diff --git a/software/swguide.tex b/software/swguide.tex index 0c89bf5..af86429 100644 --- a/software/swguide.tex +++ b/software/swguide.tex @@ -108,6 +108,13 @@ \subsection{Voltage Measurement} frequency noise components are less suppressed. A helper script exists to calculate the appropriate filter coefficients for a given bandwidth, and load them onto the FPGA (see Section~\ref{sec:lpfdesign}). +The process of time averaging over multiple periods means it is no longer necessary to sample at a high enough rate to resolve the AC excitation +waveform. Instead, we only need to sample at a high enough rate to resolve amplitude (and phase) fluctuations below the cutoff frequency of the filter +used to perform the time averaging. Bolometer sensors naturally suppress high frequency signals due to the thermal properties of the sensor, so the +signal-to-noise ratio gets very low at higher frequencies. The BOLODSP module on the FPGA downsamples the output data by a factor of 100. This +means that for an input sampling rate of 1MSPS (the maximum sampling rate of the BOLO8BLF), data will be output at 10kSPS\@. This means that the FPGA +filters should be set to a maximum of 5kHz, though in practice there is little point going above about 1.5kHz. + \subsection{Voltage Offset} \label{sec:offset} No sensor is perfect, in the sense that the bridge will never be perfectly balanced in the absence of any input power. Likewise, no electronics is perfect, @@ -318,7 +325,7 @@ \subsubsection{DIODE{\_}DROP{\_}V} \subsubsection{FILTER{\_}BANDWIDTH} As mentioned in Section~\ref{sec:voltage}, the choice of filter affects the trade-off between noise levels and time resolution. The filters themselves are designed using helper scripts (Section~\ref{sec:lpfdesign}). Those scripts take the value of this knob to design a filter with a given bandwidth. The -bandwidth should be specified in Hz. +bandwidth should be specified in Hz. The maximum supported bandwidth is 1950Hz for use with the filter helper scripts. \subsubsection{THEAT} This value, in seconds, specifies the length of time spent ohmically heating the sensor during calibration. It should be long enough to ensure the sensor