1.1. A minimal Electronics Setup

The following items will be needed to build our simple setup:

• CAEN V785 ADC module

• VME Crate

• SBS Bit3 PCI/VME bus interface

• NIM Crate

• NIM Spectroscopy amplifier of some sort

• NIM Discriminator

• Two channels of NIM Gate and delay generator

• NIM Signal splitter

• LEMO to Ribbon cable converter

• VME CAEN V262 I/O module

• Pulser

• 50ohm LEMO terminator

• An assortment of LEMO Cables

• A 34 conductor ribbon cable with 3M connectors on each end

• An oscilloscope

Before starting be sure that you can secure all of these items, many are available through the NSCL electronics pool. Figure 1-1 shows the complete setup of the system, due to the varying amount of familiarity of readers to the electronics, a brief description of the component and what the signal coming out of it should look like will be given.

We will first look at the signal from the pulser. A pocket will work well for this. A pocket pulsers will give a small negative pulse, but we will invert it later. The signal directly from the pulser will look like figure 1-2 when viewed on a scope. The pulser only works when terminated with 50 Ohms.

The pulser will run directly into a NIM based amplifier. This amplifer serves two purposes. First it will amplify our signal helping it stand out against any noise we might have in the channel. It will also invert the negative pulser signal giving us the positive signal that is required by the V785. The signal coming from the bi-polar output of the amplifier will look like figure 1-3. Adjust the gain on the amplifier until you output signal has an amplitude of about 2V.

After you are happy with the signal from the amplifier plug the bipolar output into the splitter. The splitter will simply split the signal much like a "T" but will keep the 50Ω termination needed by the pulser.

One of the splitter outputs will go into a LEMO to Ribbon Cable con- verter. This can be either a NIM based module or a freestanding adapter. Either way the function is simple and will allow you to put the signal onto a flat ribbon cable to be plugged into the CAEN V785.

The other output from the splitter will go into a discriminator. This could be either a leading edge or a constant fraction discriminator. The job of the discriminator is determine when a signal has occurred and to put out a logic signal when it does. The discriminator will have a threshold knob, be sure that the threshold is set above any background noise that may be present so that the discriminator only trips on the pulser signal. It will be useful to look at the both the logic pulse and the signal from the amplifier at the same time when setting the threshold value. Figure 1-4 shows what you will see.

When the threshold is set to a reasonable level, connect one of the dis- criminator outputs to the start of one channel of your gate and delay gen- erator. The gate and delay generator will generate a gate when it receives a logic signal from the discriminator. When looking at the signal from the amplifier and the gate at the same time on the scope, the signal pulse should fall within the gate as seen in figure 1-5. If the gate is occurring too early you can use the gate and delay generator to delay the gate or to make the gate last longer. If the gate is too late you need to delay the signal pulse, that can be done using a delay module, or by adding cable delay. When you are happy with the gate timing, plug it into one of the LEMO connectors on the CAEN V785 labeled gate, put a 50 Ohm terminator in the other one.

The second output of the discriminator will go to a second gate and delay channel; this combined with the CAEN V262 will trigger the computer when there is data on the V785. This channel will be run in latched mode meaning that it will be given both a start and a stop signal for the gate. The output signal will start being "true" when the start signal is received and stops being "true" when the stop signal is received. The start signal is the signal from the discriminator. The discriminator output will go to the IN0 of the CAEN V262 I/O module. The stop signal is generated by the computer to indicate it has responded to the trigger. That signal is present at the SHP2 output of the V262 and should be connected to the stop on the gate and delay generator.

At this point your setup should be complete. Now is good time to make sure that your setup is the same as Figure 1-1. If everything is setup correctly the BUSY and DRDY lights on the V785 should be lit up.