
GTS1063 and GTS1250 GBIC Test Systems Instruction Manual
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Appendix C: Recovering a Clock from an External
Electrical Signal
Consider the information in this appendix when recovering a clock from an
external signal.
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Rx DATA connectors used as electrical inputs
There are applications in which you may need to recover a serial clock from an
electrical data stream. An example of such an application might be: GBIC
modules are often powered on a separate circuit board with 50-ohm input and
output connectors; this is a common application, especially among GBIC
manufacturers that wish to calibrate and/or test a GBIC module under well
controlled environmental conditions, such as temperature. In applications where
the GBIC module cannot be installed into the GBIC Receptacle (such as for
testing 1x9 GBIC modules), the +Rx Data and –Rx Data MIN–LOSS outputs
can act in reverse as electrical inputs.
Overriding RX_LOS to Enable the Clock Recovery Circuit
If there is no GBIC present, the GBIC Test System detects the RX_LOS line
being pulled HIGH and shuts down the recovered clock circuitry (to optimize
noise and power when no signal is present). To use the clock recovery circuitry
without a GBIC in the receptacle, hold the RX_LOS signal LOW by driving the
RX_LOS signal (pin 11) of the REMOTE INTERFACE connector low (or
simply short this pin to GND)
Proper AC Input Coupling: Single-ended or Complimentary
The internal Min-Loss 75-to50 ohm conversion network is a passive network of
resistors with a differential comparator measuring the difference between the
+Rx_Data path and the –Rx_Data path (see the diagram in Figure 5 on page 14).
This network is DC coupled to the front panel +RX DATA and –RX DATA
connectors; due to the DC coupling, any external single-ended input signal
applied to just one of the +/–RX Data connectors must be AC coupled through
an external DC-block, and the unused RX DATA connector must be DC
terminated with a 50-ohm termination (use the termination attached to front
panel). Two separate complimentary signals may also be input into both the RX
Data connectors using similar AC coupling on both inputs (in which case the
signal input to +Rx DATA must have its compliment signal input to the –Rx
Data connector with matching phase).
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