In our spectrophotometry systems the rotary filter changer is usually controlled either directly from the main system front panel or through software via our COMPUTER INTERFACE module. In either case we would suggest testing the rotor control thoroughly before installing the filter wheel into the light path. If using the rotor with our spectrophotometry software it should be first tested without connection to the computer.
The rotor control electronics can be supplied housed either in a double-height rack that can accept a variety of plug-in modules for photometric measurements, or in a single-height rack (which also has space for up to three modules) for other applications. The double-height rack also has one double-height module slot, which is intended to accommodate our computer interface module, whereas all the other slots are single-height, and accommodate the other system modules. Both rack sizes share a common front panel. Most modules are approximately 60mm wide, and can be installed in any single-height module slot in the enclosure. The input amplifier and photon counting modules are only half this width, and it is normally recommended that half-width modules are installed in the upper half of the enclosure, which has a correspondingly closer spacing of card guides and connectors. However, we generally recommend that the slot nearest the system control panel in the upper half of the enclosure is not used to take an input amplifier module, since there is the possibility of a small level of interference from the adjacent digital electronics.
Modules should be installed or removed ONLY when the system power is off. Although many modules have input and/or output connectors on their front panels, signal interconnections between modules are made via the system backplane.
The top left section of the enclosure contains the system power supplies and the rotor control electronics. The rotor plugs into the 12-pin socket on the front panel of this section. This socket provides both the power to the rotor and the connections to the various sensors that are used for measurement and control of rotor speed and position. There are also two BNC sockets on the front panel, the lower of which carries a TTL-level output pulse derived from one of the rotor position sensors. This pulse occurs once per rotor revolution. The rising edge occurs midway between the highest and lowest filter positions, i.e. between positions 6 and 1 in the standard rotor, whereas the falling edge of the pulse has no special significance. This pulse has a variety of uses, such as providing a sync pulse for oscilloscope display of some of the optical signals, particularly the composite out signal from the INPUT amplifier module as described later.
The rotor speed can be synchronised either to an internal reference (frequency set by the edge switches on the front panel) or to another reference frequency. When any other reference frequency is used, the EXTERNAL indicator is illuminated. This frequency can be provided from a module (i.e. the computer interface or tape recorder interface) via the system backplane, from the external control 'D' connector on the rear panel (specification provided separately) or from an external TTL-level reference connected to the BNC socket labelled EXT. When providing the signal via the BNC socket, the system should be switched to external control by depressing the external indicator light, which also acts as a switch of the push-on, push-off type. When a module such as the computer interface provides the reference frequency, the corresponding switchover is done automatically via a control line on the system backplane, and the indicator switch should be illuminated but in the off position.
The speed is continuously variable between about 2.5 and 300 revolutions per second (Hz), and in general a speed within the range of 10-100 Hz will be most appropriate.
We suggest that you avoid running the rotor at speeds close to the mains electricity frequency or to multiples or submultiples of it, i.e. for 50Hz mains avoid 12.5, 25, 50, 100 and 150Hz rotor speeds. The reason is that any electrical, or more likely optical (e.g. from room lights), pickup - by the system at the mains frequency will generate interference, and this interference will contain a component at a very low frequency (and hence be difficult to remove by filtering) when the rotor is running at those speeds.
Under internal control, the rotor speed can be set using the edge switches and the speed range toggle switch on the front panel, to the nearest 1Hz for speeds above 100Hz, or to the nearest 0.1Hz for speeds below 100Hz. When the rotor is spinning at exactly the set speed, and so that the rising edge of the rotor output pulse (see above) is within a few degrees of the rising edge of the reference signal, the green SYNC light is illuminated, and the correct speed is shown on the front panel LED display. If the selected speed is changed, the green sync light will be extinguished until the rotor has stabilised to the new speed. If the rotor is not in sync, or if it is synchronised to an external reference, the LED display always shows the speed to the nearest 1Hz below (e.g. when spinning at 43.87 Hz the display will show 43), but when an external reference frequency is used the rotor will always synchronise PRECISELY to it. Note that the external control interface ('D' connector on the rear panel) can be used to set the frequency in two ways. Either a TTL-level frequency, or the appropriate steady bit pattern, corresponding to the BCD-encoded speed setting that is normally provided by the front panel switches. In the latter case, the rotor is still effectively under control of the internal frequency reference (which has now just been set externally), so fractional speeds below 100Hz can be displayed in this mode.
As well as spinning smoothly and continuously over a wide speed range, the rotor can also be stopped at any filter position and moved in stepwise fashion from any filter position to any other. Operation in this mode can be selected either from the front panel controls or via external control inputs. To select this mode from the front panel, the red STOP switch (which also acts as an indicator in the same way as the EXTERNAL switch and indicator) should be depressed. The rotor will then rapidly come to a stop and automatically go to filter position 1. In this mode the LED display will now show the current filter position.
To change the filter position, the appropriately numbered pushbutton on the front panel should be depressed (unlike the STOP switch and EXTERNAL switch, these switches have a momentary action, so they immediately release again). Alternatively, to advance the rotor by plus or minus one filter position, the up-arrow or down-arrow pushbuttons can be used instead. Please note that although our standard rotor has six filter positions, eight filter position pushbuttons are provided on the front panel. The reason for this is that the Cairn system is designed to accept rotors or other filter-changers with anywhere between two and eight positions, so we provide eight pushbuttons to cater for the maximum limit. Selecting a filter position higher than the highest actual position (i.e. positions 7 or 8 in the standard 6-filter rotor) will cause the rotor to go to the highest actual position instead.
When the rotor is moving to a new filter position in this mode, the green SYNC indicator will briefly go out (a corresponding signal is also available on the 'D' connector), to indicate that the rotor is currently not at a valid filter position. For the standard rotor fully loaded with filters, this period is approximately 35msec for moving to an adjacent filter position, or 60msec for moving to the opposite filter position. The rotor never moves further than half way round during any one movement, as the shortest path is always taken. For moving between opposite filters, the clockwise direction is always chosen, i.e. for moving from 1 to 4, the rotor goes via 2 and 3, not via 5 and 6. The rotor always sends out precise positional information to the control electronics, which automatically returns it to the selected position if any attempt is made to displace it (try it if you wish!).
Just as for continuous spinning, these functions can also be controlled externally, either via our computer interface module or via the 'D' connector. In this case, the STOP switch on the front panel should be off, to allow the equivalent external control. The STOP indicator will now display the external control status instead.
In addition to the above, the double-height system enclosure contains a digital control board which derives a sequence of control pulses from the single pulse provided by the rotor. These control pulses are used to control the various system modules for photometric measurements. A voltage-controlled oscillator provides the input to a series of divider and decoder stages, and the final output is fed to one input of a phase-sensitive detector. The other input to the phase-sensitive detector is then provided by the rotor, and any phase difference between the two signals generates an error voltage that is fed to the voltage-controlled oscillator. The result is that the voltage-controlled oscillator frequency is an exact multiple of the rotor spin. Control signals corresponding to individual filter positions can thus be derived from the frequency divider/decoder stages with great precision, even though only one reference pulse is available per rotor revolution. Furthermore, the divider/decoder stages can be reprogrammed by a jumper link to provide pulse sequences appropriate for rotors having from 2 to 8 filter positions. Finally, it should be noted that the frequency to which the electronics synchronises is the actual rotor speed, not the speed selected by the internal frequency generator or by the external reference frequency reference. Therefore, even under conditions where the rotor is not spinning at the set speed, e.g. when accelerating or decelerating, the system will still operate correctly.
