Re: Which probe is better suited for 2465B?


Jim Ford
 

Totally makes sense.  I was refuting Jean-Paul's assumption that the 20 pF was across the 50 ohm input.  We agree.  Thanks, Tom.              JimSent from my Verizon, Samsung Galaxy smartphone

-------- Original message --------From: Tom Lee <tomlee@ee.stanford.edu> Date: 2/23/21 11:09 AM (GMT-08:00) To: TekScopes@groups.io Subject: Re: [TekScopes] Which probe is better suited for 2465B? 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.CheersTom-- Prof. Thomas H. LeeAllen Ctr., Rm. 205350 Jane Stanford WayStanford UniversityStanford, CA 94305-4070http://www-smirc.stanford.eduOn 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@ix.netcom.com>> To: TekScopes@groups.io> Sent: 2/23/2021 10:41:46 AM> Subject: Re: [TekScopes] Which probe is better suited for 2465B?>>> Rebonjour,  the 485 spec is 50 Ω /1 MΩ//20 pF>>>> https://w140.com/tekwiki/wiki/485>>>> 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>>>>>>>>>>>>>> >>

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