Automatic commit. Mon Oct 15 12:00:09 WST 2012

[matches/honours.git] / thesis / appendixA.tex

diff --git a/thesis/appendixA.tex b/thesis/appendixA.tex
index 9735646..1a8d089 100644 (file)
--- a/thesis/appendixA.tex
+++ b/thesis/appendixA.tex
@@ -95,11 +95,13 @@ Ideally, the electron gun should produce a constant emission current for fixed e
 \subsection{Leak Current}
 ``Leak'' currents are currents flowing through the electrodes of the gun. These can be reduced by optimising the potential of the gun electrodes, but are almost impossible to eliminate entirely. The leak current is expected to be equal to a constant (and usually large) fraction of the emission current.
 
-A measurement point was included for the total leak current through most of the gun electrodes.
+A measurement point was included for the total leak current through most of the gun electrodes. 
 
-\subsection{Finite Resistance of Ammeters}
+\subsection{The Ammeters}
 
-An ideal ammeter
+An ideal ammeter has no input resistance. In reality, it is not the current that is measured, but the voltage accross a fixed input resistor. This voltage can either be amplified, or the resistance increased, for measuring a smaller current.
+
+The 602 and 610B electrometers both provide a large range of scales and amplifier settings for current measurement. Using a low scale setting increases the input impedance, which increases the potential drop accross the ammeter. However, using a large amplifier gain increases noise; hence there is a trade off. For the 602 and 610B electrometers, a significant drift (typical +5\% of scale in 10min) in the zero level was also observed at high amplifier gains, whilst low gains appeared more stable (+10\% noted after 2 days).
 
 \section{Data Aquisition and Automation}
 
@@ -108,6 +110,12 @@ An ideal ammeter
 \subsection{DAC Control of Initial Energy}
 
 
+\section{Vacuum Chamber and Sample Holder}
+
+\subsection{Vacuum Chamber}
+
+\subsection{Sample Holder}
+
 \section{Sources of Error}
 
 \subsection{Accuracy of ADCs}
@@ -120,7 +128,7 @@ Based on these figures, an estimate of $\pm 1$ count for the error of the ADC se
 
 \subsection{Noise due to Instruments}
 
-The Keithly 602 electrometer quotes a minimum noise value of $10mV$ and maximum of $, whilst the 610B electrometer quotes  
+The Keithly 602 electrometer quotes a minimum noise value of $10mV$ and maximum of , whilst the 610B electrometer quotes  
 The noise was found to increase with the amplifier gain setting on each electrometer. Figure \ref{fig_610B_noise} shows the 
 
 
@@ -143,7 +151,18 @@ Software averaging alone is not always sufficient to reduce noise. If the sinuso
 To solve this difficulty, a physical low pass filter was added at the input for the differential ADC. The cutoff of the low pass filter is:
 Although some sinusoidal noise was still observed after the addition of the filter, it was reduced to a level that made software averaging feasable.
 
-\subsection{
+\subsection{Time variance of measured current}
+
+There are two effects:
+\begin{enumerate}
+       \item When the initial energy is changed, the sample current trends towards an assymptote. The trend is exponential with a very large time constant (~2min to ~5min).
+       \item The general shape of successive I-V curves changes noticably
+\end{enumerate}
+
+Possible causes:
+\begin{itemize}
+       \item Capacitance of Sample holder
+\end{itemize}
 
 \end{document}