Oscilloscope
By Robert Reed
Probes and Probing
So, you made a great deal on eBay for a
quality wideband oscilloscope along with
some decent after-market probes, and
UPS has just dropped it off at your
doorstep. You anxiously tear open the
package, power the scope up, and spend
the next two hours at your test bench
familiarizing and checking it out with your
function generator. Everything works and
appears to be correct — great! Now it’s
time to examine the operation on a host
of prototypes you have completed recently. Time to see the real deal on how
these circuits are actually performing — things that your old worn-out Heathkit
scope’s results could never have been trusted for.
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Eeks : Problems!
The first circuit you look at with one of your 10X
probes is a wide band amplifier. The results are disastrous.
The frequency response is terrible; rising and falling in
amplitude across the spectrum and with a much lower
bandwidth than expected. In a cascaded section, one amp’s
output is too high and the following amp’s output is actually
lower than the one feeding it. Yikes! You switch to another
probe and get the same results! You also notice that at the
higher frequencies, moving the probe or cable changes the
displayed amplitude. Next, you frantically look at another
prototype containing an oscillator and a couple of buffers to
an output jack. Connecting a probe from channel 1 to the
output jack verifies that a signal is present there. You
connect a probe to the scope’s channel 2 and probe a
section in the oscillator and see no signal at all. How can
that be as you previously saw an output at the output jack?
Looking back at channel 1, the former signal is also
missing. Moving the channel 2 probe to a different section
of the oscillator, you now see a signal on both channels and
at this point are somewhat confused. One last prototype
that is supposed to produce fast rise time pulses is probed
and the rise time is much slower than expected with a lot of
overshoot and ringing at its leading and trailing edges. Oh
no, you are thinking. My prototype designs are terrible and
the new scope is defective! Well, don’t throw your protos in
the trash bin and send the scope back to the seller just yet.
44 November 2010
If your scope passed your initial tests, it’s probably working
okay. If you have done similar protos in the past, then for
the most part, they are probably doing at least a fair job.
There is a very good chance that the problems lie in your
probes and/or your probing!
Examining the Source
of the Problems
You are probably thinking that since the scope has a
300 MHz bandwidth and the probes are rated at a 250
MHz bandwidth with a 1.4 Nsec rise time, they should give
good performance up to 200 MHz and beyond — which
they will under the right conditions. The key word here is
right. Let’s cover a few things before we dive into probes.
An oscilloscope’s rated bandwidth is the point where the
displayed signal amplitude vs. frequency drops to - 3 db
(70%) of its DC or very low frequency value when a leveled
sine wave source is feeding the vertical pre-amp(s). In
scopes that follow a Gaussian response curve (and almost
all do), this will follow a very flat frequency response to 1/3
of its rated bandwidth and then drop at an increasing rate to
its - 3 db point (i.e., 300 MHz bandwidth; DC to 100 MHz
flat response). However, this doesn’t mean that there won’t
be peaks and dips along the way. In quality scopes, these
are minimized. A probe with the same bandwidth spec as
