[TCML] Experiment to measure effects of RF grounding

[Jim Lux]

Lau, Gary wrote:
> Well, having just tried it, maybe there is some element of rocket science involved in scoping mains hash.  I have a fancy Tektronix TDS-series digital scope, and a variety of Tek probes.  Even with the shortest probe, with no wires connected to the tip of the probe, and the probe ground clip tied just to the tip of the probe, just running my coil near the scope or probe provides a nice display of the bang events rather than a flat line.  Unless I can get a scope probe to accurately read zero with the input shorted, it would be futile to try to measure line hash.  Comments and suggestions welcome!
> 
> I still plan to document any correlation in streamer length to various grounding schemes, but I was hoping to do the hash measurements at the same time.


Are you looking for common mode or differential mode signals?

First off, with the ground clip tied to the probe tip, you've made a 
nice loop antenna, feeding a 1 Meg input, so you'll get plenty of 
signal. I'll bet you can also see local AM radio stations and your cell 
phone's transmitter.

You need to build a good low impedance measuring fixture that can 
directly feed a 50 ohm input, and that is appropriately magnetically 
shielded so that it doesn't pick up the signals propagating that way.

If you're looking for differential mode signals, you have to come up 
with some way to ignore the underlying line frequency signal at 120VAC, 
and unfortunately, it's not that clean.

For common mode, it's a bit easier.  Say you had a transformer with two 
120VAC windings.  Hook it up across the line with the windings in 
series, so they form a centertap, and measure the voltage from there to 
ground.  (you might have to deal with the 60V line frequency offset)

Another way to attack the measurement problem is to feed the system 
through a suitable isolation transformer.