In message <[email protected]>, dated Thu, 4 Dec 2008, Grace Lin <[email protected]> writes:
>The impedance varies a lot over the frequency range from 9kHz >(1,750.095 ohm) to 30MHz (566.456 ohm). Is this variation acceptable? I think it's not unusual. The probe is basically a 1500 ohm resistor (but it appears to have a series capacitor, causing the impedance to rise at very low frequencies), and its impedance reduces at high frequencies because there is stray capacitance in parallel with the resistor. >I plan to use this probe to measure disturbance voltage at load >terminals per CISPR 15. The second paragraph under Introduction of the >3701 manual (http://www.ets-lindgren.com/manuals/3701.pdf) states "The >Model 3701 Line Probe presents a nominal resistance of 1500 ohms to all >voltages on the line. The Line Probe also attenuates noise voltage, so >a correction factor must be added to the EMI receiver readings to >obtain the proper emission levels on the line." To me, 566.456 ohm is >too far away from 1,500 ohm. It doesn't matter very much if the source impedance of the disturbance voltage at the point where you measure it is much lower than 566 ohms. For example, if the source impedance were 50 ohms resistive, the 566 ohms (if purely resistive, which it isn't) would cause the voltage at the measured point to be somewhat less than 1 dB low, while the 1500 ohms would cause it to be about 0.3 dB low (very rough figures) compared with the open-circuit voltage. So the variation of probe impedance gives around a 0.5 dB error **at the measuring point**. But there may be a compensating term hidden in the tabulated insertion loss figures. > >Question 2: >Is the insertion loss the correction factor mentioned in the paragraph >quoted above? For example, if the EMI receiver reading is 20 dBuV at >9kHz, should I add 36.078 dB to it as my test data/result? Yes: in principle the insertion loss is the loss due to the probe impedance in series with the 50 ohm input impedance of the receiver. However, if you calculate the insertion loss assuming all impedances are resistive, you get somewhat different values, so you should use the tabulated values. -- OOO - Own Opinions Only. Try www.jmwa.demon.co.uk and www.isce.org.uk Either we are causing global warming, in which case we may be able to stop it, or natural variation is causing it, and we probably can't stop it. You choose! John Woodgate, J M Woodgate and Associates, Rayleigh, Essex UK - This message is from the IEEE Product Safety Engineering Society emc-pstc discussion list. To post a message to the list, send your e-mail to <[email protected]> All emc-pstc postings are archived and searchable on the web at: http://www.ieeecommunities.org/emc-pstc Graphics (in well-used formats), large files, etc. can be posted to that URL. Website: http://www.ieee-pses.org/ Instructions: http://listserv.ieee.org/request/user-guide.html List rules: http://www.ieee-pses.org/listrules.html For help, send mail to the list administrators: Scott Douglas <[email protected]> Mike Cantwell <[email protected]> For policy questions, send mail to: Jim Bacher: <[email protected]> David Heald: <[email protected]>
