Fractional-Delay TDC Architecture for Sub-Inverter Timestamp Resolution
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Solution Overview
Problem
Existing time-to-digital converters (TDCs) face challenges in achieving high resolution due to dependencies on propagation speed changes of delay line inverters, leading to suboptimal timestamp normalization and increased phase noise in phase-locked loops (PLLs).
Innovation Solution
A novel TDC architecture incorporating a fractional-delay element circuit, a first and second delay line timestamp circuit, a time difference equalization circuit, and a programmable delay element, which generates a time-shifted facsimile of the input signal with a controlled time-shift of half an inverter propagation delay, combined to produce a high-resolution timestamp with finer resolution than either timestamp alone.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If a conventional delay line TDC is used, then the circuit is simple, but the timestamp resolution is limited by the propagation delay of delay elements
Solution Approach 1:
The TDC is divided into multiple independent delay line timestamp circuits (DLTCs), each producing a timestamp. By segmenting the measurement function across multiple parallel circuits with different time shifts, the overall resolution exceeds the propagation delay of individual delay elements. The fractional-delay element creates additional time-shifted versions of the input signal that are processed by separate DLTCs.
Solution Approach 2:
The invention adds a temporal dimension to the measurement by creating multiple time-shifted copies of the input signal. Instead of measuring time along a single delay line, the system creates parallel measurement paths with different time offsets (0, T/4, T/2, 3T/4 where T is the period), effectively adding a dimensional aspect to the timestamp measurement that enables super-resolution.
2Reliability
If delay line inverters are used, then the implementation is straightforward, but propagation speed changes cause normalization errors and increase phase noise
Solution Approach 1:
A feedback mechanism measures the actual propagation delay of the delay line inverters and uses this information to normalize the timestamps. The system continuously monitors the propagation speed changes and adjusts the normalization accordingly, compensating for process variations and environmental effects that would otherwise cause errors and phase noise.
Solution Approach 2:
The invention changes the parameter being measured from absolute time delay to normalized time delay by dividing by the measured propagation delay. This parameter transformation makes the measurement immune to propagation speed changes, eliminating the source of normalization errors and reducing phase noise in the PLL.
3Measurement precision
If multiple time-shifted signals are processed, then higher resolution is achieved, but the device complexity increases
Solution Approach 1:
Multiple DLTCs that process different time-shifted signals are merged into a unified timestamp generation system. The outputs of parallel DLTCs are combined through logical operations to produce a single high-resolution timestamp, reducing the complexity of having completely independent circuits while maintaining the resolution benefits of parallel processing.
Data Source
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AI summary
A time-to-digital converter (TDC) can have a resolution that is finer than the propagation delay of an inverter. In one example, a fractional-delay element circuit receives a TDC input signal and generates therefrom a second signal that is a time-shifted facsimile of a first signal. The first signal is supplied to a first delay line timestamp circuit (DLTC) and the second signal is supplied to a second DLTC. The first DLTC generates a first timestamp indicative of a time between an edge of a reference input signal to the TDC and an edge of the first signal. The second DLTC generates a second timestamp indicative of a time between the edge of the reference input signal and an edge of the second signal. The first and second timestamps are combined and together constitute a high-resolution overall TDC timestamp that has a finer resolution than either the first or second timestamps.