Amazing. Long term memory I still intact. Agree with all Mr. Woodgate's comments. Particularly about stray capacitance shorting out 1500 Ohms. From personal experience, it is difficult to construct 1500 Ohms at rf in a small envelope. Ken Javor
Phone: (256) 650-5261 > From: John Woodgate <[email protected]> > Date: Fri, 5 Dec 2008 10:10:59 +0000 > To: <[email protected]> > Subject: Re: 3701 Line Probe > > 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]> - 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]>
