Hello all,

I am continuing work on my PIC-based digital storage osc. and I am actually
in the building phase right now, although the front end is not completely
finished. This brings me to my question, what should I make the input
impedance? I can remember when I asked a few other questions about this
device on the list before, at least one person said something like "if you
do [blah blah blah, etc.] it will then have a nice 50 ohm input impedance"
While 50 ohms is good for many devices where I want the load matched to the
generator, lets say I want my scope to see the output of an oscillator with
Zout = 1K, I will see little or nothing if my Zin(scope) = 50 ohms!
Besides, what happens when my 50 ohm scope gets attached to a signal which
is riding on a 12 V DC level? My scope will have to dissipate 2.88 Watts!
It is true that the cable going to the probe will probably have a
characteristic impedance of around 50 ohms, but if I compensate the
probe(or the internal input nework), I should be able to compensate for any
added reactance due to the cable, right?

I expect the scope's front end to have abt a 50 MHz BW. My input network
must connect the BNC input connector to a MAX4107 300MHz BW op amp which
has an input which looks like a 1Meg resistor in parallel with 2pF of cap.
This input network will have to provide variable attenuation.

I would appreciate any suggestions that anyone has regarding what input
impedance is best. I would think the higher, the better.

Thanks,

Sean

+--------------------------------+
| Sean Breheny                   |
| Amateur Radio Callsign: KA3YXM |
| Electrical Engineering Student |
+--------------------------------+
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