[SI-LIST] Re: question about voltage dividers

Yes a very usable 10:1 low impedance probe can be made by soldering an 1/8
watt, 453 ohm resistor to a length of 50 ohm coax. Connect this to the
'scope and set it to 50 ohms.
The Tektronix version (P6159 I think) has a BW of 9 GHz. The homebrew
version can get to a few GHz with a very short GND wire (I mean 1/2 cm).

One might think that 500 ohms is too much circuit loading. A 1 pF active
probe has a lower input impedance above 300 MHz. While the 500 ohm prove
will tend to make the signal risetime look a bit better the active probe
will tend to make it look slower.

For checking the fidelity of active probes I parallel them with the
Tektronix 500 ohm probe. The idea here is that the 500 ohm probe is telling
you what the node really looks like while the active probe is adding
aberations.

Agilent has a 1000 ohm low-Z probe. It is not as nice as the Tek but does
come with replaceable resistors. The Tek probe will break with a sideways
force of only 3 lb. Everyone who uses one for the first time breaks a probe.
Is this why the probe kit includes two of them? It also comes with a 50 ohm
probe that makes a fair TDR launch probe.

Picoprobe, if I remember correctly, has some high frequency passive probes
to 5k ohm input. These are wafer probes. These have very little loading of
course. And they have active probes to 20 fF (yes fF). The FET probes are 40
fF. Everyone who uses one of these for the first time zapps or breaks one.
Oh well. They're only $100 for the probe amp thing.

    Dave Cuthbert

On Wed, Mar 4, 2009 at 8:06 AM, Andrew Ingraham <a.ingraham@xxxxxxxx> wrote:

> > Here is an example of a 10:1 voltage divider with an input resistance of
> > 10M
> > ohms. 9M ohm resistor on top and 1M ohm resistor on the bottom.
>
> This is just like those passive oscilloscope probes we used to use, back
> when our 'scopes were full of tubes and 100 MHz was a pretty good scope
> bandwidth.  Those scope probes were OK for audio and low RF frequencies,
> but
> not much beyond.  (And you should never trust the advertised bandwidth of
> many of those 10 Megohm 10:1 probes!  Especially with the long ground
> cliplead they come with.)
>
> These days most high speed scopes use a 50 ohm input rather than 1 Megohm
> at
> the front panel, so those older probes wouldn't work anyway even if you
> tried to use one.  But you can make your own passive divider probe that has
> better bandwidth than those older probes, using much smaller resistances
> than 9M/1M, and some work quite well for today's bandwidths and risetimes,
> even without compensating capacitors because of the smaller resistances.
> (I think Doug Smith has some tips for rolling-your-own compensated one, at
> his website www.emcesd.com .)  Of course these lower resistance passive
> divider probes also load down the circuit you are probing, a lot more than
> the 10 Meg probes did.
>
> Andy
>
>
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