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.
Jim Wallace, Skope, 727-919-3590
>"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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>>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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>>>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
Member #: 1627
Registered: 1996-2001
Posted:
47
Name:
Jim Wallace
Name:
Jim Wallace
Company:
Skope Splicing and Communications, LLC
Occupation:
Director of Certification
Location:
New Port Richey, FL
There is 1 reply to this message
