A question regarding probe point on high speed digital circuits
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From: langwadt on 17 Jun 2010 13:50 On 17 Jun., 03:52, John Larkin <jjlar...(a)highNOTlandTHIStechnologyPART.com> wrote: > On Wed, 16 Jun 2010 13:24:58 -0700 (PDT), Javad Benhangi > > > > <benhan...(a)gmail.com> wrote: > >Hi all, > >I have a question regarding probe point on high speed digital > >circuits. I m working with a chip that has 16 bit LVDS parallel data > >bus (including the clock that is LVDS). I m looking for a way to > >observe the 34 data signal and find the exact cause of not working the > >circuit. > >I m designing the board and I m looking for the best way to implement > >some test/probe point on the high speed signals. Of course some > >exposed tracks/VIAs is not enough to observe them because the need for > >proper termination. Since the number of signals is too much (34 > >signals) and sounds impossible to have an SMA connection point for > >each of them. If I use conventional oscilloscope probes adds too much > >noise. > >So what is the best way to observe such kind of signals? > >Since the signaling is differential is there a way to observe the > >signals using no differential probes? > > We sometimes add a "Mictor" connector to our boards in a wide, nasty > signal path, like between an FPGA and some complex chip like a PCI > Express bridge. Then you can plug one of those cute little USB logic > analyzer heads right into it. What I don't know is if there are any > good LVDS logic analyzers. > > TYCO AMP 2-5767004-2 38 PINS > > is one we use. > > John use the fpga as the analyser, I always add a few extra connectors with uncommittet pins to an fpga design. Then it is easy to route out any signal you'd like to see ofcourse this only works when you want to see the signal in the digital domain, if you want to see for example an LVDS signal in analog you need to probe the signal itself. but then one or two of the signals should be enough assuming they are all routed, terminated etc. in the same way. -Lasse
From: Joerg on 17 Jun 2010 14:24 Bill Sloman wrote: [...] > Since the buffer presents a largely capacitative load to the line, you > can compensate for it by narrwoing the trace - say from 50R to 75R - > for a cm or two around the buffer connection. > Well, if the capacitance is a problem there's a solution as well but it ain't for the faint of heart: http://www.infineon.com/dgdl/bfp620.pdf?folderId=db3a30431400ef68011425b291f205c5&fileId=db3a30431400ef680114274deb27072a Always got some here in a drawer. Just be careful, these can oscillate without you even knowing it. -- Regards, Joerg http://www.analogconsultants.com/ "gmail" domain blocked because of excessive spam. Use another domain or send PM.
From: Archimedes' Lever on 17 Jun 2010 14:55 On Thu, 17 Jun 2010 11:24:30 -0700, Joerg <invalid(a)invalid.invalid> wrote: >Always got some here in a drawer. Just be careful, these can oscillate >without you even knowing it. How would you know then? How do you know the claim is true, for that matter. Are black holes visible? I say yes. One must simply know how and where to look.
From: Joerg on 17 Jun 2010 16:35 Archimedes' Lever wrote: > On Thu, 17 Jun 2010 11:24:30 -0700, Joerg <invalid(a)invalid.invalid> > wrote: > >> Always got some here in a drawer. Just be careful, these can oscillate >> without you even knowing it. > > > How would you know then? How do you know the claim is true, for that > matter. > People in the RF biz often have analyzers available. Then there is experience: A sure-fire way to know it's fishy is when you slowly crank up the supply voltage and suddenly there is a jump in current. For that, I have mounted high-quality 20-turn potmeters on some lab supplies. This is actually an age-old method in the RF-world. Lots of other tricks as well, such as wideband detectors. Since you mentioned in another thread that you work on sat receivers I am a bit surprised now ;-) > Are black holes visible? I say yes. One must simply know how and > where to look. Nah, got to look for that tell-tale halo :-) Other times the announcement of oscillatory behavior can be more vigorous, as in PHUT ... *BANG* -- Regards, Joerg http://www.analogconsultants.com/ "gmail" domain blocked because of excessive spam. Use another domain or send PM.
From: Bill Sloman on 17 Jun 2010 18:25
On Jun 17, 8:24 pm, Joerg <inva...(a)invalid.invalid> wrote: > Bill Sloman wrote: > > [...] > > > Since the buffer presents a largely capacitative load to the line, you > > can compensate for it by narrwoing the trace - say from 50R to 75R - > > for a cm or two around the buffer connection. > > Well, if the capacitance is a problem there's a solution as well but it > ain't for the faint of heart: > > http://www.infineon.com/dgdl/bfp620.pdf?folderId=db3a30431400ef680114.... > > Always got some here in a drawer. Just be careful, these can oscillate > without you even knowing it. 5GHz transistors were what I could buy when I last used the trick, and they oscillated if you didn't put the "base-stopper" resistor in sereies with the base, and close to it. Not having played with a 65GHz transistor, I don't know what it takes to tame one of them. -- Bill Sloman, Nijmegen |