[Amps] A Meeting Ground

[Bill Fuqua]

This has always been an issue. Particularly when high-Q circuits are involved.
One time while helping the accelerator tech try to find out why our 20 MHz, 
10KW bunching amplifier signal was getting into the sensitive 
instrumentation I noticed that in haste they only installed every other 
screw on a panel shielding a very high-Q tuned circuit from the outside 
world. This was a 3 turn ( really looked more like twist) water cooled coil 
with the ends of the tuned circuit going to defection plates used to 
deflect 5 MegaElectron Volt protons at a 20 MHz rate.

Well, I took an insulated screw driver and put it between the shield (where 
a screw was supposed to be) and the chassis. And there was an RF arc. The 
there were screws on either side only about 6 inches away and we were 
producing an arc half way between them on a 1/8 inch thick aluminum plate 
which they assumed was grounded.  Needless to say, they reinstalled the 
screws and the problem went away.

I have noticed some homebrew amplifiers I have purchased have the plate 
tuning capacitor grounded to the front panel. This is bad. Since all the 
fundamental and all the harmonic currents from the plate of the tube flow 
thru this ground path and can produce a good deal of radiated TVI off the 
front panel.

Also, it is bad when the plate capacitor return path to cathode on the 
chassis is common to the input circuit. Even though the circuits are on 
opposite side of the chassis there can be a good deal of coupling.

Just some thoughts.

73
Bill wa4lav


At 12:25 PM 11/4/2004 -0500, StephenTetorka at cs.com wrote:
>Hi All:
>
>This radiant discussion on parasitics invites me to ask a question: "where 
>does all the 'mish-mash' go to when a chassis ground is employed?"
>
>Signals and voltages that are DC, AC and RF are all permitted to happily 
>co-join and intermix - and to be politically correct, regardless of gender 
>(positive or negative cycle) -  on this conducting element.
>
>The fact that one chassis 'ground' can easily be a potential different 
>from another 'ground' is an issue for another occasion.
>
>Regards,
>Steve
>WA2TAK
>
>
>
>
>_______________________________________________
>Amps mailing list
>Amps at contesting.com
>http://lists.contesting.com/mailman/listinfo/amps