[time-nuts] Re: 10 MHz Distribution Best Practices?

[ed breya]

As always on this subject type, I must comment on the utility of 
managing ground loops with common-mode chokes on cables between 
equipment. I think this is the simplest, most beneficial thing you can 
do for good quality signal distribution.

Good cables, shielding, and grounding are all fine and good, but do not 
address the ground loop currents that inevitably flow between any 
interconnected pieces of gear. Adding CM chokes raises the common-mode 
impedance of the cable(s), greatly reducing the loop currents. You can 
see these parts all the time, inside equipment, or added or built into 
cable assemblies and such - almost always to fix EMC issues. The 
simplest are the clip-on type ferrites that are easily added to almost 
any cable type.

These are mainly to suppress RF and HF emissions from SMPSs and digital 
circuits and such, but they can also be effective even for mains 
frequency (and harmonics) ground loop control, with the right cores and 
some simple tricks. The reason is that the voltage levels and impedances 
are quite low (millivolts and milliohms), so adding even a small 
inductive reactance can be quite effective, percentage-wise.

If you have mains frequency ground loop problems, you can try cores with 
as high an A sub L as possible, and even better, multiple turns to get 
the squaring benefit. There are ferrites available that should do well, 
depending on the situation. I have even used regular silicon steel wound 
or laminated line frequency transformer cores as CM chokes in certain 
situations where very large line rejection was needed.

For narrow-band distribution of 10 MHz for instance, another option is 
to use transformer coupling of the signal, so the low frequency ground 
current and interference is eliminated, and a high frequency CM choke 
can take care of the rest.

Ed