Hi Jim,
Lower frequency scopes merely throw a 50 ohm terminator across the input to provide a 50 ohm mode, but the 485 has two separate attenuator sections, one for 1M, and a different one for 50 ohms, with a relay switching between them, as necessary. Because the entire 50 ohm signal path is different, the capacitance can be (and is) designed to be inherently lower, as it has to be to make the bandwidth spec.
Cheers
Tom
--
Prof. Thomas H. Lee
Allen Ctr., Rm. 205
350 Jane Stanford Way
Stanford University
Stanford, CA 94305-4070
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On 2/23/2021 11:00, Jim Ford wrote:
Well, 20 pF is about 22.7 ohms at 350 MHz, so that would affect the 50 ohm input.? So I assume Tek left the 20 pF shunt cap off the 50 ohm input.
Jim Ford
------ Original Message ------
From: "Jean-Paul" <jonpaul@...>
To: [email protected]
Sent: 2/23/2021 10:41:46 AM
Subject: Re: [TekScopes] Which probe is better suited for 2465B?
Rebonjour,? the 485 spec is 50 ¦¸ /1 M¦¸//20 pF
I assume the 20 pF applies to either Zo.
Best measurement of Cin is the classic TEK 130 LC meter, very easy to use and accurate, with a 1, 3 pF scale.
We also use the HP 4232A LCR meter.
Very interested to hear the 485 Cin actual data. I bet its >0 (:-:)
Bon Journee
Jon
