On 02 Mar 2014 16:13:41 -0800, you wrote:
what I'm mostly looking for is very high input impedance (so that I can probe things 'as if I wasn't there') and also good high freq response so that the wave that is shown is closer to what is really at the tip of the probe.
I don't want to spend $200+ on each probe. my scope only goes to 500mhz but someday, who knows, maybe I'll end up with a 7104 or something (I was very close to buying one just a few weeks ago). however, it needs a 50ohm probe and special plugin (vertical) to reach 1ghz. if I ever decide to buy a scope like that, I'd like to have a probe that works with it as well as my 2400 series stuff.
Many active probes have surprisingly low input resistance. At high frequencies,
the shunt capacitance of a high impedance passive probe is more significant than
the its input resistance so the later tends to become unimportant. If you can
get by with more attenuation, maybe a x100 passive probe would work.
How much bandwidth do you want? Up to 100 MHz you can certainly fabricate your
own active probe. I am including an attachment showing an active probe design
from Troubleshooting Analog Circuits by Bob Pease. Page 3 of Linear Technology
application note 21 shows a similar active probe design. I would probably
combine the two:
The same circuit is on page 9 of Technology application note 9. I know they
had another similar circuit somewhere using a fast FET hybrid but offhand I am
not sure where:
If you do go with an active probe, they are designed to drive 50 ohm inputs so a
7104 with 7A29 will be quite at home with them assuming you can power the
probes. Most oscilloscopes faster than 100 MHz have switchable 50 ohm
terminations but for those that do not, a 50 ohm feedthrough terminatior can be
used.
David Hess