| Guenter,
I think I can offer an explanation for the different cable loss
figures. You state that using 50/125 fibers gives you a loss budget of
6 dB - a difference of 5 dB from the loss budget you would have with
62.5/125 fibers.
But in your calculations you used a 6 dB adjustment instead of a 5 dB
adjustment. If 5 dB is used, the calculation becomes:
Result: -16.6 dBm Launch Power + 5dB adjust (50/125 <=> 62.5/125)= -21.6dBm
-27,0 dBm Rcv Power - (-21.6dBm) = 5.4 dB resulting Fiber Loss
Which agrees fairly well with the other measured loss of 5.1 dB.
Richard Kirk
FDDI Adapter Engineering
LEVERS::KIRK
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Note 1051.0 FDDI Rcv Power Measurement ?? No replies
VNASWS::HONISCH "Guenter Honisch ACT Austria" 51 lines 6-AUG-1993 05:18
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At a recent Fiber Certification we tried several methods to determine
the Loss of a Fiber Path consisting of several patched segments.
The fiber is a european 50/125 NA 0.2 Type which gives me a budget of
6 dB with FDDI
besides OTDR and Powermeter we used a FDDI Bridge as a source to
doublecheck our numbers.
In the Handouts of the FOFT Training they mention a Way to measure
Output Pwr of FDDI Devices by a Powermeter and calculate the Loss on
the fiber.
The Values given are : -14 dBm to -31 dBm for receive
BUT as far as I know FDDI Xmits a 50% Square Wave except in Halt State
*************************************************************************
Does this mean i have to adjust these Values by 50% (-3dBm) ???
so that I should read -11 dBm to -28 dBm for receive ???
*************************************************************************
____ -------
| B |- PATCHFIBER -| |
| R | |------------------| |anritsu| 16,6 dBm
| I |- 62.5/125 -| |
| Dge| -------
____
____ 62.5 -------
| B |- Fiber -| |
| R | |-|--/\/\/\/\/\/---| |anritsu|
| I |- 50/125 -| | -27,07dBm
| Dge| -------
____
Result: -16.6 dBm Launch Power + 6dB adjust (50/125 <=> 62.5/125)= -22.6dBm
-27,0 dBm Rcv Power - (-22.6dBm) = 4.4 dB resulting Fiber Loss
Using normal Loss metering without a Bridge we got 5.1 dB
(in these cases we used TWICE the needed Length to be safe)
Are these assumptions valid??
Guenter
|
| Re power measurement procedure:
FDDI signalling actually has a duty cycle between 40% and 60% (not fixed
at 50%, though 50% is the long term average). "Quiet Line State", which
is the state of a disabled line, is an exception: that simply has the
transmitter turned off, i.e., 0 power. (You should not call this "halted"
since that causes confusion; there is "halt line state" which is quite
a different thing.)
The FDDI standard states that power level measurements are done while
the station is transmitting "halt line state", which is a sequence of H
symbols -- corresponding to a 12.5 MHz squarewave signal. So the test
signal indeed has a duty cycle of exactly 50%. The power specs are written
for that test signal; you should not adjust them further to take into account
the 50%, since that is already included in the standard numbers.
paul
|