I have been using a PMT to detect pulses of light, and logic pulses were involved, but I'm not quite clear how they work. Essentially the PMT was connected to a counter and discriminator, and it was possible on the discriminator to change the width of the pulse and the threshold voltage. I understand that the threshold voltage filters out pulses below a certain amplitude.

I'm not sure how the logic pulse works and I'm obviously not googling the right thing, because I can't find any info on them. It's somehow related to how many times a pulse is counted. But I'm not sure whether a wider pulse would cause more or less triggers, and I no longer have access to the equipment so I can't simply check. If anyone can work out what I'm describing and point me in the direction of resources that explain it, that would be much appreciated.

I have a feeling that while logic pulse was the term I was told to use, it may be the wrong one. I found an incredibly short reference to something called a 'gating pulse' which sounds like what I had, but I can't find info on that on google either.


A discriminator is used to convert from analogue signals to digital/logic signals. As you say, the threshold is used to filter out signals below the threshold.

The "width" setting changes the width (or sometimes only the minimum width ) of the output pulse. This is useful to match the speed of the receiving electronics or if you wish to reject pulses that happen close together in time.

The width can sometimes affect the count rate. If a second pulse arrives within the "width" of another pulse, the discriminator may simply ignore it. In other types of device, the discriminator may merge the two pulses together, creating one long pulse. However, the counter usually will only count edges (usually rising edges), and will therefore only count once. How exactly it works depends on the exact kind of device you have and how it was configured.

The gating input usually acts to "turn on" the counter if you wish to only accept counts within a defined time interval (or "gate").


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