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It has been approved that there is no significant
difference (<0.3dB) between the continuous mode sensitivity
and burst mode sensitivity for a good designed burst mode receiver in
the GEPON and GPON application. Theoretically, there is no any deference between
the BM sensitivity and CM sensitivity, if the discharging time for the large
signal and charging time for the small signal is fast enough in the
amplification circuit in the burst mode receiver.
There are several factors that affect the
burst mode receiver sensitivity in the real application.
1. Receiver settling time defined by the standard
and application.
a. In the 802.3ae, the receiver settling time is
defined as 400ns.
b. In the GEPON application in Asia, most customers
tighten the receiver settling time to 64ns or 32ns.
c. In the ITU-T G.984.2 GPON application, the
receiver settling time is not defined clearly, but 984.2 defined the guide time
as 32bits, preamble as 44 bits, delimiter as 20bits. Normally, the received
upstream signal needs to be recovered in the first 30 bits of the preamble, and
the CDR needs at least 14bits to lock the signal.
Generally, the longer the receiver settling time
allowed, the better the BM receiver sensitivity (less difference with the
continuous mode senstivity) and BM dynamic range.
2. The coding scheme of the data
a. In the 802.3ae and GEPON application in Asia,
the coding is 8B/10B. Therefore, a low time constant signal path without
RESET is good enough to pass the 8B/10B code and meet the RX settling time
without extra RESET circuit.
b. In the ITU-T G.984.2 GPON application, the
coding is PRBS2^7-1 scramble, and the BM receiver needs to pass the 72bits CID.
There is no simple circuitry to pass the scramble signal without error and meet
the short RX recovering time at the same time.
3. RESET vs. no RESET design
a. In the GEPON application, RESET for the receiver
is not needed in the burst mode receiver, due to the 8B/10B coding scheme and
the longer RX Settling time requirement.
b. To handle the scramble signal with longer CID in
the GPON application, (maybe the same case for the 10GEPON application with
either 64B/66B or 32B/33B coding as proposed), It seems that a RESET is
necessary for the industrial temperature application (-40ºC
to +85ºC).
Even for the comercial temperature application, the OLT receiver without RESET
will generate problems when the temperature and dynamic range
varies.
c.
There are several GPON 1.25G OLT receiver design without RESET. As commented by
Maurice, all the OLT receivers without RESET need to get the optical signal from
the ONU TX with very high extinction ration (normally higher than 15dBm).
The high ER is very difficult to keep through the operating temperature range.
It is noticed that in the 10G XFP or XENPAK application, the ER is defined only
as 6dB or 4dB (or the optical modulation amplitude is used to replace the ER in
order to avoid a higher ER value).
4.
BM dynamic range
For
the GEPON and GPON application, the burst mode dynamic range (difference between
two consecutive signal) is either 15dB, 21dB or 25dB, depending on the
requirements in the applications.
The
BM receiver sensitivity (or the sensitivity differnce between the BM
receiver and CM receiver) is a function of the coding scheme, the RX settling
time allowed in the standard, the BM dynamic range defined in the standard, and
certainly the BM receiver design approach (with or w/o
RESET).
Best
regrds,
David
David
Li
Ligent Photonics, Inc. 2701 Dukane Dr., Suite 102 St. Charles, IL 60174 Phone 630-513-7226 ext 15
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