Nanosecond range pulse generators for quality control and diagnosis of Cherenkov telescope cameras

Resumen

The main goal of this thesis consists in the development of compact nanosecond-range pulse generator technologies for the Cherenkov Telescope Array (CTA) project. This kind of instrument is used for detecting the Cherenkov radiation that has been generated by gamma rays when entering in contact with the atmosphere. The duration of the Cherenkov light is of about a few nanoseconds. As a consequence, the need for compact pulse generators emerged soon with the aim of testing the telescope acquisition chain. Additionally, they are required for performing both laboratory and field tests in several parts of the telescope, such as the camera photodetectors or the camera calibration. The first pulse generator here presented is based on Step Recovery Diodes (SRDs), which are characterized by their very abrupt turn off. The topology carried out consists in a pulse sharpener composed by the SRD, a short-circuited stub and a diode Schottky. Within this design pulses of more than 4 V amplitude and widths of the order of one nanosecond have been achieved. Additionally, a width jitter in the ps range has been obtained. As well it is worth to mention that the width of the pulses is simply controlled by just modifying the length of the stub, what provides with high versatility this pulse generator. In particular, a four channel prototype has been developed with pulses of 4 V peak amplitude and 1.3 ns FWHM at each output, as required by the specifications. This prototype was successfully used for testing the receiver boards of the MAGIC II telescope camera. Additionally, this design provided a basis for the development of a 96 fanout pulse generator which was successfully used for analyzing the readout system of the MAGIC upgrade, and it is proposed for testing the Dragon Boards of the CTA camera (CTA readout boards), which are based on the same chip. Another topology of pulse generator has been proposed in this work, the Signal On Transition (SOT) pulser. In this design a control signal determines the path of a current source by the switching of four transistors. When it changes from high to low state or vice versa, and just in the middle of that transition, the current is driven throughout a specific path where a resistor has been placed. The voltage drop across the resistor produces the output pulse. Pulses of more than 4 V peak amplitude and less than 10 ns FWHM into a 50 ohms load have been achieved. It is expected to improve to a great extent the characteristics of the pulses obtained by carrying out an integrated circuit implementation. A special characteristic of this pulse generator is that the pulse width is controlled by means of the control signal slope, so in combination with the appropriate electronic it becomes a variable width pulse generator. With such aim a variable transition time control circuit (TTCC) has been developed, based on the charge and discharge of a capacitor by means of two variable current sources. In this design the rising and falling times are independently controlled by varying the value of the current sources, which is performed by means of potentiometers. Additionally, it is also possible to activate/deactivate the pulses when required by just enabling/disabling the SOT circuit current source. That allows creating any desired pulse pattern. Furthermore, replacing the analog potentiometers that determine the control signal slope by digital ones and adding the digital circuitry necessary for managing the SOT circuit current source (e.g. a transistor for turning on and off the current source) allows it to be remotely controlled by a computerized system. Thus, a new branch of application in automation processes is opened. Despite all the applications mentioned above, it is believed that the scope of the work here presented is not restricted to CTA, but can be applied in a wide variety of other fields thanks to its versatility, compactness and reduced cost.