Point one.
To measure the switcbox crosstalk at 450Ohm any 50 ohm instrument is good,
just a 9:1 step up transformer is what needed if already the other 9:1
transformer is part of the switchbox, as it was in my case.
I did measure the box with a spectrum analyzer and its tracking generator
feeding all the unused ports and using the tracking generator at maximum
level. Checking what was at the 50Ohm port (the transformer output applied
to the analyzer input, the signal of the tracking matched 450 Ohm and
applied to the unused ports) I didn't measure anything bigger than -45dB,
across the band 1.5-8 Mhz.
Aware that my result doesn't prevent much worse situations in cases of bad
wire layouts inside a switchbox box, this shows that it's not impossible to
obtain good isolation at 450 Ohm.
Point two.
If the common mode trouble is here intended as the advice to use separate
grounds for the beverage and for the line (transformer windings isolated)
then I agree, and sure I wouldn't ground the line close to the transformer
and the beverage gnd.
What I don't understand is instead which sort of problem can cause a single
ground for n beverages when unused ones (wires) are left floating. Unless
one wants to ground unused beverages, whose thing I don't think is necessary
at all, one ground for each side of the transformer is enough, as well as a
single transformer is all what is needed.
73,
Mauri I4JMY
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