FreeLists / si-list / [SI-LIST] testing LVPECL signal with 50ohm system

["Dan.Zhu" <dan.zhu@xxxxxxxxxx>]

Hi,

I have a question on how to test LVPECL signal with 50ohm system like digital 
oscilloscope or Spectrum Analyzer (not 
probeing, just SMA connector on board ->cable-> scope). 

Background -
For the LVPECL(VCC=3.3v & VEE=0v) output, normally a 50ohm termination resistor 
is used between LVPECL output pin and 
a voltage source of Vtt(=VCC-2v) to get a common-mode PECL output level of 
VCC-1.3v.  When testing a standalone PECL 
device, people can lower the supply/gnd of pecl device by -2v so VCC'=1.3v, 
VEE'=-2v -> Vtt'=0v, then a digital 
oscilloscope with 50ohm input resistor(to GND) can be used to replace the 50ohm 
termination resistor and test the 
signal directly. 

Question -
My question is that if due to some reason, the supply of PECL device can't be 
changed(for example, the pecl device is 
integrated on a mixed-signal IC and  change of supply of PECL device is not 
allowed), there is any other way we can 
still test the PECL output signal directly(not probeing, just SMA connector on 
board ->cable-> scope)?


Thanks for your suggestion

Best Regards,

Dan Zhu

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