The nature of a CDN is that its purpose is to provide a 150 Ohm source impedance for the injected rf. The requirement is specified as an open circuit potential and a 150 Ohm source impedance. Since rf signal sources and amplifiers for our line of work have 50 Ohm source impedance, the CDN provides another 100 Ohms.
If you were to load a CDN properly for calibration, you would load it with 150 Ohms. That way the spectrum analyzer is getting the benefit of some voltage division. If the open circuit limit is 10 Volts, then the injected potential into a matched load would be half that, 5 Volts, and the portion of that 5 Volts that the analyzer would see would only be 1/3, or 1.67 Volts, which would not cause damage. ---------- >From: Mike Hopkins <mhopk...@thermokeytek.com> >To: "'Jacob Schanker'" <schan...@frontiernet.net>, Mike Hopkins <mhopk...@thermokeytek.com>, "'Colgan, Chris'" <chris.col...@tagmclaren.com>, EMC Forum <emc-p...@ieee.org> >Subject: RE: EN61000-6-2 >Date: Wed, Feb 20, 2002, 5:14 PM > > > 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. 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