Xref: utzoo comp.arch:17461 sci.electronics:13263 sci.physics:13857 comp.lsi:1116 Path: utzoo!attcan!uunet!husc6!wuarchive!brutus.cs.uiuc.edu!ux1.cso.uiuc.edu!aries!mcdonald From: mcdonald@aries.scs.uiuc.edu (Doug McDonald) Newsgroups: comp.arch,sci.electronics,sci.physics,comp.lsi Subject: Re: Electro-optic bus Message-ID: <1990Aug1.123603.21380@ux1.cso.uiuc.edu> Date: 1 Aug 90 12:36:03 GMT References: <1985@trlluna.trl.oz> Sender: usenet@ux1.cso.uiuc.edu (News) Reply-To: mcdonald@aries.scs.uiuc.edu (Doug McDonald) Distribution: comp Organization: School of Chemical Sciences, Univ. of Illinois at Urbana-Champaign Lines: 44 In article aglew@oberon.crhc.uiuc.edu (Andy Glew) writes: >>>>A laser might not be the best - they have a delay from zero curent... >> >>Depending on area etc it can be up to 10 ns for lasers. LEDs are a >>little quicker. > >Thanks for providing the ballpark number! > >Now, is that just the time for the electro-optic conversion? Does >anyone have any ideas what the latency for reception by a native Si >optic->electronic receiver would be? Especially taking into account >the time to amplify from whatever signal levels the optical receiver >produces, to normal Si logic levels? > >Let me do some informal calculations (mainly to prompt people who know >better to correct my (mis)estimates): > > Say it's 5ns for the electro-optic latency. > Say another 5ns for the reception and amplification to CMOS levels. > Add 1ns for transmission time in the optical medium > Add another 4ns for the point-to-point electrical interconnect. > >That gives maybe a 15ns latency. > >Well, that's nothing great... you never go wrong building a >low-latency interconnect, and electronics can do better than this. > >How many signals could be crammed onto this 15ns latency electro-optical star? >-- Well, the rise time for a cheap ($0.10 [sic]) silicon detector (1N914) at 1.06 microns is 75 psec. That is 0.075 nsec. I would guess the amplification to 1-volt levels could be done easily (by high speed circuit standards) in 150 psec. For money you can get it down into the 10s of picoseconds. The risetime of emitter diodes (and emitter lasers too, if you always run above threshold, which is not exactly fun) is probably in the same ballpark. Please don't quote different times for the risetime of a 1N914 - that is the measured value, measured by me with a 50 psec light pulse and Tektronix's fastest sampling scope. It differs from the normal electrical response time because inm this case the thing remains always back-biased. There are commercial diodes that are that fast into the visible. Doug McDonald