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Registered Member #2463
Joined: Wed Nov 11 2009, 03:49AM
Location:
Posts: 1546
Is your scope earth at the same point as the 324 earth? If not-you will get noise (hum). If the scope can operate in differential mode (A-B) using two probes, tips only, with no ground clip, this might help.
Sorry for double post before. I clearly do not know how to work it.
Registered Member #1772
Joined: Tue Oct 21 2008, 05:23AM
Location: Athens, OH
Posts: 71
The scope ground and the 324 ground are not connected to the same node on the bridge if that's what you mean. If you're talking about where the machines themselves are grounded (i.e. mains ground, copper pipes, wire going into the ground), the 324 is just grounded to the case and the scope has a 3 prong plug going into the wall. If I connect the scope directly to the 324, the signal is crystal clear. It's only distorted when I use the AC bridge.
I'm not sure I did the differential mode correctly. I connected one channel to node B and the other to node C and did not connect the scope ground. The signal shown was not recognizable.
Could my problem be that the resistance is too high for R2? My values are R2=1kOhm, C1=330pF, and R3=0-2kOhm.
Would shortening the wires help eliminate some of the interference? Would wire with better insulation help much? I've got about 60 feet of 15kV wire left over from my tesla coil that I could use.
Registered Member #2463
Joined: Wed Nov 11 2009, 03:49AM
Location:
Posts: 1546
If the scope is mains powered, you must use differential mode. lead length is not to critical at audio frequencies. You need a variable resistance in series with the bridge reference capacitor to balance out D D = R/Xc , R being resistive losses at the frequency of measurement in the capacitor under test. To avoild the noise, build a small battery powered 400 Hz sine wave oscilator and use that as your exiter. An opamp configured as a wien oscillator giving 5 volts will do. With the D control, you can measure D in your dielectric.
Registered Member #1772
Joined: Tue Oct 21 2008, 05:23AM
Location: Athens, OH
Posts: 71
It seems that my problem is just poor shielding. I changed the probe on the oscilloscope to one with better shielding and that's made a huge difference! I also unplugged some of the electronic equipment nearby which made a slight but noticeable improvement. I can measure to within about 3pF! If I run a test several times and average the results I feel fairly confident that my results are within about 1pF. I'll replace some of the wire with my old tesla coil wire and see if I can make it a little better before I start taking data. Thanks a lot for your help everyone!
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Dielectric Constant Meter
