maximum input frequency

A single maximum input frequency cannot be specified for a Time-to-Digital Converter (TDC). Although this term is widely used, it does not adequately describe the performance of a TDC.

Unlike a frequency counter, a TDC does not measure periodic signals but records discrete events (hits or timestamps). Consequently, the event rates that a TDC can handle are not determined by a single parameter, but by the interaction of several technical characteristics.

For this reason, the technical specifications of our TDCs include several parameters, each describing a different aspect of the system's performance:

• Minimum Pulse Width specifies the minimum pulse width required for an input pulse to be detected reliably.
• Double Pulse Resolution defines the minimum time interval between two events on the same input channel that allows both events to be resolved reliably.
• Readout Rate specifies the maximum event rate that the complete TDC system can process continuously and transfer to the host computer.
• Multihit capability and the internal FIFO buffers determine how many closely spaced events can be temporarily buffered before they are processed and read out.

These specifications complement one another and should not be considered independently.

A common misconception is that the Double Pulse Resolution directly defines the maximum input frequency. In reality, the Double Pulse Resolution describes only the temporal resolving capability of the input stage. It specifies how closely two events may occur on the same input channel while still being detected as separate events. It does not describe how many events per second can be processed continuously.

To accommodate short-term peaks in the event rate, cronologic TDCs employ multi-stage FIFO buffers. These buffers temporarily store bursts of high event density, effectively decoupling event acquisition from subsequent data processing. As a result, applications with highly fluctuating or pulsed event rates can be handled reliably.

Only when these buffers become full does the Readout Rate become the limiting factor. It determines the maximum event rate at which data can be processed continuously and transferred to the host computer. This is precisely why our technical specifications include both the Double Pulse Resolution and the Readout Rate.

When selecting a suitable TDC, both parameters should therefore always be considered:

• The Double Pulse Resolution determines how closely two events may occur in time while still being resolved individually.
• The Readout Rate determines how many of these events the complete system can process continuously.

In summary: The performance of a cronologic TDC cannot be characterized by a single maximum input frequency. Instead, it is determined by the combination of temporal resolution, event buffering, and sustained readout performance. Together, these parameters define the event rates that can be processed reliably in a given measurement application. An overview of these specifications can be found in the technical data of our TDC products.

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