GB2086591A - A periodic averaging correlator - Google Patents

Abstract

The correlator is arranged to establish when the time constant of a repetitive time-varying function x(t) reaches a predetermined level in response to variations in one of the factors of the function x(t). The correlator includes an integrator (16) and an analogue store (24) for periodically sampling and storing the output level of the integrator. A switching assembly (6) is switched by a control circuit (28) to correlate the function x(t) with a function y(t) by feeding in succession to the integrator the sum of the time varying function x(t) and the stored voltage level, and the sum of the inverted time-varying function x(t) and the stored voltage level. The achievement of a peak in the stored voltage level will indicate when the time constant function x(t) reaches the predetermined level. By repeated operation of the switching assembly an averaging action takes place so as to eliminate noise. In addition the level of the stored signal at its peak value will be directly proportional to the amplitude of the function x(t). <IMAGE>

Description

SPECIFICATION A periodic averaging correlator The present invention relates to periodic averaging correlators.

According to the invention, there is provided a correlator for establishing when the value of the time constant (T) of a repetitive time-varying function x(t), in which the time constant is progressively varied, reaches a predetermined value TO, the correlator comprising an input terminal for receiving the time-varying function x(t), a store for storing a voltage level, an integrator, and control means for correlating each occurrence of the function x(t) with a step function y(t) having positive and negative areas of equal magnitude, and for feeding the sum of the stored voltage and the correlated waveform x(t) . y(t) to the integrator, the control means feeding the result of each integration to the store to replace the voltage value stored by the store.

According to the invention, there is further provided a correlator for establishing when the time constant of a repetitive time-varying function x(t) of variable time constant reaches a predetermined value, comprising an input terminal for receiving the time-varying function x(t), a store for storing a voltage level, an integrator, control means for cyclically feeding the integrator with the sum of the function x(t) and the stored voltage during a first part of each cycle, with the sum of the inverse of the function x(t) and the stored voltage during a second part of each cycle equal in duration to the first part, and with an invariable signal during a third part of each cycle, and during the third part of the cycle replacing the level of voltage stored by the store with the instantaneous level of the output voltage from the integrator.

According to the invention, there is yet further provided a method of establishing when the value of the time contant of a repetitive time-varying function x(t) reaches a predetermined value upon varying a factor of the function x(t), the method comprising the step of storing a voltage level, and the step of processing the function x(t) on each occurrence thereof, then integrating the processed function, the processing step including combining the function x(t) with a step function y(t) having portions of opposite polarity but with an overall integrated value of zero, integrating the sum of the stored voltage level and the combined function, and at the integration replacing the stored voltage with the result of the integration.

According to the invention, there is still further provided a method of establishing when the time constant of a repetitive time-varying function x(t) of variable time constant reaches a predetermined value, comprising on each occurrence of the function x(t) processing the function and then integrating the processed function, the processing step including combining the part of the function occurring during a first time period with the instantaneous integrated level at the end of the integration of the immediately preceding function x(t), and combining the inverted function x(t) during a second period with the same said instantaneous integrated level.

Periodic averaging correlators embodying the invention will now be described by way of example with reference to the accompanying diagrammatic drawings, in which: Figure lisa block diagram of one of the correlators; Figures 2 and 3 are waveform charts of signals appearing at various points of the correlator of Figure 1; Figure 4 is a chart of different correlation functions; Figure 5 is a block diagram of another of the correlators; and Figure 6 is a waveform chart of signals of the correlator of Figure 5.

Imperfections or traps in semiconductor materials provide a capacitive effect when the semiconductor is subjected to an electrical field. Thus, when a semiconductor sample is subjected to a voltage pulse the sample exhibits a time-varying capacitive effect. The effect foilows an exponentially-decaying law and the decay rate or time constant will vary as a function of temperature. In order to establish the nature of the traps in the semiconductor there is a need to make measurements at different temperatures in order to establish a match with a known standard.

The match is achieved by cross-correlating the output waveform x(t) of the sample with a selected known waveform y(t) having the same periodicity (T) as the waveform x(t).

The result C of the correlation can be expressed by the equation:

The function x(t) is exponential and can be expressed by the equation: x(t) = A exp (-V) + n(t) (2) where n(t) is a noise function A is the maximum amplitude of the function x(t) and T is the time constant of the function x(t).

