Rutgers University Experimental Nuclear Physics

Test Pulser

Contents

• Credit
• Uses
• Description
• Ordering of boards on Pulser Cable

The test pulser described here was conceptually designed by Stan Sherman of the Rutgers University Department of Physics & Astronomy electronics shop. Detailed design of the pulser for CEBAF has been done by Mark Jones of the College of William & Mary.

The uses of the test pulser include:

• Calibration of the multiplexing widths, for use in demultiplexing. The pulser can provide a clean input signal to the readout board, resulting in a 1-2 ns determination of the widths of the multiplexed pulses.
• Remote efficiency check. As a turn on check of the chamber, or as a check during an experiment, in particuular if some wires appear to be inefficient, The pulser can be used to measure efficiency at getting a signal in the TDC as a function of pulse voltage. This feature allows remote checking of the chamber electronics functionality and correctness of wiring. It does not check the state of the gas within the chamber, or whether high voltage is being applied to the high voltage boards -- both of these problems would presumably lead to inefficiencies in large numbers of straws, rather than single straws.
• Gain / threshold check. One can determine the signal voltage for each channel at which the logic starts sending out pulses to the TDC. If one knows the attenuation of the signal vs distance in the pulse signal cable, all connections are good, and the signal is about the same shape as a real data signal (so that the gain is the same as designed) then one can check for consistency with the threshold that is set.
• Debugging aid, at the chamber. Tracking down miswiring or electronics problems at the chamber is difficult with either cosmic rays which are not intense, or a source, which is not very directional. The pulser only fires one channel in each group of eight, and allows one to check continuously along the electrical circuit to see where the pulse goes away, and thus where the problem occurs.
• Timing check. While offsets for different triggers and test pulse cable length make it difficult to determine absolute t0's, relative t0's within groups of 8 or more straw can be determined, since there are only small differences in the wire lengths between boards and straws, and between test pulse connectors to stacks of HV boards. These differences will generally be confirmed to be at the sub ns level.

The FPP system comes with a test pulse feature. The pulser is designed to send a signal into the high voltage / termination boards. The pulse then propagates through the straw into the radout boards, from which it is sent to level shifter borads and then ultimately to the TDCs. Due to the multiplexing of the readout boards, only one wire in each group of eight wires can be fired at a time, since all of the eight are combined into the same readout channel.

The pulser is generated and output with 50 ohm impedance. An RF transformer is used to connect to the 100 ohm twisted pair cable that runs through the chamber. The pulser signal is coupled through a 20 kohm resistor to the low leg of the 1500 pF HV capacitor. A 1 V fast signal in the test line leads to an ~ 8 mV signal at the HV card pin. Connections and the ~ 3 m attenuation length of the straws leads to a signal of about half this amplitude for a typical straw at the readout board, at the input to the amplifier. The input to the comparator is about a factor of 24 larger than the amplifier input; this number is frequency dependent.

It is important NOT to use fast logic signals (rise time ~ 1 ns) for the pulser. The fast rise time leads to a large amount of noise broadcast and pickup. Typically the noise level at the amplifier inputs will be close to the signal size, and exhibit 1-2 ns period oscillations. This leads to a voltage dependent output width, as sometimes the first channel in a group of eigth to trigger is a noise channel rather than the channel with the actual signal. The typical effect is that there is only a small range of voltages, perhaps 0.5 - 1.0 V in the signal cable, over which one can generally (but not always) see a width in the output corresponding to the signal channel.

Shaping the pulse so that rise time is about 20 ns leads to a pulse slower than that of the data, but appears to largely eliminate the fast RF noise associated with the fast rise time pulse. The dynamic range over which the pulser system operates dependeably increses (preliminary tests) to about 0.5 - 4.0 V.

The cable always connects from upper boards to lower boards, within a stack. The signal amplitude in the cable varies by about a factor of two, being smallest for the far end of the cable.

Revised May 6, 1996 Norma Lucero