A question regarding probe point on high speed digital circuits

Hi all, I have a question regarding probe point on high speed digital circuits. I=92m working with a chip that has 16 bit LVDS parallel data bus (including the clock that is LVDS). I=92m looking for a way to observe the 34 data signal and find the exact cause of not working the circuit. I=92m designing the board and I=92m 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?

Reply to
Javad Benhangi
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When I wanted to probe a high speed signal, I put in a test point that was buffered by an emitter follower - using 5GHz broad-band transistors (BFR92, BFT92 or the like). You needed a base stopper - around 33R - in series with the base, as well as the resistor in series with the emitter, so it did take up board space, but you can probe such a buffered test point with a scope probe without messing up the transmission line carrying the signal itself.

-- Bill Sloman, Nijmegen

Reply to
Bill Sloman

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

Reply to
John Larkin

I can only see your posts when someone answers because you are posting via Google. Anyhow, you can use any connector you like and that is small enough, then come off with coax or at least twisted pair where one wire is always grounded. 3M used to make "coax ribbon". But most important is to provide 1k SMT resistors from the connector to your signal tap-off points. With 50ohm coaxes this results in a roughly 20:1 voltage divider because you must terminate the coax properly. Twisted pair would be around 100ohms.

Good idea. I also used that in reverse, for clock distribution. The RF transistor would make sure that the clock distribution line was loaded ever so slightly at each tap. The register or whatever needed to be clocked was connected to the emitter.

For Javad it probably won't work because that's three parts per line. Unless he's willing to solder 0402 and SC75 packages.

--
Regards, Joerg

http://www.analogconsultants.com/
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Reply to
Joerg

ent

I did mention that it was a bit space hungry. He may not need to monitor every track in his bus: monitoring one will tell him if the bus is working, and monitoring two would give him some idea of the spread on the propagation delays and edge speed.

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.

-- Bill Sloman, Nijmegen

Reply to
Bill Sloman

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

Reply to
langwadt
[...]

Well, if the capacitance is a problem there's a solution as well but it ain't for the faint of heart:

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Always got some here in a drawer. Just be careful, these can oscillate without you even knowing it.

--
Regards, Joerg

http://www.analogconsultants.com/
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Reply to
Joerg

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.

Reply to
Archimedes' Lever

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 ;-)

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/
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Reply to
Joerg

.
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

Reply to
Bill Sloman

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...
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Reply to
Nico Coesel

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).

Why? He isn't an engineer.

Of course they're not visible, but they do cast a shadow. ;-)

Or burn a finger. BTDT (uA709 in college).

Reply to
krw

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.

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.

...unless something isn't working.

Reply to
krw

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.
Reply to
Michael A. Terrell

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.

Reply to
krw

Archie? He claimed to have worked in a myriad of specialty areas of electronic engineering and a 30 year career.

It's complete BS of course, the wishful delusional pretense of a mental case.

Reply to
Greegor

The Philbrick KW/2 had a pair of 12AX7 glass things with glowy thingys. :)

--
Anyone wanting to run for any political office in the US should have to
have a DD214, and a honorable discharge.
Reply to
Michael A. Terrell

He's been a janitor for a large defense company, with access to all the labs. Notice that he never says what *he* did, just what the others have done.

Dilusions of adequacy.

Reply to
krw

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