By selecting the function y(t) to be in the form of a bipolar pulse having a positive period At of from time to tot1 and a negative period At t of the same duration from time t1 to t2 then, provided the period to tot2 falls within the cycle period T, equation 1 (assuming the absence of any noise i.e. n(t) = 0) can be written as c = A (tIT) exp (-toIT).(1 -exp(- AtIt)2 (3) When noise is present, i.e. when n(t) + o, then if the correlation is repeated over a number of cycles and averaged, the noise element will be eliminated.

It will be appreciated that if the value of the time constant varies (as result of varying the temperature of the semiconductor sample) the value of C will pass through a maximum.

Thus the maximum value of c (cmax) will identify the temperature at which the time constant of the transient x(t) matches a reference value TO.

If to is arranged to be equal to 0 then it can be readily shown that to = 4At15 (4) it can also be shown that Cpeak = (0.4)aha where Aa =7M= a constant and creak is the peak value of c.

The described cross correlation thus allows an experimenter making repetitive measurements of the function x(t) for a semiconductor sample while slowly and progressively varying the temperature of the sample to determine the temperature at which the rate constant of the function reaches a predetermined value To and also to measure the maximum amplitude A of the function while amplitude is directly proportional to the value of peak.

It will of course be appreciated that the function y(t) must be selected in accordance with the value of the predetermined time constant z0, see equation (5).

A correlator for correlating the waveforms x(t) and y(t) using a selected function y(t) is shown in Figure 1.

The correlator shown in Figure 1 includes an input terminal 2 for receiving an input signal x(t). The input terminal 2 is connected through a resistor4 to a first switch S2 of a switching assembly 6. The input terminal 2 is also connected through a resistor 8 to an inverting amplifier 10 with feed-back resistor 12.

The output of the inverting amplifier is connected through a resistor 14 to a second switch S, of the switching assembly 6. The outputs of the switches S, and S2 together with a third switch Sf of the assembly 6 are connected to a common input of an integrator 16. The integrator 16 is in the form of a differential amplifier 18 having a capacitor 20 connected between the output and common input and a resistor 22 connected between the other input and earth.

The output of the integrator is connected to an input of an analogue store 24. The output of the analogue store 24 is connected through a resistor 26 to the switch Sf of the switching assembly 6.

A control circuit 26 provides trigger pulses for triggering the excitation of the semiconductor sample and control pulses for the switching assembly 6 and the analogue store 24. The control circuit operates in cycles of period T and during each cycle produces four distinct pulses as will now be described with reference to Figure 2.

During each cycle from time t = 0 to time t = to no pulses are produced. Thereafter a first pulse is produced from time to tot1 to close the switch S2 (see waveform A), a second pulse is produced from time t1 tot2 to close the switch Si (see waveform B) and a third pulse is produced from time to to t2 to close the switch Sf (see waveform C). Finally a trigger pulse is produced between time t = t2 and t = t' to trigger the analogue store 24 (see waveform D).

When the analogue store 24 receives a trigger pulse it stores the value of the voltage from the output of the integrator and thereafter supplies a voltage of the same level at its output until a new trigger pulse is received.

In operation it will be seen that from time to tot2 the switch Sf is closed and the voltage Vf stored by the analogue store 24 contributes to the integration of the function x(t) during the period to tot1 and the inverted function of x(t) during the period t1 tot2. From time t2 in the first cycle until time to in the next cycle the output Vf' from the integrator does not change. Within this interval is generated the trigger signal which will cause the store to store the new output voltage Vf'. Thus the new output voltage Vf' will contribute to the integration in the next cycle.

The voltage difference Vf' - Vf between the voltages generated at the output of the store during the final period t2 tot' of successive cycles of the control circuit can be expressed by the equation

where At is the duration for which each switch S2 and S is open; Rf is the resistance of the resistor 26; C is the capacitance of the capacitor 20; and R is the value of each resistor 4 and 14.

This equation can be resolved into the form:

where T = Rf.C and c is given by equation (3).

