The characteristic impedance of the cal fixture is only important if the length is a significant fraction of a wavelength, which is rarely the case.
In most cases, it suffices to place a 10 or 20 dB pad on either end of the fixture, so that the long runs of coax between signal source and fixture, and from fixture to receiver, see 50 Ohm terminations near the fixture. Finally, since the receiver is acting as a 50 Ohm load on the load end of the fixture; the value read there is 34 dB above the current flowing in the jig. If you place 34 dB higher attenuation between the load end of the jig and the receiver, than between the probe output and receiver, then the difference between what you read off the jig vs. off the probe is precisely the probe transfer impedance in dB above one Ohm. The above presupposes matched lengths of the same type of coax coming off the jig and probe, both into a coax switch which is connected to the receiver. If you run your measurement this way, the accuracy of your calibration depends only on the accuracy of the extra 34 dB pad inserted on the jig load end, and the linearity of the log amp in your receiver. The receiver itself needs no calibration, as long as it is linear over a dynamic range numerically equal to the probe transfer impedance. The only other important number is the frequency at which the measurement is made; but synthesized rf sources are practically dirt cheap these days so that isn't a problem. Ken Javor Phone: (256) 650-5261 > From: <[email protected]> > Date: Wed, 15 Apr 2009 16:08:07 -0600 > To: <[email protected]> > Conversation: Current Probe Calibration Test Fixture > Subject: Current Probe Calibration Test Fixture > > Hi All, > > Have any of you ever tried to build your own current probe calibration > text fixture? I understand the goal is to control the impedance of the > current path in the fixture to ~50 ohms, and still be able to clamp the > probe around the signal conductor and maintain the impedance. > > Thanks, > Cody > > - > ---------------------------------------------------------------- > 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]>
