Building logic analyzer probes

[Ivan]

I recently purchased a Gould 450B LA. It came with no pods. Each pod
terminal on the LA is a BB-25 connector. I want to use a ribbon cable
to connect DB and a pin header to connecto to my PCBs. My questions
are:
1. What are the issues with this attempt to use homemade pods?
2. Do passive probes have termination built in?
3. How much do shielding and probe length matter?
4. How would I test whether they work well enough?

Note that this is for hobby work...

Also, if someone has to probes for this LA they are willing to sell,
please email me.

 

[mike]

I recently purchased a Gould 450B LA. It came with no pods. Each pod
terminal on the LA is a BB-25 connector. I want to use a ribbon cable
to connect DB and a pin header to connecto to my PCBs. My questions
are:
1. What are the issues with this attempt to use homemade pods?
2. Do passive probes have termination built in?
3. How much do shielding and probe length matter?
4. How would I test whether they work well enough?

Note that this is for hobby work...

Also, if someone has to probes for this LA they are willing to sell,
please email me.

The answer depends on a lot of stuff.
Are you doing synchronous or asynchronous acquisition? How fast?
Voltage levels, logic families, threshold voltages?
How much circuit loading can you tolerate?
Do you care if the data is correct?

Probe design is a big mass of problems.
Signal fidelity/reflections/terminations/thresholds/sensitivity/noise
immunity/cable repeatability/
channel skew
Probe skew
clock skew
cable skew...cable length mismatches can be the biggest factor
Pattern sensitive delays/crosstalks
Ground loops between probes
Input protection

Setup time
DUT loading
Crosstalk between channels
Ground induced crosstalk at the source.
Coupling crosstalk back into the DUT.

One rule of thumb: Take the inductance of the channel lead and the
ground lead. Resonate that with the input capacitance. At that
frequency, your probe is a dead short on your DUT. It ceases being
a good probe well below that frequency. All that ground currrent is
induced into your DUT and all the other acquistion channels.
Now multiply all this by the
number of channels you want to work reliably...

Making a probe that loads the circuit, blows up when you short it to VCC
or static zap it and almost works sometimes
isn't too hard. Making multiple probes that always give you the
right answer is real hard. There's a reason why probes cost a LOT.

When you get fired, some administrator takes your logic analyzer and
sends it to equipment disposal. The probes go to the wire box and are
never seen again. Then the LA ends up on ebay with no probes and the
buyer posts here. I recommend against. Cycle repeats...

Figger out what you're gonna do about probes BEFORE you buy an ebay
LA. Finding probes without a matching LA is rare. Maybe buy a dead one
with probes.
mike
Former logic analyzer designer.

--
Bunch of stuff For Sale and Wanted at the link below.
laptops and parts Test Equipment
4in/400Wout ham linear amp.
Honda CB-125S
400cc Dirt Bike 2003 miles $450
Police Scanner, Color LCD overhead projector
Tek 2465 $800, ham radio, 30pS pulser
Tektronix Concept Books, spot welding head...
http://www.geocities.com/SiliconValley/Monitor/4710/

 

[Ivan]

I know that what I am trying to do might sound naive to someone who
spent considerable time with probe design intricacies. I don't want to
build professional quality probes. It was OK by me to invest $50 in
the LA and some time into trying to build probes, since there is a lot
one can learn trying to make them work.
If I can get it to work at 25Mhz (synchronous), for 3.3 and 5V CMOS,
I will also have an instrument I can actually use it for some
projects. Is this achievable? Which books and web sites should I
consult before attemting this?

 

[mike]

Mike,




I am wondering, is there any "correct" way to deal with this problem
(generally speaking, outside of the LA probe question)?

I was always under the impression most equipment invariably ties the
system GND to saftey GND. I am being plagued by this problem on some
projects I am working on where there is cross-talk.

Is the only solution to have equipment that talks to each other do it
over opto-isolated or magneticlly isolated links?

Thanks,
Jay.

If you're sending data, you have a few solutions.
Use enough signal strength that the common mode noise doesn't bother you.
Use differential signals with enough common mode compliance to ignore
the ground loops.
Or eliminate the path as you suggested with magnetic or optical coupling.

Now, what do you mean when you say crosstalk?
If you're generating the signals that cause the crosstalk then it's
easy...don't do that. If you use differential drivers and stuff
the extra output into ground, you can often mitigate the ground loop
problem by keeping the ground current independent of the logic state.
But that's really a different issue from big noise on the power line.

Safety ground doesn't have to connect to signal ground. You can float
the whole thing if you want. As I recall, all the safety guys care
about is if you put big amps into a metal case, it goes out the
safety ground. A relatively small resistance between safety ground and
signal ground can make a lot of difference in induced noise.

Grounds are always an issue. I don't know of any magic formulae.
mike

--
Bunch of stuff For Sale and Wanted at the link below.
laptops and parts Test Equipment
4in/400Wout ham linear amp.
Honda CB-125S
400cc Dirt Bike 2003 miles $550
Police Scanner, Color LCD overhead projector
Tek 2465 $800, ham radio, 30pS pulser
Tektronix Concept Books, spot welding head...
http://www.geocities.com/SiliconValley/Monitor/4710/