We have developed a new PC-based program to run the instrument. With this program, we will have complete computer control over all timing and phase-cycling functions of the instrument. Where the original instrument required knob-turning (adjustments which by nature cannot be made systematically and repeatably in the course of an experiment or from one experiment to the next), the timing and phase-cycling functions of the new instrument will require only some occasional calibration at most and will otherwise be automated, systematically and repeatably supervised by the computer.
We have built into the instrument the ability to produce up to four pulses and four delays. The durations of each of these pulse sequence components are independently variable and under the control of the computer program mentioned previously. In contrast to the old instrument which could perform three different pulse sequences, the new instrument can produce up to 128 different pulse sequences (counting only the variability of the currently implemented set of four delay and four pulse durations).
We have installed fast programmable digital delay generators (Stanford Research, Sunnyvale, CA) to produce the precisely timed events (<1 ns resolution) which comprise the pulse sequences and gate the acquisition of data. These digital delay generators are interfaced to the controlling computer by way of a GPIB bus. By simply adding more delay generators to the GPIB bus, making some trivial modifications to the computer program, and making small additions to the pulse control circuitry, pulses and delays (in groups of two), may be added to the new system up to approximately 30 pulses and 30 delays, each of independently variable duration. This is in contrast to the old system which was capable of producing at most three pulses of fixed duration.
We would also note that with the implementation of a GPIB bus in the system, straightforward computer control of other pieces of the instrumentation could be instituted relatively easily. Instruments which may in the future be controlled in this way include our fast digital storage oscilloscope (LeCroy Corp., Chestnut Ridge, NJ) and a digital frequency meter.
In addition to the computerizing of the instruments timing functions, we have placed the relative phases of the pulses under computer control. This will add substantially to the variety of possible pulse sequences already alluded to. These capabilities allow for better rejection of instrumental artifacts by the use of phase cycling techniques such as those which have been heretofore more common in pulsed NMR experiments. While the original instrument could cycle pulse phases between relative phases of 0 and 180 degrees, the new instrument can cycle pulse phases among 0, 90, 180, 270 degree relative phase shifts. Each of the new instrument's pulses will have independently specifiable phases or phase cycling sequences. All these are under the user-configurable control of the computer program we are developing.
In view of the plethora of new pulse sequence possibilities, we have equipped the controlling computer program with the ability to load in new pulse sequence descriptions designed by the users. Every variable of the pulse sequence may be "programmed" in this way. The different pulse sequence descriptions may be loaded from the computer's disk with the effect that the instrument is instantly reconfigured for the desired pulse sequence. This is in contrast with the old system, where what little variability did exist in the pulse programming required both manual instrumental adjustments and changes to the computer program settings.
We have constructed a completely new fast logic circuit to manage certain aspects of the interface between the computer and the rest of the new instrument. The functions of this circuitry include fast control of the digital phase shifter, gating of the power amplifier, and the assembling of the pulses produced by the digital delay generators into the pulse train used in the experiments. This circuitry is easily extendible to accommodate the addition of more delay generators (and hence more pulses and delays for the pulse sequences) should the need arise.
The figures depicts schematically the original design of our pulsed EPR spectrometer and the new design we have implemented. Areas in which important design improvements have been made are highlighted. The key point to note about the two designs is that in the new design, the user no longer controls the delay generators or the pulse controller manually, but through the computer program which is directly responsible for controlling these components. In this way, these components may be controlled and adjusted to a greater degree in the course of an experiment, without the user's direct intervention, and with complex variations of the experimental parameters taking place automatically and, if need be, repeatably.
The present state of pulsed EPR spectroscopy and the studies it is used in are such that there have been a steady supply of new applications to consider. And the new applications envisioned by advances in instrumentation, not to mention those posed by analogous advances in the more mature field of pulsed NMR spectroscopy have created a climate in which there is much room for improvement upon almost any conceivable design of pulsed EPR equipment. This is certainly true of our newly reconfigured system. While the original spectrometer, when first conceived, represented a significant achievement in terms of instrumentation, it was also an excellent platform upon which to perform the previously described upgrades. With these upgrades, we believe our instrument will have almost unique currency in the field of pulsed EPR spectroscopy on a world-wide basis. But since in reality the recent upgrades are part of an ongoing program to maintain a world-class machine, we hasten to add that the next generation of upgrades and enhancements is already being envisioned. Among the elements of the likely next wave of upgrades are the following:
Here is a picture of our pulsed S-Band spectrometer.
Here is a low-resolution schematic of the Pulse and Phase Controller we have constructed as a part of the recent upgrade of this machine.
Here is a block diagram of the pulsed S-Band EPR spectrometer.
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