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29/03/2016

Understanding signal levels and cable loss characteristics

Understanding signal levels and cable loss characteristics

Cable is full of acronyms, we can ramble off dozens of them and some we’ll know, some we will vaguely know and some we have no idea.  My thoughts are that Cable is a career; it is a progressive career, start as an installation or service technician and work your way up, maintenance, head-end, digital technician, supervisor, manager, etc,...but start out slow and learn as you go along, ask questions, understand the answers, experiment in the field, never stop learning. 

As a service technician in Cable TV responsible for signals from the tap to the TV or modem, there are items you need to know, items you should know and finally items that are nice to know. 

Items that are nice to know are the little things that will lead to your progression within the company, a QAM signature for example, when is it good and when does it need repairing.  Following my “need to know” items below, I believe you can be very successful without understanding the “nice to know” items. 

Items that you should know are requirements you should learn and understand, but initially are not detrimental to your success as a service technician.  These are items such as velocity of propagation (VOP), you need to know the value if you are using a time domain reflectometer, (TDR), but it is usually supplied.  You should understand the impedance of cable, how it is determined and the detriments if it changes, but again, you can be successful initially without fully comprehending what these terms refer to. 

What I believe to be the single most important guarantee of success, at least initially, in the industry is to understand signal levels and cable loss characteristics.  This is where we get back to basics. 

If you can know your signal levels you can determine if there is a fault.  Speak with your system’s network technician; understand what the pilot levels are of the plant, the levels coming out of your line extender.  If you go and measure a tap and your pilot levels are 18 dB on 4 and 18 dB on 70 do you have a problem?  What if that tap is a 14 tap in the middle of a cascade of taps, it is likely fine, what if it is a 23 tap fed directly off a line extender?  What if it is again a 14 tap, but this time it is connected to an 8dB directional coupler which in turn is connected directly to the line extender.  

Understanding your pilot levels, understanding the skin effect of cable and then applying the math in a rule of thumb manner, (as part of my personal disclosure I have to confess here, part of my left thumb was amputated when I was 16 so for me a rule of thumb is actually a rule of half thumb), but I digress, using a rule of thumb will give you an idea as to what you should have coming off any tap in your system.  For example, where I worked we typically used channels 4 and 70 as our pilot channels and we set them up for line extender outputs of 37 and 44 dB.  Typically we fed a 23 tap to a 20 to a 17 to a 14 to an 11 and then fed another line extender.  In an aerial application each tap was between 100 and 150 feet away from each other.  The rule of thumb that I used was that each span of cable would lose 1dB of signal at channel 4 and 3 dB of signal at channel 70.  The taps themselves had an insertion loss, another rule of thumb I used was 0.5 dB of insertion loss on taps above the value of 17, a full dB on a 17 tap and 1.5 dB on taps less than a 17. 

In the chain of line extender, 23 tap, (connected directly), pole span to a 20 tap, pole span to a 17 tap, pole span to a 14 tap and pole span to an 11 tap I had an expectation of the levels I would see.  At the 23 tap I would have 14 over 21, the 20 tap would be 16 over 21, the 17 tap would be 17 over 20, the 14 tap would be 19 over 20 and finally the 11 tap would be 20 over 19.  If levels were plus/minus by 2dB using approximates and the actual accuracy of the meter I would be satisfied.  This would allow me to determine if there was a mainline problem or if levels were good at the tap.  

Once I had my tap levels determined I could troubleshoot the subscribers drop to the house.  Again, going back to basics, I always recalled arguing with my high school grade 11 algebra teacher...”when am I ever going to use this stuff in the real world”?  As a cable TV tech I used algebra quite often, another rule of thumb was four times the frequency equals twice the loss.  This was due to the skin effect of cable, the higher the frequency the less cross sectional area of a cable the electrons flowed on. 

So for me, in addition to using channels 4 and 70, I also measured channel 36, the reason being was that channel 36 is approximately 4 times the frequency of channel 4.  So now if at my tap I had levels of 14 on channel 4 and 17 on channel 36 and 21 on channel 70 I could pretty easily verify the integrity of the cable to any point measured.  Knowing that channel 36 would lose 4 times as much as channel 4, I would check at the side of the house; it did not matter how long the cable drop was.  If I had 14 on channel 4 at the tap and I had 8 on channel 4 at the side of the house I had lost 6 dB of signal.  If I lose 6 on channel 4 then I should lose 12 on channel 36, so in my example above I would expect to see 5dB at the side of the house on channel 36 and significantly more on channel 70.  The signals in my example here are obviously going to be low, especially as we move through higher frequencies and through a splitting network, perhaps it is time to upgrade the cable to an RG-11 and possibly consider using a house amplifier or some other similar device. 

Finally, as I developed as a technician I began to use other methods to check the integrity of the cable.  I could have good levels on 4, 36 and 70 but have an issue with channel 46 for example, that is when I began to learn and understand sweep, as a service technician we usually did not have access to system sweep, but our meters had “sweepless sweep” built into them and I could use that feature to check for suck outs on the cable. 

Anyway, that is getting things a little more advanced and for now I just wanted to keep it simple, bring it back to the basics. 

Thanks all, and have a great day

 

Cable Professional | Operations Management | RF Technical Trainer | Employee Engagement Champion

 

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