Optoscan - CAIRN RESEARCH - #29

Control Modules

Yet another of the many features of the microprocessor control unit is that if it is supplied in a suitable electronics rack (either single-height or double-height), it can also control our established range of photometry modules. These were originally developed for fluorescence photometry with our spinning rotor system, where they can extract the signals obtained from each of the individual optical filters in the wheel, but they also included a much simpler form of operation that could be used when only a single excitation wavelength was required. However, modules supplied from 1992 onwards support an additional mode of operation which is particularly appropriate for use with asynchronous wavelength-changing systems such as the Optoscan. Since the subject of optical signal detection is a particularly important one, it is worth discussing in some detail in order to explain why our modules work in the particular ways that we have chosen.

The simplest form of photometric detection is for the detector (usually a photomultiplier, but possibly a photodiode in some applications), which generates a signal-dependent current, to drive a current-to-voltage converter. This provides an amplified and low-impedance output signal for connection to subsequent signal-processing circuitry. The converter circuit is normally combined with a low-pass filter - which may be variable as on our PMT amplifier - in order to reduce the high-frequency noise on the signal.

It is important to match the filter frequency to the subsequent data acquisition frequency, for the following reason. If (as is usually the case) the acquisition system makes each measurement at some effectively instantaneous point in time, as opposed to averaging it over a finite period, then successive samples of a noisy signal may differ substantially from each other, and in extreme cases one can just be measuring the noise spikes. Low-pass filtering reduces the instantaneous fluctuations, so that the samples increasingly represent signal rather than noise.

On the other hand, excessive filtering has two disadvantages. First, the signal can now change only slowly compared with the sampling frequency, so this means that there is a lot of redundancy in the recorded file, as successive samples will always be so similar. However, this is not necessarily a problem, whereas sampling at too low a rate certainly is.

The second disadvantage is more serious, since the filtering may be removing useful information as well as noise from the data. In a system with a single excitation wavelength, the effect is clear to see, and as long as the bandwidth and sample rate are both set to "reasonable" values, we see little advantage in doing things any other way. As a broad recommendation, single pole filtering, with a sample frequency at least 10 times the filter frequency, gives perfectly satisfactory results. We call this the "averaging" mode of the input amplifier.

When this is likely to be the preferred operating mode, we may instead supply the input amplifier with a wider front panel, to make room for two additional controls (although the circuit board is identical). In this form we call it the PMT amplifier, the additional controls being the filter frequency (this is set by jumper links on the pcb when no front panel control is provided), and a variable DC offset control. For multiple-excitation applications, an offset control in this location is not appropriate, since different offsets may be required at different wavelengths, for which our gain/offset module should be used.

However, averaging mode is NOT suitable for multiple-excitation applications, since the filtering would tend to smear out the signals for the different excitation wavelengths, causing them to "bleed" into each other. Under these circumstances, signal detection by integration is greatly preferable.

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