Coincidence and coincidence window
When two or more events strike at the same time, a coincidence occurs.
In the case of a coincidence detector, such as the Tausand Abacus, signals arrive at two different ports. A coincidence between ports A and B is taken when an electrical pulse arrives at port A and another electrical pulse arrives at port B, within a time frame known as the coincidence window.
An advantage of the method used by Tausand Abacus against other coincidence detection methods is that is irrelevant which signal arrives first; it can be first A followed by B or first B followed by A. Both scenarios are read equally as a coincidence, if they arrive within the coincidence window.
The same applies for the measurement of 3-fold or 4-fold coincidences. The order of arrival is indistinct, as all 3 (or 4) pulses arrive within the coincidence window they will be taken as a coincidence.
In the following examples, the background grid corresponds to a 5ns scale. Swapping A and B roles leads to the same results.
Simple coincidence example
Coincidence window 20ns. Two 5ns pulses arrive in A and B within the coincidence window. Therefore 1 coincidence between A and B is detected.
Simple no-coincidence example
Coincidence window 20ns. Two 5ns pulses arrive in A, and one 5ns pulse arrives in B. The pulse on B arrives too late to be considered a coincidence with the first pulse from A, but is close enough to the second pulse in A, arriving about 5ns later, to be considered a coincidence with it.
Maximum repetition rate example
Coincidence window 5ns. Synchronized bursts of 5ns pulses at 100MHz arrive at both input ports, A and B. Every couple of pulses is considered a coincidence, and therefore a total of 100MHz coincidence rate is achieved. This is the limit case of maximum repetition rate for the Tausand Abacus AB1004.
Different pulse widths coincidence
Coincidence window 5ns. A single 20ns pulse arrives on port A while a burst of 5ns pulses at 100MHz arrives on B. Only 1 coincidence is detected, corresponding to the pulse in A arriving within the same coincidence window with the second pulse in B.
Coincidence window auto-reset
When a coincidence is detected within the selected coincidence window, the system auto-resets the timer to expect a new arrival on either port to fire a new coincidence detection process. Therefore, the system remains enabled to receive a new detection, avoiding unnecessary holding processing times.
Coincidence window 1000ns. In this example, although a long coincidence window of 1000ns is set, during a lapse of 75ns two valid coincidences arrive and are correctly detected.
Coincidence window auto-extend
When several pulses arrive at the same port, let us say A, before the coincidence window timer is done, each new pulse resets the coincidence timer allowing an auto-extend of this timer expecting for a detection on the other port, let us say B.
Coincidence window 20ns. The coincidence timer starts with the first pulse in A, and is auto-extended two times due to later pulses in A. In this example the short 5ns pulse in B arrives at the last moment that would be detected as a coincidence with A. If the pulse on B arrives 5ns later, the scenario would not rise a coincidence detection.