A question regarding probe point on high speed digital circuits
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From: Nico Coesel on 17 Jun 2010 18:29 John Larkin <jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote: >On Wed, 16 Jun 2010 13:24:58 -0700 (PDT), Javad Benhangi ><benhangi0(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. AFAIK Tektronix has LVDS probes for their logic analyzers. If you bring enough money there are also connectorless press-on probes. Samtec also offers connectors that only need gold plated pads on the PCB. -- Failure does not prove something is impossible, failure simply indicates you are not using the right tools... nico(a)nctdevpuntnl (punt=.) --------------------------------------------------------------
From: krw on 17 Jun 2010 19:13 On Thu, 17 Jun 2010 13:35:50 -0700, Joerg <invalid(a)invalid.invalid> wrote: >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. About a year ago I ran into a case where one audio channel was bleeding into another (about 3dB down). It didn't take long to figure out that one of the op-amps feeding the other channel of the codec was oscillating, swamping its input amplifier (which was supposed to be muted). >Since you mentioned in another thread that you work on sat receivers I >am a bit surprised now ;-) Why? He isn't an engineer. > >> 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 :-) Of course they're not visible, but they do cast a shadow. ;-) >Other times the announcement of oscillatory behavior can be more >vigorous, as in PHUT ... *BANG* Or burn a finger. BTDT (uA709 in college).
From: krw on 17 Jun 2010 19:18 On Thu, 17 Jun 2010 10:50:43 -0700 (PDT), "langwadt(a)fonz.dk" <langwadt(a)fonz.dk> wrote: >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 I try to design in a couple size larger FPGA than I need so I can build a logic analyzer into the FPGA. In production it can be stripped out and a smaller FPGA, sometimes even a smaller package can be used. I keep a few engineering models around with the larger chips for future debug. Routing signals out to unused pins works, too. That's really useful before the FPGA is functional. >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. Or do a lot more work. I also use a paperclip wound around a scope probe to grab a close ground. Decoupling caps, on the back of the board, are a good place to probe ground, and vias work for probe tips, sometimes. >but then one or two of the signals should be enough assuming they are >all routed, terminated etc. in the same way. ....unless something isn't working.
From: Michael A. Terrell on 17 Jun 2010 21:20 "krw(a)att.bizzzzzzzzzzzz" wrote: > > On Thu, 17 Jun 2010 13:35:50 -0700, Joerg <invalid(a)invalid.invalid> wrote: > > >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. > > About a year ago I ran into a case where one audio channel was bleeding into > another (about 3dB down). It didn't take long to figure out that one of the > op-amps feeding the other channel of the codec was oscillating, swamping its > input amplifier (which was supposed to be muted). > > >Since you mentioned in another thread that you work on sat receivers I > >am a bit surprised now ;-) > > Why? He isn't an engineer. > > > >> 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 :-) > > Of course they're not visible, but they do cast a shadow. ;-) > > >Other times the announcement of oscillatory behavior can be more > >vigorous, as in PHUT ... *BANG* > > Or burn a finger. BTDT (uA709 in college). Just be glad it wasn't a Philbrick Op Amp. :) -- Anyone wanting to run for any political office in the US should have to have a DD214, and a honorable discharge.
From: krw on 17 Jun 2010 23:44
On Thu, 17 Jun 2010 21:20:00 -0400, "Michael A. Terrell" <mike.terrell(a)earthlink.net> wrote: > >"krw(a)att.bizzzzzzzzzzzz" wrote: >> >> On Thu, 17 Jun 2010 13:35:50 -0700, Joerg <invalid(a)invalid.invalid> wrote: >> >> >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. >> >> About a year ago I ran into a case where one audio channel was bleeding into >> another (about 3dB down). It didn't take long to figure out that one of the >> op-amps feeding the other channel of the codec was oscillating, swamping its >> input amplifier (which was supposed to be muted). >> >> >Since you mentioned in another thread that you work on sat receivers I >> >am a bit surprised now ;-) >> >> Why? He isn't an engineer. >> > >> >> 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 :-) >> >> Of course they're not visible, but they do cast a shadow. ;-) >> >> >Other times the announcement of oscillatory behavior can be more >> >vigorous, as in PHUT ... *BANG* >> >> Or burn a finger. BTDT (uA709 in college). > > > Just be glad it wasn't a Philbrick Op Amp. :) Well, we had a couple of analog computers that I took care of in college, too. The opamps in those ran pretty hot too, but they had those glass things with the glowy thingys in them to warn you that they were on. ;-) The servo multipliers and sine converters were neat. |