After a plurality of cycles at each temperature of the sample, in which due to averaging any noise factor is eliminated, the output Vf of the analogue store can be expressed by the equation:

Thus by plotting the value of Vf versus temperature an experimenter can determine from the plot not only at what temperature the predetermined time constant To is reached (the point at which c reaches creak) but also from the value of C peak the maximum amplitude of the transient x(t), given that all the constants as known.

It will be appreciated that other forms of the function y(t) can be used to achieve a similar result.

Forms y'(t); y"(t) and y"(t) are shown in Figure 4. In each case it will be noted that the amplitude and duration of the positive and negative sections of the waveform are selected so that the total area under the positive sections is equal to the total area under the negative sections.

In the correlator shown in Figure 5, parts similar to those in Figure 1 are similarly referenced.

In Figure 5 the control circuit 26 is programmed to switch the switch assembly in accordance with the correlation function y'(t) of Figure 4. Thus instead of just three time switching instants as in the Figure 1 correlator, there are now four time switching instants, to,t1,t2 and t3, to produce two positive-going pulses and one negative one, the negative pulse having twice the amplitude of the positive pulse.

The waveforms A, B and C of Figure 6 illustrate the switching sequence of the switches Si,S2 and S1. In orderto simulate the larger amplitude of the negative-going pulse, the resistances R1 and R2 of the resistors 14 and 4 are so selected that

Claims (11)

1. A correlator for establishing when the value of the time constant (t) of a repetitive time-varying function x(t) in which the time constant is progressively varied, reaches a predetermined value TO, the correlator comprising an input terminal for receiving the time-varying function x(t), a store for storing a voltage level, an integrator, and control means for correlating each occurrence of the function x(t) with a step function y(t) having positive and negative areas of equal magnitude, and feeding the sum of the stored voltage and the correlated waveform x(t).y(t) to the integrator, the control means feeding the result of each integration to the store to replace the voltage value stored by the store.

2. A correlator according to claim 1, wherein the function y(t) comprises a positive and a negative step, the steps being of equal magnitude and duration.

3. A correlator for establishing when the time constant of a repetitive time-varying function x(t) of variable time constant reaches a predetermined value, comprising an input terminal for receiving the time-varying function x(t), a store for storing a voltage level, an integrator, control means for cyclically feeding the integrator with the sum of the function x(t) and the stored voltage during a first part of each cycle, with the sum of the inverse of the function x(t) and the stored voltage during a second part of each cycle equal in duration to the first part of each cycle, and during the third part of the cycle replacing the level of voltage stored by the store with the instantaneous level of the output voltage from the integrator.

4. A correlator according to any one of claims 1 to 3, wherein the store comprises an analogue store having an input connected to the output of the integrator and a trigger input responsive to a trigger signal from the control means to replace the stored voltage level with the output voltage from the integrator.

5. A correlator according to any preceding claim, wherein the control means comprises a switching assembly operative selectively to feed said function x(t), the inverse of the function x(t) and the stored voltage level to a common input of the integrator, and a control circuit for generating switching pulses to be fed to the switching assembly.

6. A correlator according to any preceding claim, wherein the integrator comprises a differential amplifier and an integrator capacitor connected between the output and the input of the amplifier.

7. A correlator according to any preceding claim, including an inverter connected to said input terminal operable to invert said function x(t).

8. A method of establishing when the value of the time constant of a repetitive time-varying function x(t) reaches a predetermined value upon varying a factor of the function x(t), the method comprising the step of storing a voltage level, and the step of processing the function x(t) on each occurrence thereof, then integrating the processed function, the processing step including combining the function x(t) with a step function y(t) having portions of opposite polarity but with an overall integrated value of zero, integrating the sum of the stored voltage level and the combined function, and at the integration replacing the stored voltage with the result of the integration.

9. A method of establishing when the time constant of a repetitive time-varying function x(t) of variable time constant reaches a predetermined value comprising on each occurrence of the function x(t) processing the function and then integrating the processed function, the processing step including combining the part of the function occuring during a first time period with the instantaneous integrated level at the end of the integration of the immediately proceding function x(t), and combining the inverted function x(t) during a second period with the same said instantaneous integrated level.

10. A correlator substantially as hereinbefore described by way of example with reference to Figures 1 to 4 of the accompanying drawings.

11. A correlator substantially as hereinbefore described by way of example with reference to Figures 5 and 6 of the accompanying drawings.