I'm looking at the types of amp plug-ins on Tek history sites. Why aren't
there any over 150MHz with 1Mohm inputs? All but the slowest are just
50ohms. I know there are faster scopes with high impedance inputs.


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"Tom Del Rosso" wrote in
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I'm looking at the types of amp plug-ins on Tek history sites. Why
aren't there any over 150MHz with 1Mohm inputs? All but the slowest
are just 50ohms. I know there are faster scopes with high impedance
inputs.


Why,I don't know.


7A26 was 200Mhz,IIRC,1Meg input Z.
Maybe 250Mhz,in a 7904/7104.


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Jim Yanik
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Jim Yanik typed
in ,

Why,I don't know.


7A26 was 200Mhz,IIRC,1Meg input Z.
Maybe 250Mhz,in a 7904/7104.

Can you use a FET probe with a faster amp?


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"Tom Del Rosso" wrote in
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Jim Yanik typed
in ,

Why,I don't know.


7A26 was 200Mhz,IIRC,1Meg input Z.
Maybe 250Mhz,in a 7904/7104.

Can you use a FET probe with a faster amp?

I believe so.



What are you trying to measure?
TEK makes FET probes for use with 1megR
inputs(P6202,IIRC had a switchable 50 ohm termination on it's BNC.),but
above some freq,the environment is low-Z anyways.
All you end up with is picking up noise.

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Jim Yanik
jyanik
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Jim Yanik typed
in ,


What are you trying to measure?
TEK makes FET probes for use with 1megR
inputs(P6202,IIRC had a switchable 50 ohm termination on it's
BNC.),but above some freq,the environment is low-Z anyways.
All you end up with is picking up noise.

Thanks for your help Jim.

I was just hoping I could take advantage of the full 400 MHz of a 7854 with
something more than 50 ohms impedance. So the P6202 can do it with a 50 ohm
input, but does it require a special BNC for power or can it be hot-wired?


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50 Ohm input

If you work out the reactance of a typical 10x passive probe (for a 1M input
scope) with (say) 20pF in parallel with 10Meg, the actual loading is mostly
down to the capacitance at higher frequencies: At 100MHz the reactance would
be about 83 Ohms.

Things only get worse as the frequency increases.

The 10x and 100x "50 Ohm" probes (which function as if an almost pure
resistive load with about 1pF additional loading when connected to a 50 Ohm
input) actually impose a much lower loading on the circuit than a classic
10x (or even 100x) designed for a 1M input.

So that is why Tektronix chose to fit 50 Ohm inputs to the higher frequency
plugins.

PS If you're using a P6202A or a P6201 then they have a power connector
(Lemo) that plugs into an outlet on the back of the MF. The original P6202
uses an external mains PSU.

Dave

Thanks a lot Dave.


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