TimNJ and the group,
In this circuit:
![](https://www.eevblog.com/forum/projects/diy-dm-cm-seperator-for-emc-lisn-mate/?action=dlattach;attach=1050704;image)
The function of T1 is the same as in the one I built. R
int is 50
![Ohms \$\Omega\$](https://www.eevblog.com/forum/Smileys/default/omega.gif.pagespeed.ce.G89vbH4KGC.gif)
so when the spectrum analyzer is connected the common mode impedance is 25
![Ohms \$\Omega\$](https://www.eevblog.com/forum/Smileys/default/omega.gif.pagespeed.ce.G89vbH4KGC.gif)
. This is R
int // Spectrum analyzer.
T3 in this design is a common mode choke. It references the differential signal between A-B to ground. So it is a balun, balanced to unbalanced. The differential impedance between A and B should be 100
![Ohms \$\Omega\$](https://www.eevblog.com/forum/Smileys/default/omega.gif.pagespeed.ce.G89vbH4KGC.gif)
. T3 should be built with a 100
![Ohms \$\Omega\$](https://www.eevblog.com/forum/Smileys/default/omega.gif.pagespeed.ce.G89vbH4KGC.gif)
transmission line.
T2, with a turns ratio of 1.41 : 1, has an impedance ratio of 2:1 matching the 100
![Ohms \$\Omega\$](https://www.eevblog.com/forum/Smileys/default/omega.gif.pagespeed.ce.G89vbH4KGC.gif)
to the 50
![Ohms \$\Omega\$](https://www.eevblog.com/forum/Smileys/default/omega.gif.pagespeed.ce.G89vbH4KGC.gif)
of the spectrum analyzer.
The transformer would have to be approximated to 3 : 2 or 7 : 5
In the one I built R2 and the 50
![Ohms \$\Omega\$](https://www.eevblog.com/forum/Smileys/default/omega.gif.pagespeed.ce.G89vbH4KGC.gif)
load, the spectrum analyzer are essentially in series, making a 100
![Ohms \$\Omega\$](https://www.eevblog.com/forum/Smileys/default/omega.gif.pagespeed.ce.G89vbH4KGC.gif)
differential load. There is a 6dB reduction in the signal, half the signal on R2 the other half on the spectrum analyzer. But I don't need the third transformer. There is also better symmetry.
Does that help?
Regards,
Jay_Diddy_B