[SI-LIST] : Termination and Test Equipment leads

Peter Baxter (peter.baxter@tantau.com.au)
Wed, 11 Nov 1998 17:27:39 +1100

Hi,

The Si-List talks a lot about the termination of transmission lines but
I'd like to know about test leads.

On a Bus Analyser, Logic Analyser, Oscilliscope and other test
equipment, they have "high impedance" connections to the "Circuit Under
Test" with one metre of cable (test lead) running to the test
instrument. If this was a terminated transmission line (say 100 ohms),
the load apon the "circuit under test" would possibly upset the CUTs
operation, even if the CUT has the current to drive the transmission
line.

If the test lead is un-terminated and you are monitoring a 100MHz square
wave, surely you must get reflections up and down the cable. Also, the
capacitance of the test lead, being 40pF or so, must affect the edges of
the square waves, which on an Oscilliscope lead is trimmed but not on
Analysers.

Do most of these test leads (more specifically Bus Analysers rather than
Oscilliscopes) have powered buffers/line drivers up the CUT end of the
cable to drive a low impedance termination transmission line to the test
equipment or do they do something else? Oscilliscopes also have input
termination switches 50ohm/1M ohm so I'd think that they'd be
un-terminated as such.

Somebody suggested that they might use a lossy transmission line, so
that as the signals reflect, they get attenuated? Out of curiosity, I
measured an HP Analyser lead and found that its signal leads had 200
ohms DC resistance from end to end with the grounds having effectively
zero ohms. The input capacitance up the CUT end must still be important
though?

As the cable length is short (1 metre versus say 3 metres), does it not
matter?

Is my understanding of the concept completely wrong?

Regarding ESD protection, Bussman has a product called SURGE-X which are
suitable for protecting high speed signal lines as they have very low
capacitance (2pF or so). As they are fairly new, there is little written
on them except advertising material. Has anyone had any experience with
them or any comments regarding their application or miss-application?

Thanks

Peter Baxter

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