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.
These examples are for Tausand Abacus AB1002 and AB1004 devices, having 5ns resolution.
Tausand Abacus AB1502, AB1504, AB2502 and AB2504 devices behave similarly, but having 2ns resolution.
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.