1. Nick Shipman - Monday, 4 April 2011
Progress Report
Nick Shipman - Monday, 4 April 2011

2. What I did this week Finished GUI
Wrote a program in Matlab to find the values of Capacitance and Distance offset that minimise the sum of the squares of the differences between measurement and simulation.
Made measurements on VNA using phase information.
Soldered together a inductor and capacitor in series along with connectors for the VNA.
Thought about the significance of the large distance offset and sent samples of to be polished to see if this can explain it.
Simulated on Maxwell a flat tip with a protrusion and a wonky tip.

3. Success at last

4. What could explain the large distance offset?
The observed large distance offset required for the measured data to agree with the simulation could be explained by:-
a protrusion on the tip (or several protrusions);
or the anode being slightly ‘wonky’, or not perpendicular to the cathode.
Both these could result in the average distance of the tip (which is what the capacitance is essentially measuring) being further from the cathode than the closest point (what we measure at the moment by going into electrical contact)
How large is large?
The measured distance offset of 13 um is large compared to the 20um gap size.
However it could be explained by the anode being misaligned by 0.3 degrees, or a protrusion of 13um. A 13um protrusion is also believable I can see the surface isn’t even with the naked eye.

5. Comparison of Capacitance measured with the VNA using the zero phase crossing and the minimum of S12
Still haven’t cracked this quite…

6. Comparison of flat tip simulation with no defects “wonky” and with a protrusion
‘wonky’ real life
‘wonky’ simulation
protrusion real life
protrusion simulation

7. Comparison of flat tip simulation with no defects “wonky” and with a protrusion
‘wonky’ real life
‘wonky’ simulation
protrusion real life
protrusion simulation

8. What I will do next week
Try a couple more measurements with polished tip and VNA
Still need to replicate bridge meter results with VNA
Design a circuit to measure the capacitance
Investigate possibility of measuring surface roughness with capacitance measurement