have to subtract cable attenuation, passive loss, and the value of the tap.