Hi Victor,
Interesting reading about cmc models and ways of establishing by measurements and maths of obtaining values for the various elements of the models form the links you provided, thanks.
Dominguez-Palacios et al (DP) paper uses S21 Voltage measurements, whilst the Hal document works on S11 impedance measurements. Fig 2(a) of DP paper is equivalent to HAL Fig 3 (a) ¨C T0 connection, or as you describe in your notes CM0 as you find no appreciable difference to CM3.
For DM, the DP paper makes use of two baluns. The equivalent in the HAL document would be fig 3 (b) = T1 connection, an arrangement close to my original suggestion for measuring CMC DM, the differences being what started our excellent discussions and something yet to be ascertained is if there is any appreciable difference of techniques. For sure if one considers a CMC filter (extra capacitance on the far side), yes I fully agree that the techniques are not comparable.
Thus, I see that the ¡®open¡¯ measurement of the DP paper (fig 2 (c) is part of a series of tests of a particular method for the purposes of modelling, not an alternative to more conventional means of establishing performance of a cmc. Just as are the additional configurations of the HAL document Fig 3 (c), (d) & (e) with T4 being equivalent to DP paper ¡®open¡¯.
Interesting to note that in the HAL document, the conclusion is that results not only depend on components (tolerances?) but also on respective positioning (lay-out).This reminds me of a quotation I heard many years ago (can¡¯t remember where/who it came from) =¡±Theoretically, practice and theory are the same, in practice, they are not!¡±
I have also been refreshing and improving my knowledge on transmission line transformers (TLT¡¯s) and found for me a very useful reference = Ruthroff transmission line transformers, Guanella baluns, lowpass and bandpass filters, and balanced transmission lines using multilayer technology, Riaz R Sobrany, M.Eng.
Although aimed at microwaves and MICC type technology, chapter 3 covers basic theoretical operation of various Ruthroff TLT¡¯s and chapter 4 Guanella types. Of interest to me in particular is his notes on testing baluns(on page 136 of pdf).
¡°Connecting two identical baluns with unknown performance back to back is a very bad test procedure to use.
All that can be determined is the degree of mismatch at the unbalanced port and the insertion loss of the balun. This test procedure is incapable of revealing values for the mode conversion parameters which means that the CMRR cannot be calculated, and even worse, provides no conclusive evidence that the balun actually functions as a balun at all!
A procedure to test a balun using a conventional 2-port VNA which determines the CMMR but does not measure the insertion loss or the port reflection coefficients is detailed in [113]. This test procedure requires three precision resistors and the accuracy of the measurement is dependent upon the accuracy of these resistors.¡±
With ref 113 being ¡°O. M. Woodward, ¡°Balance Quality Measurements on Baluns,¡± IEEE Transactions on Microwave Theoiy and Techniques, vol. 31, no. 10, pp. 821-824, Oct. 1983.¡± I have not located this, so no further comment on the technique at present.
This to me raises questions concerning the validity of using baluns in the measuring chain; what exactly is being measured and is making back-back compensation measurements valid, especially if one is using home made baluns. The same might well apply to commercial ones.
I did a little research in to commercial baluns, ¡°mini-Circuits¡± produce a very wide range of rf transformers (500+), there may be something suitable there, I have yet to explore possibilities
Summarising so far on the way to measure a cmc depends rather on the objective, is it to find parameters for a model or to evaluate performance, different approaches are required. There is no universally applicable model for cmc¡¯s it depends if low frequency or rf application. Your own experiments and studies support this when you say ¡°The 'equivalent' circuit maps the best my remeasurements is the model of Guanella (to ideal transformers; a transmission line to represent the DM and a reactance+resistance to represent the CM)¡±.
Kind regards
Ed, G8FAX
