Timebase Ground-Loops - why place the isolation transformer at the input instead of the output?
A load impedance&mdash;the isolation transformer in this case&mdash;should always be connected at the end of a transmission line, not at the line's beginning. This is related to transmission line theory (signal reflections, etc.).

I noticed that the Mini-Circuits isolation transformer mentioned in the article has a characteristic impedance of 75&nbsp;&Omega;. Considering that Mini-Circuits also makes a 50&nbsp;&Omega; isolation transformer (note 1), I'm wondering if perhaps the DISTRIBUTION AMPLIFIER (DA) mentioned in the article has a 75&nbsp;&Omega; output impedance, and 75&nbsp;&Omega; coaxial cables were used to connect the DA's output signal to the isolation transformer.

If I recall correctly, the EXT REF IN inputs (which the author labels as EXT TB IN) on the 3325B and 3585B required a 10&nbsp;MHz signal (or a subharmonic down to 1&nbsp;MHz) terminated into 50&nbsp;&Omega;. (NB: I don't recall the input impedance of the EXT REF IN jacks, so I'm not sure if an external 50&nbsp;&Omega; feed-thru termination is required or not.) Assuming I'm correct about the DA being a 75&nbsp;&Omega; system, I would also insert a 75/50 matching pad (impedance converter) between each isolation transformer (IT) and the EXT REF IN input on the 3325B and 3585B.

[DA]--75-ohm coax--&gt;[(bal)IT(unbal)]--&gt;[75/50 pad]--&gt;[50-ohm feed-thru term?]--&gt;[(EXT REF IN) 3325B]



NOTE 1)&emsp;FTB-1-1*A15+ BNC(F,F) or FTP-1-1*C15+ BNC(M,F)