Beginners: Why you need a x10 probe

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David Beierl

Dec 23, 2010, 7:17:41 PM12/23/10
to, Stevan C., JYE Tek
The primary purpose of a good x10 probe is not to reduce the voltage
presented to the scope, though it does that. Indeed, that is often
an inconvenience, and it's one of the reasons why the increased
sensitivity of the 082 is such a good thing. The primary purpose is
to reduce the capacitance presented to the load, so as not to
overload high-impedance circuits at high frequencies.

Let's take the DSO 062 as an example. Its input impedance is
specified as 1 megohm, although I suspect it may be a bit
lower. Very good, you say, a megohm. However the input capacitance
is not specified, and together with its BNC jack, RCA adapter and
included RCA probe (which by itself has around 110 picofarads) it
presents something approaching 450 picofarads (aka 0.45 nanofarads,
and for us old folks, 450 micro micro farads) to the load. Doesn't
sound like much, but let's look what it does:

At 10 kHz the scope impedance is ~35 k-ohms!
At 100 kHz it's ~3,500 ohms!
At 1 MHz it's~ 350 ohms!
So obviously this capacitance is deadly if we're not working with
low-impedance circuits, and we have to isolate it from the
circuit. That's where the probe comes in.

The x10 probe that JYE Tech sells, when hooked to the scope and
properly adjusted, shows about 50 pf to the load.
At 10 kHz this gives an impedance of about 320 k-ohms, with ~32k at
100 kHz and 3k2 at 1 mHz. Lots better. Not bad for USD20.

If you're willing to spend USD100 on a Tektronix 200 mHz probe you
can have 17 pf, which will give you ~936, ~93.6 and ~9.36 k-ohms respectively.

If you need higher impedance than that you'll probably have to go to
an active probe of some sort, although if you've got USD1400 kicking
around in your pocket you can get a Tektronix probe with a
capacitance of 0.17 pf and frequency response around 4 gHz. It's a
very specialized device, though.

The formula describing this is thus: XC(capacitive reactance,
expressed in ohms) = 1/(2 x pi x f x C) where f is the frequency in
Hz and C is the capacitance in farads. If you don't want to work it
out yourself there are many calculators on the web. Here's one: Note that it takes C in microfarads.

Merry Christmas,

David Beierl -- Providence RI USA
Atlas 618 6"/3" lathe ca. 1941, shiny-new Taig mill.

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