HP used to have some information on each probe type's actual impedance in the instructions packaged with them; it was educational, to say the least. A 10 meg probe turned into a rather greater load at 200 MHz. Then, too, the 'scope's -3 dB point was much modified by the probe one hung on it.
You can still find engineers troubleshooting their 1 ns rise time logic with 100 MHz probes hung on 4 Gsample digitizing scopes. Some other time we might talk about 15 cm (6 inch) probe grounds. Cortland ====================== Original Message Follows ==================== >> Date: 02-Apr-01 20:48:03 MsgID: 1078-26768 ToID: 72146,373 From: "Douglas C. Smith" >INTERNET:[email protected] Subj: Heisenberg's uncertainty principle and circuits Chrg: $0.00 Imp: Norm Sens: Std Receipt: No Parts: 1 List-Post: [email protected] Date: Mon, 02 Apr 2001 19:43:17 -0700 From: "Douglas C. Smith" <[email protected]> Organization: D. C. Smith Consultants Subject: Heisenberg's uncertainty principle and circuits Reply-To: "Douglas C. Smith" <[email protected]> Hi All, Often, when making measurements, the loading on the signal by the measurement is not taken into account. In addition to probe bandwidth, its input impedance is just as important. Many high performance probes on the market are useable to only a fraction of their stated bandwidth because of very low input impedances, and this includes many FET probes. That spec of less than a picofarad of input capacitance seems to imply low circuit loading, but this is not always the case. This month at http://www.dsmith.org the Technical Tidbit is on error caused by probe loading. It is a real eye opener. But this is just the "tip of the iceberg" on measurement limitations. Doug -- ------------------------------------------------------- ___ _ Doug Smith \ / ) P.O. Box 1457 ========= Los Gatos, CA 95031-1457 _ / \ / \ _ TEL/FAX: 408-356-4186/358-3799 / /\ \ ] / /\ \ Mobile: 408-858-4528 | q-----( ) | o | Email: [email protected] \ _ / ] \ _ / Website: http://www.dsmith.org ------------------------------------------------------- ------------------------------------------- This message is from the IEEE EMC Society Product Safety Technical Committee emc-pstc discussion list. Visit our web site at: http://www.ewh.ieee.org/soc/emcs/pstc/ To cancel your subscription, send mail to: [email protected] with the single line: unsubscribe emc-pstc For help, send mail to the list administrators: Michael Garretson: [email protected] Dave Heald [email protected] For policy questions, send mail to: Richard Nute: [email protected] Jim Bacher: [email protected] All emc-pstc postings are archived and searchable on the web at: http://www.rcic.com/ click on "Virtual Conference Hall," ====================== End of Original Message ===================== ------------------------------------------- This message is from the IEEE EMC Society Product Safety Technical Committee emc-pstc discussion list. Visit our web site at: http://www.ewh.ieee.org/soc/emcs/pstc/ To cancel your subscription, send mail to: [email protected] with the single line: unsubscribe emc-pstc For help, send mail to the list administrators: Michael Garretson: [email protected] Dave Heald [email protected] For policy questions, send mail to: Richard Nute: [email protected] Jim Bacher: [email protected] All emc-pstc postings are archived and searchable on the web at: http://www.rcic.com/ click on "Virtual Conference Hall,"
