Dr Bruce Griffiths wrote: > Didier Juges wrote: > >> Dr Bruce Griffiths wrote: >> >> >>> Didier Juges wrote: >>> >>> >>> >>>> Bruce, >>>> >>>> I have read about this, noise performance also is not good for analog >>>> transmissions, causing very limited dynamic range. >>>> >>>> That's probably why they use either FM or digital coding in just about >>>> all applications. >>>> >>>> I just did not think the jitter would be so bad, even with a laser >>>> transmitter. >>>> >>>> In my case, we are just trying to send a reference signal, but it must >>>> be clean, so unless a clean-up PLL is used, forget about fiber. >>>> >>>> Thanks, >>>> >>>> Didier >>>> _______________________________________________ >>>> time-nuts mailing list >>>> [email protected] >>>> https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts >>>> >>>> >>>> >>>> >>>> >>> Didier >>> >>> I presume you are directly modulating the laser. >>> This is hopeless you get chirping and all sorts of other effects that >>> are virtually impossible to tame. >>> An external electrooptic (LiNb03) modulator is the only way that >>> actually works and has a halfway decent performance. >>> >>> Bruce >>> >>> >>> >>> >> Bruce, >> >> I am talking about an integrated 4 GB/s transceiver from Avago: >> AFCT-57R5. It is a small plug-in module, so they probably directly >> modulate the laser. I understand when the laser turns on, there are >> probably a bunch of transient effects until the beam is stabilized. An >> external modulator would alleviate these problems, but the cost is most >> likely out of the question for this market. >> >> Both the transmitter and the receiver each have *average* jitter >> specifications of about 60 pS, from logic level signals. >> >> So even with a better modulator, the receiver still would create way too >> much jitter. It is probably a pin diode. I guess the answer would be an >> heterodyne receiver? >> >> Probably performance would be much better if it were not measured at the >> end of 10 kM of monomode fiber, but they don't say. I guess they are >> designed for an application where it's 10kM or you don't need it. >> >> >> Didier >> >> _______________________________________________ >> time-nuts mailing list >> [email protected] >> https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts >> >> >> > Didier > > There is no virtually possibility of direct modulation working > sufficiently well. > If an external modulator is unaffordable, then forget it. > All successful RF over fiber applications uses external modulators. > > Integrated lasers + modulators cost somewhere in the region of $US1000 > or a perhaps a bit more. > The microwave InGaAs photodiode and matched amplifier at the other end > has a similar cost. > > End to end, the phase noise floor of around -130dBc/Hz at L band. > This may well be good enough. > > Bruce > > Bruce,
Thanks for the rough order of magnitude. I believe the cost you are indicating may not be out of line. It will save a remote synthesizer, so that would be a significant saving, over simply sending a 10 or 100 MHz reference. I will check with my friend and if he is interested, I may call you back for more details. Thanks a lot. Didier _______________________________________________ time-nuts mailing list [email protected] https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
