Derek, I do this all the time for the MIL-STD-461E LISN, which should be the same as for CISPR, and the 5 uH LISN used in RTCA/DO-160. I use an obsolete spreadsheet designed for the Mac, or I'd just send you the file, but here is how the calculation goes:
First column is frequency in MHz. Second column is LISN inductor in uH. Third column is LISN input side filter capacitance in uF to ground (actually to resistor to ground in the 50 uH LISN). Fourth column is LISN input side resistor to ground off the capacitor in Ohms. Fifth column is not a spy but a calculation of radian frequency. Sixth column is a calculation of series impedance of filter cap and LISN inductor: = (C5 * C2) - (1 / (C5 * C3)). Seventh column is the summation of the line-to-ground resistor and the result of the sixth column, but because this involves adding a real and imaginary number, you use the square root of the sum of the squares: =SQRT(SQR(C6) + SQR(C4)). The eighth and final column is the LISN impedance itself, obtained by the parallel combination of 50 Ohms and the impedance calculated in column seven. Parallel impedance of two circuits is the product over the sum, but again the sum has to be taken as the square root of the sum of squares, since 50 Ohms is resistance and column 7 is reactance: (50 * C7) / SQRT(SQR(50) + SQR(C7)). That's it. If you use 50 uH, 8 uF, and 5 Ohms, you come up with precisely the LISN impedance curve shown in MIL-STD-461E. If you use 5 uH, 1 uF, and 0 Ohms, you come up with the 5 uH LISN impedance curve. Now Mr. Woodgate is correct; this model calculates the specification impedance, it does not account for all the stray or parasitic values. But the whole point of designing a LISN is to control those parasitics so you end up with what's in the spec, +/- 20%, typically. -- Ken Javor Phone: (256) 650-5261 From: "Derek Walton" <[email protected]> List-Post: [email protected] Date: Wed, 8 Aug 2007 02:13:06 -0500 To: "IEEE EMC Discussion Group" <[email protected]> Subject: LISN Curve Limit Lines Hi All, I'm curious if anyone has developed a mathmatical equation that describes the upper and lower limit lines of a LISN's Impedance curve. Would save trying to figure the data values from a graph. Thanks, Derek Walton L F Research - ---------------------------------------------------------------- This message is from the IEEE Product Safety Engineering Society emc-pstc discussion list. Website: http://www.ieee-pses.org/ To post a message to the list, send your e-mail to [email protected] 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] All emc-pstc postings are archived and searchable on the web at: http://www.ieeecommunities.org/emc-pstc ______________________________________________________________________ This email has been scanned by the MessageLabs Email Security System. For more information please visit http://www.messagelabs.com/email ______________________________________________________________________ - ---------------------------------------------------------------- This message is from the IEEE Product Safety Engineering Society emc-pstc discussion list. Website: http://www.ieee-pses.org/ To post a message to the list, send your e-mail to [email protected] 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] All emc-pstc postings are archived and searchable on the web at: http://www.ieeecommunities.org/emc-pstc
