Probes (was Re: GSM-noise "buzz" issue)

[Werner Almesberger]

Andy Green wrote:
> I looked at this last night, but my setup is too crap to get a sensible
> result.  All I have around is some old scope probes terminating in BNC,
> so I cut one off and used that cable from the tracking generator (it's

Aiii ... if that's from a capacitative probe, that's going to be evil.

Here is a very nice article about the various probe types and their
characteristics:

http://www.sigcon.com/Pubs/straight/probes.htm

A bit off-topic: DIY probes are a very good thing to have. In my case,
it's not about ringing or bandwidth limits, but I need them to win the
daily battle against EMI from the microwave antenna clusters, cellular
and other stuff, surrounding my lab at eye level. Here's an example of
a basic resistive probe ending in a 2-pin socket for a 0.1" header:

http://people.openmoko.org/werner/rigol/r-probe.jpg

And here you can see one attached to a header soldered to a GTA02:

http://people.openmoko.org/werner/sdio-test-setup.jpg

For the scope end, if the scope doesn't have a built-in 50R termination,
you can either build it into the BNC connector or use a 50R feed-thru
terminator, e.g.,

Digi-Key 501-1036-ND
http://www.tequipment.net/CalTestCT2944.html
http://www.dilette.com/main/dilettesales/DL-22_feed-thru_terminator.html

For the cable, another option besides DIY would be to buy a pre-made
BNC-RG174-BNC cable and to cut it in half. That's actually cheaper
than buying the individual components (well, if S&H don't drive the
price up):

http://www.l-com.com/item.aspx?id=206

In any case, I found that a spool of RG-174 cable and a few BNC
connectors are very good things to have around.

- Werner