The input and PMT amplifiers comprise of the same board configured
with different front panels. The larger panel on the PMT amplifier has variable
filter and offset controls. This is intended for single excitation experiments
where the output of the amplifier corresponds directly to the desired signal.
In multiple excitation systems there is no reason to apply offset to the output
of the amplifier, because this raw signal is actually a composite of different
signals at different wavelengths. The input amplifier retains full
functionality of the PMT amplifier, through internal jumper links, but it's
smaller panel size helps to save space in the system rack. For the purposes of
most of the following discussion the two modules can be treated as the same.
The input amplifier can be configured to measure positive- or negative-going signals according to the setting of the jumper J1. This jumper sets the polarity of the first stage so that the output is always positive-going. The input impedance in either of these configurations is 1 megohm. J1 can also be used to select a virtual ground input stage configuration, in which the input stage acts as a current-to-voltage converter. This mode is intended for direct interfacing with a photomultiplier tube. The photomultiplier power supply should be negative with respect to ground, and the tube cathode connected to the negative supply. The tube anode is connected directly to the amplifier (in)put on the front panel. In this mode the anode current is converted into a positive-going voltage in the input stage.
The gain of the input stage is set by J2. In either the positive or negative voltage mode configurations, the gain is either x1 or x10, and in virtual ground mode the gain is either 0.1V or 1.0V per microamp. This signal is coupled to the next stage via the front panel potentiometer, allowing the effective gain to be varied continuously down to zero from these limits. The amplifier stage following the potentiometer has a gain of x1, x10 or x100, according to the setting of J3. This gives a maximum overall voltage gain of x1000 or, 100v per microamp in virtual ground mode, which should be more than sufficient for all applications. The module is normally supplied with J1 set to virtual ground mode, and J2 and J3 both set to x10.
When the rotor is spinning, the amplified input signal is sent to a low-pass filter and a pulsed (switching) integrator. An integration method is used to extract the maximum information content from the input signal, since the total area of the optical signal waveform from each filter is thereby measured. A conventional integrator would give a maximum output that varies inversely with the rotor speed, but the pulsed integrator automatically compensates for this effect. The integrator is activated for several hundred brief constant-length periods, uniformly distributed in time throughout each filter position. Thus the total actual integration period is independent of the rotor speed. The low-pass filter, which has a corner frequency that automatically varies with the rotor speed, smoothes the input signal over a period that is long relative to the integrator sampling rate but still short relative to the rotor speed. This ensures that the noise performance of the integrator is equivalent to that of a conventional integrator design.
A further feature is that the input signal is briefly sampled by an additional amplifier during the dark period between each filter. This signal is filtered to average it over a number of rotor revolutions, to provide a reference against which the integration is performed. The input module thereby compensates for photomultiplier dark current or for any steady background illumination component. In practice this is very useful, as it greatly reduces the interference from room lighting. However, the room lighting will also contain components at the mains frequency and multiples thereof, which can still cause some interference (as explained previously). During system testing, it may be useful to disable the background subtraction, by moving jumper J5 to the off position. This allows the response of the system to a steady light signal to be evaluated (normally, a steady light signal would give a zero output because of the background subtraction).
The integrated output voltage is automatically reset by a control pulse on the system backplane. Other control lines on the backplane ensure that the integrated output from each filter is stored in the appropriate sample-and-hold amplifier in the output module, just before the integrator is reset. The (out)put is also available on a BNC socket on the front panel, and when the system is functioning correctly, it will consist of a series of sigmoid curves corresponding to the integral of the signal from each optical filter in turn, with an abrupt reset to zero between each integrated signal. For testing, and/or as an alternative in normal operation, the BNC socket can instead be connected to the output of the preamplifier section, so that the (unintegrated) amplified input signal can be observed. This is done by plugging the lead from the BNC socket onto the SK2 pins instead of SK3.
When the rotor is stopped, there are three possible operating modes, which are set by jumper J7. The module switches to the chosen mode automatically when the rotor is stopped, on receipt of a control signal on one of the lines on the system backplane. For use in simplified configurations (e.g. dual-emission only systems) in which this control signal may not be present, jumper J8 can be switched to simulate the permanent receipt of the control signal, thereby enforcing the selection of the chosen stopped mode. The stopped modes are intended to cover the situations where the system repetitively samples the same optical wavelength, or switches discontinuously between different optical wavelengths (in which case the different wavelengths may be sampled for different periods), or some combination of these alternatives.
The first of the three stopped modes is the same pulsed integration system as described above, and is intended for sampling periods similar to those used for the rotor, i.e. about 1-100msec. The second mode is a conventional integration system, which is intended for signal measurement over longer periods, between about 100msec-10sec, as may be appropriate for a discontinuously-operating filter changer instead of a continuously-spinning rotor. This mode may be particularly useful for compatibility with camera imaging experiments, where measurements are often made on a similar timescale in order to achieve acceptable signal-to-noise ratios. The gain of the continuous integrator has deliberately been made much less than that of the pulsed integrator, on account of the expected longer measurement periods, and if required it can be decreased by a further factor of ten by changing the position of jumper J9.
In both the integrating stopped modes, the output normally has a sawtooth appearance, and continuous outputs can be obtained from the output module, just as when the rotor is spinning. However, if the amplifier is sequentially measuring the same optical wavelength (i.e. no filter changing is taking place), then it is possible to obtain a steady output directly from the input amplifier, by activating its optional sample-and-hold stage. In this condition, the output is held at the final value of each integration until the next integration has been completed. This mode of operation is selected by moving jumper J6 into the hold position.
The third stopped mode gives the average value of the input signal, low-pass filtered with a corner frequency given by J4. The corner frequency range is 1, 10, 100 or 1000Hz, according to the setting of the first jumper link, and the actual frequency can be reduced to 0.3 of the selected value (giving 0.3, 3, 30 or 300Hz) by removing the second jumper link. This mode is intended for repeated measurements at a single filter wavelength, and it has the advantage that (unlike the other two stopped modes) no external control pulses of any kind are required, making it particularly suitable for dual-emission systems.
The remaining jumpers, J9 and J10, affect the output signal from the module. The spectrophotometer can support up to four input modules, and there are therefore four alternative output lines available on the system backplane. These lines are labelled A, B, C and D, and the signal connection is set by J9 (which connects the output signal to pin A19, C19, A18 or C18 of the module DIN connector). The output signal is also available on the output BNC socket on the module front panel. When only one input module is used, it is recommended that J9 should be set to send the output to the A line, and this is the normal factory configuration. Similarly, a second input amplifier would normally have its output connected to the B line.
The other jumper, J10, allows subtraction of a signal present on one of the other output lines. This facility is intended for making certain differential measurements between two photodetectors operating at the same wavelength, but under normal conditions this jumper should be left in the off position to disable the subtraction.
When a computer or tape recorder interface module is installed in the system, it can place its own data on any of the four output signal lines. At the same time, a control signal disconnects all the input amplifiers from the backplane. This allows either the computer or the tape recorder interface to replay data back into the system, which can then be processed exactly as if it were coming from the input amplifiers.
One final point to note is that the output from this module forms the input to other modules. Therefore, the A B C and D signal lines referred to above are known elsewhere in the system as the A B C and D input channels.
