This module provides either a linear or a logarithmic ratio of any
two filter signals. The two input signals are selected by the edge switches at
the top right of the module, labelled N (for numerator) and D (for denominator)
respectively. Each switch has ten positions, numbered 0-9. As explained
elsewhere, there are two alternative output signal lines on the system
backplane, labelled A and B, for each of the eight possible filter positions. A
jumper link on the board determines for each position of each switch whether
the A or B signal line is selected when that switch position is selected.
Switch positions 1-8 correspond to filter outputs 1-8, and position 9 connects
the input to the A or B input signal line (i.e. to the output of an input
module), depending on the setting of the jumper link for this position of each
switch. Position 0 of each switch selects an internal 1 volt reference signal,
and there is no jumper link associated with this position. There are thus two
sets of nine jumper links altogether, which are labelled J1-J9 for the
numerator switch and J10-J18 for the denominator switch. As labelled on the
board, the A signal line for any switch position is selected by installing the
jumper between the central pin and the pin to its left (nearest the front of
the board), and the B signal line is selected by placing the jumper between the
central pin and the pin to its right.
Just below these jumpers is another jumper, labelled J19. This connects the ratio amplifier output to either the A or B "amp" output lines, which allows its output to be displayed on the meter module. The (out)put is also available via the BNC socket on the module front panel.
The reason for this jumper arrangement is that the A and B allocation for any one switch position on the ratio amplifier module is unlikely to be changed very often, whereas the filter signals may be changed rather frequently. This arrangement simplifies the front panel control layout and makes the module easier to use. For example, when only one photodetector is in use, there will be no need to change the jumper settings at all once the system is set up.
Next to the two edge switches is a ten-turn (balance) potentiometer. This should be set to its central position (dial set to 500) in normal operation. As the control is advanced in either direction from this position, the relative values of the numerator and denominator are either increased or decreased so that unequal input signals can be made to produce a unity ratio. The module uses components of the highest possible accuracy for performing the ratiometric conversion, and the balance control can also be used to ensure that full accuracy is achieved under conditions when equal input signals must produce a unity ratio. This is done by setting both the numerator and the denominator switches to position 0 (1 volt internal DC reference), and adjusting the balance control to produce a ratio of exactly unity. In the linear ratio mode, a unity ratio corresponds to an output of exactly 1 volt, whereas in logarithmic mode the output should be exactly zero (so this is the easiest way of making the adjustment).
The balance potentiometer is primarily intended for use in the logarithmic mode, as is appropriate for making absorbance measurements. It allows differential absorbance changes to be measured between any two filter wavelengths, without any need to equalise the two signals before sending them to the ratio amplifier module. In the logarithmic mode, the output signal is inverted so that an increase in differential absorbance (i.e. a reduction in the numerator signal relative to the denominator signal) gives an increase in output voltage. The output in logarithmic mode is also amplified by 10, to give extra sensitivity. The balance control can zero the output for any input signal ratio, provided that both inputs are positive.
The linear ratio mode is appropriate for applications such as fluorescence measurements, and the output in volts corresponds directly to the input signal ratio if the balance control is in its central position. The balance control in this mode can be used to vary the ratio if required, but this is not normally recommended. The type of adjustment that is normally required in this mode is the subtraction of a DC voltage from each signal input, to compensate for the autofluorescence component. Since each filter signal has to be compensated independently, this facility is not included in the ratio amplifier module. Instead, one or more gain/offset modules can be configured to perform this function for as many filter positions as are required. As discussed in the section describing that module, the compensated outputs are normally made available on the corresponding B signal lines for that filter position, with the original signals still available on the A signal lines. When the system is supplied with both ratio amplifier and gain/offset modules, the ratio amplifier is normally configured to accept the B signal lines for those filter positions for which an offset amplifier stage has been provided. For the remaining filter positions the ratio amplifier will normally have been configured to accept the outputs directly from the A signal lines.
The ratio amplifier module also has a variable low-pass filter on the output. The cutoff frequency of this filter is switchable between 100 Hz. and 0.3 Hz., or the filter can be disabled altogether by setting the slope switch (just below the frequency selector rotary switch) to its central off position. The other two positions of this switch select one-pole and two-pole filtering respectively. Two-pole filtering provides a greater attenuation of out-of-band signals, but can cause some overshoot of signal transients. One-pole filtering is less effective but has a much better transient response. This is the recommended choice unless the ratio signal is varying only slowly in comparison with the filter cut-off frequency.
