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wrote:
Hi folks,

The other day, I was trying to troubleshoot a 100MHz amaplifier in a
piece of equipment ( actually, a Cushman CE-3 Spectrum Monitor ). I
was using a 150MHz tektronix scope with a 10X probe.
I was getting different results with each measurement, it was driving
me crazy.

My friend Bob, the RF guru, said "Your trouble is that you're
using the HI-Z input to the scope. For VHF, Hi-Z is crap. It
just doesn't work. Especially if you have a pigtail for the
ground." Guilty as charged...

He then asked if the scope had a 50-ohm input ( which it does ).
"Then just make up a probe with a 450-ohm series resistor, and
some little ground clip. That's all you need. It will be stable, and
it will work up to about a gig. The impedances in
this RF stuff tend to be low anyway, so you don't really need Hi-Z."

Makes sense. Any way to buy a probe like this off he shelf?

I'll bet Tektronix and HP/Agilent will sell you one, but you don't want
to know what it costs.
Problem with commercial probes is that they have a rod resistor with a
probe point soldered into the end. Lots of stress, easy to break,
expensive to fix.

The resistor on the end of a coax is a pretty good solution.
Pack it into a ball-point pen. When you break it, replace the resistor.
1/4W carbon composition works well. If you try to use a metal film, you
may have to worry about the inductance. Don't know any way to tell
without measuring it.

If you want accurate/repeatable measurements at 100MHz. you need to
build the measurement point into your circuit. ANYTHING with a ground
clip will require an inch or more of wire just to let the probe reach
the circuit. Ground inductance is your enemy.

Think of it as a series resonant circuit. The tip capacitance and the
ground + tip inductance resonate at some frequency and make a short
at your circuit. Obviously, the accuracy decreases rapidly as you
approach this frequency. Doesn't take much ground wire to mess things up.
mike

- Jerry Kaidor ( )

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