I could be wrong -- need to go through the exercise and see if it makes sense.... Tell me if I'm missing something critical...
50ohm source, line, load and all connectors = no VSWR; adding any 50 ohm attenuator will not increase the VSWR. If any of the above is NOT 50 ohms, there will be reflections. So, since there seems to be some VSWR to start with, we need to assume one of the above is not 50 ohms. If the load is a high impedance, you are correct and adding an attenuator will make the VSWR look better from the source end (keeping in mind, the VSWR between the attenuator and load doesn't change). The attenuator in parallel with the high impedance load will bring the total impedance closer to 50 ohms -- not sure, but it may be that the larger the attenuator the better the match will become?? Have to think on that.... If the load is a low impedance, say 10 ohms, adding the attenuator will add impedance as far as the source is concerned, so again, the VSWR will appear to improve, and again, the VSWR between the attenuator and load remains high. Depending on the attenuator design, it also seems a bigger attenuator (more dB) will improve the VSWR more. So if I stick with that line of thinking -- adding an attenuator when the load is mis-matched will always reduce the VSWR at the source but never between the line and the load! (Obvious question: does the VSWR at the load matter?? Seems there would be some losses, and in some cases it could mean a lot, but that's for another day.) Mike Hopkins -----Original Message----- From: Jacob Schanker [mailto:schan...@frontiernet.net] Sent: Wednesday, February 20, 2002 4:01 PM To: Mike Hopkins; 'Colgan, Chris'; EMC Forum Subject: Re: EN61000-6-2 Mike: My experience tells me that an attenuator designed for the same impedance as the transmission line, will **always** improve the VSWR at the source, irrespective of how bad or good the load VSWR is. (It is most helpful to think in terms of reflection coefficients rather than VSWR directly, to appreciate this.) Your comment implies otherwise, and I wonder if you could expand on what you've said - perhaps an example of where it doesn't help (not a given)? Regards, Jacob Z. Schanker, P.E. 65 Crandon Way Rochester, NY 14618 Phone: 585 442 3909 Fax: 585 442 2182 j.schan...@ieee.org ----- Original Message ----- From: "Mike Hopkins" <mhopk...@thermokeytek.com> To: "'Colgan, Chris'" <chris.col...@tagmclaren.com>; "EMC Forum" <emc-p...@ieee.org> Sent: Wednesday, February 20, 2002 1:48 PM Subject: RE: EN61000-6-2 | | Seems an attenuator COULD improve matching and VSWR if it then became a | significant part of the load impedance; it isn't a given..... On the other | hand, adding the attenuator should NOT cause the VSWR to become very high | unless it is not a 50 ohm attenuator...... | | Mike Hopkins | | -----Original Message----- | From: Colgan, Chris [mailto:chris.col...@tagmclaren.com] | Sent: Wednesday, February 20, 2002 4:26 AM | To: EMC Forum | Subject: RE: EN61000-6-2 | | | | In my experience attenuators improve impedance matching and hence VSWR. | There must be something wrong with your set up. | | Regards | | Chris Colgan | Compliance Engineer | TAG McLaren Audio Ltd | The Summit, Latham Road | Huntingdon, Cambs, PE29 6ZU | *Tel: +44 (0)1480 415 627 | *Fax: +44 (0)1480 52159 | * Mailto:chris.col...@tagmclaren.com | * http://www.tagmclaren.com | | | > -----Original Message----- | > From: Sam Wismer [SMTP:swis...@bellsouth.net] | > Sent: 19 February 2002 16:52 | > To: EMC Forum | > Subject: EN61000-6-2 | > | > Hi Group, | > | > EN 61000-6-2 calls for severity level 3, or 10Vrms for conducted | > disturbances. This equates to 37dBm which is 7dB higher than the upper | > limit my receiver will handle (during calibration of the CDN). I've tried | > to use an attenuator and compensate for it in my readings, but this | > creates a high VSWR. Any ideas how to extend the dynamic range of my | > receiver without causing high VSWR? | > | > | > | > | > | > Kind Regards, | > | > | > | > | > | > Sam Wismer | > | > Engineering Manager | > | > ACS, Inc. | > | > | > | > Phone: (770) 831-8048 | > | > Fax: (770) 831-8598 | > | > | > | > Web: www.acstestlab.com | > | > | > | | | ************************************************************** | Please visit us at www.tagmclaren.com | ************************************************************** | | The contents of this E-mail are confidential and for the exclusive | use of the intended recipient. If you receive this E-mail in error, | please delete it from your system immediately and notify us either | by E-mail, telephone or fax. You should not copy, forward or | otherwise disclose the content of the E-mail. | | TAG McLaren Audio Ltd | The Summit, 11 Latham Road | Huntingdon, Cambs, PE29 6ZU | Telephone : 01480 415600 (+44 1480 415600) | Facsimile : 01480 52159 (+44 1480 52159) | | ************************************************************** | Please visit us at www.tagmclaren.com | ************************************************************** | | ------------------------------------------- | 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: | majord...@ieee.org | with the single line: | unsubscribe emc-pstc | | For help, send mail to the list administrators: | Ron Pickard: emc-p...@hypercom.com | Dave Heald: davehe...@mediaone.net | | For policy questions, send mail to: | Richard Nute: ri...@ieee.org | Jim Bacher: j.bac...@ieee.org | | All emc-pstc postings are archived and searchable on the web at: | http://ieeepstc.mindcruiser.com/ | Click on "browse" and then "emc-pstc mailing list" | | ------------------------------------------- | 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: | majord...@ieee.org | with the single line: | unsubscribe emc-pstc | | For help, send mail to the list administrators: | Ron Pickard: emc-p...@hypercom.com | Dave Heald: davehe...@mediaone.net | | For policy questions, send mail to: | Richard Nute: ri...@ieee.org | Jim Bacher: j.bac...@ieee.org | | All emc-pstc postings are archived and searchable on the web at: | http://ieeepstc.mindcruiser.com/ | Click on "browse" and then "emc-pstc mailing list" | ------------------------------------------- This message is from the IEEE EMC Society Product Safety Technical Committee emc-pstc discussion list. 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