Timing circuit

Details Timing circuit Timing circuit

2000-06-30

2002-08-13

Lam, Tuan T. (Department: 2816)

Electrical pulse counters, pulse dividers, or shift registers: c

Applications

Measuring or testing

Reexamination Certificate

active

06434211

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a timing circuit for timing a delay between events. The invention is suited to the analysis of arrival times between pairs of events, and the analysis and storage of continuous data streams. The invention is particularly suited to photon correlation spectroscopy measurements,
2. Related Art
Analysis of signals that vary in a characteristic manner may be carried out by various methods including: real-time digital electronic correlation, storage of a signal data stream (with later analysis by hardware/software), single stop techniques, multiple stop techniques, gating circuits and Fourier Transform of the signal.
A simple method of analysing a photon stream uses coincidence detection, where two detectors are arranged to detect photons arriving at a predetermined fixed delay (Oliver C. J., 1973, Correlation Techniques, Photon Correlation and Light Beating Spectroscopy, pages 41-74, Ed. Cummins H. Z., Pike E. R., Plenium Press NY, ISBN 0-306-35703-8). This technique allows detection of photons with a bandwidth of the order of 1 GHz (Moreno, F., Gonzalez F., Lopez R. J., Lavin A., 1988, Time-interval statistics through a Lapace transform method in quasi-elastic light-scattering experiments for low-intensity levels, Opt. Soc. Am. Vol. 13, pages 637-639). The delay between detectors and the intensity of a source of the photons must be adjusted such that there is a negligible probability of two photons arriving with a separation less than the delay time between the detectors This method effectively gives a photon correlation value for a single delay time, and requires experiments of long duration. Some improvement in operational speed may be gained by using multiple channels to measure a time interval between pairs of photons, although this will lead to distortion of the correlation.
The distortion introduced by multi-channel single stop measurements can be reduced by using multi-stop techniques, whereby a number of concurrent photons may be detected. Data collection is most effective when a period of recording is initiated by an incident photon Distortion of data is only eliminated when Gaussian sources of light are used.
Multi-stop apparatus conventionally includes time-to-amplitude converters and pulse storage devices which provide very high speed responses but are expensive. Attempts have been made to substitute these components with standard microprocessors. A multi-stop apparatus based on a hardwired computer has been produced which allows 3500 consecutive sample periods to be stored prior to analysis, although at a sampling rate limited to 0.1 MHz (Hallet F. R., Gray A. L., Rybakowski A., Hunt J. L., Stevens J. R., 1972, Photon correlation spectroscopy using a digital PDP-9 computer, Canada J. of Phys., Vol. 50 pages 2368-2372). A later version of this apparatus stored only arrival times using an 8085 processor, and was capable of operation at 1 MHz (Subrahnanyam V. R., Devraj B., Chopra S., 1987, Microprocessor based photon correlator for intensity fluctuation studies, J. Phys. E, Sci. Instrum, Vol 20 pages 340-343).
A significant drawback of multi-stop or non-stop timing circuits when used for high resolution analysis is the magnitude of raw data produced. For example, assuming a signal of 10
4
events per second and a required resolution of 1 ns, the mean number of clock pulses between events will be 10
5
. Even where events spaced by more than 2000 ns are ignored, the rate of clock cycles per second is 20×10
6
. In many applications, experimental durations range from 30 seconds to a few minutes, and problems are likely to arise due to the magnitude of data to be stored, and the processing power and/or time required to process results. This limitation is avoided by real-time correlation and pulse arrival distribution analysis techniques.
Correlators do not record the time elapsed between each of a sequence of events, but instead provide a record of the distribution of times separating consecutive events. This is done by defining a number of channels for different separation times, then incrementing a counter located at a relevant channel when an event separated by a given time from a previous event is recorded. Since correlators do not store the sequence of events, a significant reduction of the data to be stored is achieved. A drawback of correlators is that later re-analysis and/or further digital signal processing of the sequence of events is not possible since the sequence itself is not stored.
Where the correlation is measuring a signal that gives a reducing gradient with correlator delay time (i.e. an exponential or mixture of exponentials as is often the case in light scattering) it is common to space the correlator channels in a logarithmic or similar fashion (each channel spacing being double the last is often a convenient implementation in electronics). The data points at longer delay times generally exhibit greater relative errors and are given less weighting in the final fit, although all data points are measured with equal resolution prior to being transferred to a channel.
Real-time electronic digital correlators suffer from significant disadvantages. All parts of the circuitry of a correlator must operate at the shortest correlator delay time, since no data compression occurs on the data stream. This generally makes high speed correlation expensive, and practical limitations suggest hardwired electronic correlators operating above 50 MHz are not economically feasible for most applications.
On initialisation the correlator must load a sample of data equivalent to the number of channels prior to reseting the accumulators (effectively discarding this information) to operate with minimum bias/error. This is a limitation on correlator speed (to allow resetting the accumulators during a single sample time), as well as final correlator length and/or minimum experiment duration. Where the accumulator is not reset in a single sample time after prefilling significant errors or bias may be introduce particularly for short experiments and/or correlators with many channels.
A burst correlator is capable of allowing only pulse arrivals within a limited number of delay times to be detected per experiment, the number of delay times being determined by the number of channels available. This simplified arrangement allows fast correlators to be produced which operate at speeds of around 100 MHz. Whilst burst correlators operate almost in real time, an average of many results is required to carry out a reasonably accurate measurement. Burst correlators allow for rapid decays to be measured, although still requiring costly hardware, and their speed of operation is fundamentally limited by the time required for multiplication/addition processes to be performed.
Burst correlators may suffer significantly from the prefill error discussed above. Burst correlation is highly inefficient in terms of data collection as data must be read out and the correlator reset after a number of sample periods equivalent to the number of channels that have been collected.
A correlator based upon parallel processing using standard transputer boards has been developed (Bruge, Biagio, Fomrnili, 1989, New photon correlator design based on transputer array concurrency, Rev Sci Instrum, Vol. 60, No. 11, page 3425), which has similar operating characteristics to specialised hardwired commercial equipment Real-time electronic correlation is however still limited in terms of speed and cost.
SUMMARY OF THE INVENTION
It is an object of the present invention to overcome or substantially mitigate the above disadvantages, and thereby provide an apparatus capable of timing intervals between pulses in an efficient mariner.
According to the invention there is provided a timing circuit for recording the duration of intervals between a plurality of events in a data stream, comprising at least two timing channels, each arranged to generate a signal representing time elapsed between events, wherein the rate of change of the signal gener

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