Three Clocks: 10MHz, 50MHz, PMT

10MHz

This clock is used for absolute time. It is stored in a 53-bit register so will run for 28.54 years. T zero for the clock was midnight GMT, January 1, 1996. Its accuracy is maintained using GPS. GPS stands for Global Positioning System. The system involves time signals from a series of satellites and a clock (from a commercially built receiver) which is synchronized to their time. From them UTC (Coordinated Universal Time) is determined. The problem is that GPS time is very accurate but not very precise. In other words, on short time scales, the frequency and phase jump around because the time is constantly being corrected and adjusted. The exact level of jitter is not yet known. Certainly two adjacent ticks may not be 100nsec apart although it may be that there is always $10^7$ ticks in 1 second. Over longer periods of time such as an hour, though, the system is very accurate.

50MHz

This is used for relative times between events. It is stored in a 43-bit register so has to rollover every 2.036 days. The planned mode of operation is to set the clock going just once and to let it rollover as it will, even though this means it will happen within a single run. Global triggers are latched to (i.e. occur on) 50Mhz clock ticks. This is described in detail below. The clock will probably be a quartz oscillator, so will be very precise on short time scales, with an expected jitter of less than 1 nsec, but may drift badly after a couple of days. When comparing times between two events using the 50MHz clock, the 10MHz clock or event ID will have to be used to detect and correct for rollover.

For the purposes of this discussion the following terms are defined:-

50MHz clock ticks

Value of the 43-bit register

50MHz clock time

20 * (50 MHz clock ticks) i.e. time in nsec since last rollover.

PMT

This records the time of a single PMT hit relative to the global trigger (i.e. 50 MHz clock time). It consists of a simple voltage ramp, which takes 250 nsec to ramp down to zero. ^A.1

The voltage ramp goes at a constant rate and is stopped by the global trigger. So, initially a PMT time t is 250 but steadily drops the longer it takes to form the trigger. Note that if GTR is the global trigger in 50MHz clock time, GTR can only occur on clock ticks but t falls while waiting for the tick, so GTR + t remains constant.