CABL Forum - Re:870MHz S-A need answers......Unity Gain

ModernBroadband,
Due to modems and other auto level adjusting equipment there would not need to be any extra gain (or less padding) at the next amp up stream. The modems will automatically adjust to meet the proper level (back to the original level).

And, unity gain is a term from the past when all amplifiers had exactly the same inputs and outputs, therefore, every station had the same gain. This meant that engineers could easily calculate the expected noise and distortions at the ends of lines rather having to use a complex formula that would only be thrown out at when the techs would change them for their convenience. Today's systems frequently have differing inputs and outputs for simular amplifiers and can have many different amplifier in a cascade...thowing unity gain in the forward right out the door. For reverse there is the same problem. Some some systems are disigned for the input reverse test point to be the same on every amplfier. Unfortunately, the reverse input test points have different losses to the input to the hybrid...Balanced Triples and Unbalanced Triples have different losses and Line extenders have different losses...so if there is 20 dBmV at the input test point on one, it could mean 17 dBmV for a line extender or 12 dBmV on a Balanced Triple, etc. See where I am going. And if there are differing leveling hitting the hybrids and differing output pads, how can there be unity gain. In the "old" days, all input were measured at the common reverse input test point at the hybrid, but no more. Then you may have had unity gain. Those days are gone.

>I agree Skopejw and franmc but here’s why I do, in a unity gain cascade, the next amp feeding up stream to the port with the additional 8 dB of return input padding would need to provide 8 dB of "extra" gain in the form of a smaller return pad (8dB smaller) to maintain unity gain thus returning the inputs back to the original level. But there is a benefit to "matching" forward and return losses in equipment that uses completely different paths internally i.e.: the internal spitters/directional couplers used to distribute the forward signal do not provide insertion loss in the return path. You touched on it without discussing how it lowers ingress.
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>The benifit is more isolation between the network and the subscriber drops making for lower levels of subscriber line ingress and as you spoke increased modem carrier levels as the modem auto levels to maintain operational inputs to the network.
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>Here’s an example why, say you have a output port on an amp and it requires a 23 tap, now install an internal DC 8 with the down leg now feeding the tap in question.....it will have to lower in value to provide proper subscriber feed levels.....but now due to the lower value it provides less isolation on the return path between those subscriber lines and the network unless we match losses at the return input of that active feeding that tap run to ensure that all subscriber lines in the network are subject to approximately the same return isolation from the network.
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>Again agreeing with Skopejw it is usually the customer that decides the method of setup.
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>>First off, it is up to the customer to determine if they want the reverse to follow the forward losses internal to the amplifiers. However, actually, franmc is correct in that the amps "should" be set for the same losses in reverse as forward. One of the biggest problems using cable modems is that the losses in reverse are not high enough, keeping the modem levels low as they enter the tap and therefore reducing ingress immunity. By changing the pad to introduce a higher return loss, the modem must increase in level and since the ingress from the rest of the system did not change, the modem signal is now that much higher. Many systems are intoducing even more losses by way of reverse conditioners that are installed directly in the hardline to have this very effect on ingress. Any modem manufacturer will tell you they want their modems running as hot as they can. AND, running higher inputs to an amplifier REDUCES noise not increases noise. Remember, for each dB increase at the input to any active, the Carrier to Noise Ratio improves 1 dB and CTB degrades by 2 dB. HOWEVER, the input to the first return amplifier will be the same as before because the cable modems (and telephony equipment as well) will automatically adapt to the proper level, called auto-level adjusting. If the input is supposed to be +20dBmV and there is a 0 dB pad in the reverse input slot and the pad is changed to an 8 dB pad, the modem will automatically increase by 8 dB and therefore overcome the pad to keep the input to the amplifier at +20 dBmV. But now your ingress is 8 dB lower since it was attenuated by the 8 dB pad, providing an added benefit of ingress immunization.
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>>Jim Wallace, Skope, 727-919-3590
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>>>"Mimick the forward path losses"? Huh????
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>>>Why would the return network need to realize losses of the actives internal forward network? The return signal is being distributed from that leg back through the network. Unlike the the forward signal that has to be regulated by the DC-8, to balance its value in regards to its fixed originating output, to accomplish the desired downstream inputs. The return signal is being injected at various locations, through various passives, with the modem having variable output capability to accomodate all of this, allowing its signal to hit the first amplifier with as low an output as possible.
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>>>By padding the input to the return chip, you are merely adding additional attenuation the sending modem must overcome in order to hit the level it needs to provide to the return amplifier in that active.
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>>>Driving the modem output higher than would otherwise be necessary. Increasing noise to the first Return amp.
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>>>I can see no reason to pad the input to the return, in order to immulate internal forward losses.
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>>>As to padding fwd input to make output. S/A has variable multi/amps for various design needs. "Given the appropriate amplifier" and ONLY in this consideration, will the desired input levels be met by the input pad necessary for the desired output.
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>>>Just shoving pads to make output is baaaaaaaaad advice cowboys.
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>>>;)
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>>>>Splicer life is correct.
>>>>Also be aware of the return input pad requirements for the GM Duals. You must pad the return input to match any internal DC value used on the forward output. For example: If an internal DC-8 is used, the return input tap leg needs to have an 8 pad installed and the thru leg a 2 or 3 pad. This procedure mimicks the forward path losses on the return path. It is very important to do this. If no DC or two way is used, 0 pads are installed.
>>>>In our design procedure, we will also "adjust" the return input pad value incremently until we have optimized each leg for optimum output levels at the various return devices. This really helps a customer at the end of a long distribution line.
>>>>If anyone would like additional information, feel free to contact me.
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>>>>
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>>>>>pad just the input for the desired output and go. these gainmakers are the nicest equipment i have worked with to date. GREAT amps!
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>>>>>>We are having a problem with setting up the forward levels
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Re:870MHz S-A need answers......Unity Gain