In message <c55dd707.ea0d%[email protected]>, dated Thu, 4 Dec 2008, Ken Javor <[email protected]> writes:
>The roughly 29.5 dB factor is the voltage divider obtained by driving a >50 Ohm load from a 1500 Ohm source. If the impedance at 30 MHz were >really 560 Ohms, the correction factor would be 21.5 dB, not the 29.3 >dB shown. So it looks to me as if there is a typo. It would also help >to have a schematic of the device. I am away from home right now, and >don?t have access to it, but it is described in one of the old FCC or >ANSI standards. I recall there being the high value resistance, but >also an inductance paralleling the 50 Ohm output to further short out >the mains frequency signal. I don?t recall there being a blocking cap, >but it seems like there would be one of those as well. > >But all that aside, if there is a 1500 Ohm resistance in there, it is >hard to figure out how you could get a total impedance line-to-ground >of less than 1500 Ohms... Figure 6 of CISPR 16-1-2 shows a series capacitor, a 1500 ohm resistor and an inductor in parallel with the receiver input. There is some surreal math about the capacitor and inductor values that must be regarded as gross over-simplification. The inevitable stray capacitance across the resistor, which must have a significant effect at 30 MHz, is not even hinted at. There are two problems with this over-simplified circuit; - the series capacitor has to have a reactance negligible compared with 1500 ohms at 150 kHz, or at an even lower frequency according to the requirements of some standards, including CISPR 15. So it has capacitance to 'earth' and is liable to resonate with its own stray inductance and external connections below 30 MHz, becoming an inductive reactance above resonance; - the inductor has to have an inductive reactance much greater than 50 ohms at 150 kHz (or a lower frequency, as above). So it is very liable to resonate with its stray capacitance below 30 MHz, introducing error. No doubt commercially-available probes use clever techniques to avoid these problems, or minimise their effects. So it's wise to trust the published information until and unless that clearly produces an incorrect result. You can, of course, check the insertion loss with a signal generator and the receiver. Don't forget to terminate the generator output in 50 ohms, because the probe is designed NOT to provide a terminating impedance. -- 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]>
