On Sun, 12 Sep 2010 15:08:34 -0400, Joe Subich, W4TV wrote: >Bonding unit to unit ENCOURAGES common mode currents to flow through >all of the units in series. Bonding to a single LOW IMPEDANCE ground >moves the common mode and RF currents to the ground and *NOT* the >other equipment.
This is a common misconception that results from muddy thinking. It is the big lie, repeated over and over again until somehow everyone believes it. DC and low frequency currents follow Ohm's Law -- that is, the path with the lowest DC resistance. First, the issue here is AUDIO AND DC, NOT RF, and it is the injection of AUDIO noise into unbalanced connections. Let's say that you bond all equipment to your MAGIC SINGLE POINT, and it takes 3 ft of wire to get there. If two pieces of equipment are only three inches apart and you bond directly between them, CHASSIS to CHASSIS, the wire might be only 6 inches long. Assuming the same size wire, the path through that MAGIC SINGLE POINT has 12 times the resistance as compared to the direct path. That's 22dB MORE NOISE (20 log of the voltage, and the voltage is proportional to the resistance) that any circulating current would add to an unbalanced connection between that equipment. Second, the bonding together of equipment at RF is for LIGHTNING SAFETY protection. It is NOT part of a solution to RFI or RF noise. What matters is that everything be maintained as close to the same potential as possible, and that is achieved by LOW INDUCTANCE bonds, and by bonding EVERYTHING together. Providing a low resistance bond between interconnected equipment also puts a band-aid on low frequency pin 1 problems, because most current that flows between equipment takes the lower resistance chassis-to-chassis bonding path rather than the higher resistance path via cable shields and onto the signal return bus. For those who haven't heard, a "pin 1 problem" is the connection of a cable shield to internal wiring of equipment rather than to the chassis. The only proper connection of a cable shield is the CHASSSIS. When noise current flows on a signal return trace, it gets added to the signal chain at one or more points, where it appears as noise (hum, buzz, or that modulation of the V- rail), and if it's RF, it will be detected by the first semiconductor junction it encounters. Pin 1 problems are the primary causes of hum, buzz, and RFI in virtually ALL systems. When you see a connector whose shell is insulated from the chassis, or where a mic shield goes to "audio ground," it's a "pin 1 problem." It's CALLED a pin 1 problem because this issue was first clearly described and understood in the pro audio world, where the shield contact of an XLR connector is pin 1. 73, Jim Brown K9YC ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:[email protected] This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html
