Didier NIST have measured reverse isolation as high as -144dB @ 10MHz for one of their isolation amplifier designs but they gave no measurement system details.
Bruce Didier Juges wrote: > As long as one is not trying to measure extremely low signals (or extremely > high isolation), the conventional method works, and I have used it many > times, I am not sure that the method would scale when dealing with very high > levels of isolation. > > I certainly would not trust a conventional VNA for isolation of 100dB or > more, simply because such isolation is difficult to achieve in an instrument > that is supposed to be able to switch its source and receivers between the > two ports. > > The HP 8722D which we use at work is only specified with 100dB dynamic > range, and I am not sure how that applies to reverse isolation. > > Using a slightly offset test frequency while driving the device at its > normal operating frequency would only work when using a spectrum analyzer as > receiver. Most VNAs don't have the capability of rejecting large signals > close in, and separating the frequencies too much would make the test > invalid with a narrow band amplifier. I have used that method to measure the > hot output VSWR of a TWT amplifier for instance (that was interesting). Of > course, the spectrum analyzer does not give you the phase, but it's better > than nothing. > > Didier KO4BB > > >> -----Original Message----- >> From: [email protected] >> [mailto:[email protected]] On Behalf Of John Day >> Sent: Sunday, March 08, 2009 5:47 PM >> To: Discussion of precise time and frequency measurement >> Subject: Re: [time-nuts] Reverse isolation >> >> At 06:14 PM 3/8/2009, Didier wrote: >> >>> This question is directed at Bruce, but if anyone else has a >>> contribution, feel free to speak. >>> >>> Bruce, >>> >>> What is the best way to measure the reverse isolation of an >>> >> amplifier >> >>> (particularly a buffer amplifier for a 10 MHz reference), when it is >>> expected to be in the order of 100 dB or more? >>> >>> Feeding the output with a known signal and measuring at the >>> >> input with >> >>> a spectrum analyzer comes to mind, but I am sure there must be >>> something wrong with that technique, it sounds too simple. >>> >> No, not really. How else would you measure it? Reverse >> isolation is basically the reverse gain - S12 - of the >> amplifier. How does a VNA measure S12? Essentially inject a >> signal at port 2 and see how much comes out of port 1. >> >> If you want to get so picky as to determine S12 with a signal >> in the forward direction then you have a problem. Because the >> forward gain - >> S21 - is going to effectively swamp the signal going the other way. >> So this is almost impossible to measure if the signals are at >> the same frequency. In this case measure the S-parameters of >> the amplifier in its 'normal ' configuration, then de-embed >> the S-parameters of the device. Assuming the device is the >> only non-linear element you are dealing with then from the >> S12 & S22 values you can also figure out how the device >> reacts in the reverse direction. >> >> Then if you don't mind solving a large matrix you can figure >> out how the device might react to passing a signal in both >> directions. The reality is however that if the device is >> within its linear range, which it is likely to be if you want >> to distribute a reference or some such, the reverse behaviour >> of the well terminated amplifier will approach the nominal >> S12 value. The difficult part is to determine what happens >> when the amplifier is not nicely terminated! >> >> John >> >> >> >>> The presence of a signal at the input (or not) may affect >>> >> the operating >> >>> point of the amplifier, so measuring from output to input >>> >> without such >> >>> signal may not give a true result. >>> >>> Didier >>> >>> >>> >>> >>> _______________________________________________ >>> time-nuts mailing list -- [email protected] To unsubscribe, go to >>> https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts >>> and follow the instructions there. >>> >>> >>> -- >>> No virus found in this incoming message. >>> Checked by AVG. >>> Version: 7.5.557 / Virus Database: 270.11.9/1989 - Release Date: >>> 3/7/2009 6:43 PM >>> >> _______________________________________________ >> time-nuts mailing list -- [email protected] To unsubscribe, >> go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts >> and follow the instructions there. >> >> >> -- >> No virus found in this incoming message. >> Checked by AVG. >> Version: 7.5.557 / Virus Database: 270.11.9/1989 - Release >> Date: 3/7/2009 6:43 PM >> >> > > > _______________________________________________ > time-nuts mailing list -- [email protected] > To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > and follow the instructions there. > > _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
