Abstract:
A signal responsive burst period timer and counter is provided for laser Doppler velocimetry, and the like, by bandpass filtering a signal burst and testing the filtered signal for exceeding a first level (V O  -V L ) and a second level (V O  +V L ), and subsequently crossing a reference level (V O ) from the second level, and in that order, to qualify each cycle of the signal burst for timing and counting. A timing system checks the reference crossing of every qualified cycle, in accordance with the level crossing logic criteria described above, to determine that it occurs within a prescribed time interval as measured from the previous qualified reference crossing. When the sequence ends, either because a predetermined number of qualified cycles have been received or because the signal fails to meet one of the amplitude or time criteria described above, the burst period counter stops, the data is read out and the counter resets itself to process the next burst.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates to a method and apparatus for measuring the period of an oscillatory signal burst containing only a limited number of cycles and occurring at a time which is random (and not known a priori). 
     An example of such a random signal occurs in making measurements of velocity using the Doppler shift in frequency of laser light scattered by a moving particle by heterodyning the scattered light with the unshifted light to produce a beat frequency burst at the optical receiver of the system. The phenomenon of Doppler shift in frequency is also utilized by many other systems that measure frequency to determine the velocity of a target, such as sonar and radar. 
     To appreciate the utility of this invention, consider how a traditional frequency counter measures frequency. A traditional frequency counter opens a gate G for a fixed length of time T (say one second) and counts the number of level crossings, of the same sense, of an input signal occurring during the time interval the gate is open (high). (See FIG. 1) This count, N, is then displayed as the frequency of the input signal in cycles per second (Hertz). 
     It is easy to see that such an instrument is of no use unless it can be guaranteed that the input signal oscillations continue for the whole interval that the signal is accepted by the gate. This proviso is, of course, not met by an oscillatory signal that occurs in bursts, at random times and for a variable number of cycles since a burst may not last for the whole counting interval. This problem is illustrated in FIG. 2 using the same gate of fixed time T. 
     It is the purpose of this invention to provide the means of measuring the frequency (period) of an input signal consisting of a single burst of an unknown number of cycles of a typical form shown in FIG. 2. A preferred embodiment of the invention was developed to provide the means of analyzing a laser Doppler velocimeter signal in the time domain. Consequently, the motivation for the various different modes of operation, the built-in flexibility of the first implementation of the idea and the available range of values of its operating characteristics (pulse widths, time intervals, etc.) are to be understood with that specific application in mind. It should be emphasized, however, that the utility of this signal processing method extends considerably beyond this purpose. The examples of sonar and radar have already been mentioned. Similar signals are also encountered in earthquake monitoring, where the time (duration) of the signal burst is also not known a priori, and in communications utilizing bursts of frequency modulated or pulse code modulated signals whose carrier frequency for one reason or another cannot be transmitted continuously. 
     The processing electronics of the invention should be viewed generally as possessing sufficient pattern recognition capabilities to be able to recognize the type of signal burst described above and respond to it for the purposes of measuring its average frequency and/or the instantaneous period on a cycle-by-cycle basis, and the real time of the event. It is for this reason that the processor that incorporates these features, a detailed description of which follows, is denominated a signal responsive burst period counter and timer, hereafter referred to simply as the system. 
     To achieve this goal, both the amplitude information and the time between successive crossings of zero, or some other predetermined level, are utilized in order to minimize the possibility that random noise will trigger the system and yield a reading in the absence of a valid signal burst. In fact this latter feature renders this device the ideal means to label an event in time electronically, which must be detected elsewhere, for which the penalty of error (false alarm) is very high. For example, one could code the event by a succession of pulses of a predetermined number and period, e.g., 32 pulses spaced individually 13.7±0.1 μsec apart. As will become apparent in the detailed description that follows, the system can be operated in a mode that will not produce a response even if any of the following, almost similar, events have occurred: 
     i. 31 pulses, or less, individually spaced by 13.7±0.1 μsec apart. 
     ii. 33 pulses, or more, individually spaced by 13.7±0.1 μsec apart. 
     iii. 32 pulses exactly, spaced 13.7±0.1 μsec apart on the average but having at least one pair spaced by a smaller or larger interval, with another pair, somewhere else in the pulse train that is appropriately spaced such that the average spacing is within the specified limits. 
     In order to introduce the motivating principles for the invention, a brief description of laser Doppler velocimetry fundamentals will be presented. There are two common optical arrangements used when making laser Doppler velocimetry measurements. One is the dual-scatter arrangement shown in FIG. 3 in which a laser beam 10 is split by a beam splitter 11 into two parallel beams 10a and 10b of equal intensity. A single focusing lens 12 will focus both beams and will force the two beams to cross (overlap) at the focus, F. The beams have finite beam widths and planar face fronts. Consequently, as the beams go through the overlap volume, the two beams can be considered as plane waves that can form an interference pattern on the surface of a square law detector 13, with linear fringe spacing, s, given by the equation 
     
         s = λ/2sin(θ/2) 
    
     where θ is the angle subtended by the two wave vectors that are normal to the planar phase fronts. 
     If a particle traverses the overlap volume, it will scatter light in all directions and, in particular, in the directions accepted by collection optics 14-16 and narrow band optical filter in front of the detector, a photomultiplier. There the Doppler shifted amplitudes that have been scattered out of the two beams will interfere to yield a photocurrent which is modulated at a frequency, ν D , given by the equation 
     
         ν.sub.D = v.sub.195 /s = 2v.sub.195 /λ sin (θ/2) 
    
     where v 195  is the component of the velocity in the plane of the two beams and perpendicular to their bisector. Since the distance travelled by the scattered light from the two beams is the same between the scattering particle and the photomultiplier, the phase difference of the heterodyning components can be computed equally well on the position of the scattering particle as it traverses the overlap volume. It is therefore equivalent to imagine that the particle is traversing a spatially modulated intensity field and that the photomultiplier sees a temporally modulated intensity whose frequency is given by the particle&#39;s velocity component perpendicular to the fringe planes, divided by the fringe spacing. It is in this sense that Eq. 2 is derived. In addition to the modulation at the Doppler frequency, the photocurrent is further characterized by a Gaussian envelope corresponding to the intensity distribution of the overlapping beams. The particular photocurrent patterns that result depend on the way the scattering particle has traversed the overlap volume. Three typical patterns are shown in FIGS. 4a-4c. 
     In the second optical arrangement, a reference-scatter arrangement, the laser beam 10 is again split into two parallel beams as shown in FIG. 5. However, in this case, one beam, the scattering beam 10a, has most of the laser power, while the other, the reference beam 10b, has a small fraction of the laser power. The two beams are focused on a common point, F, and meet in an overlap volume with approximately planar phase fronts. A scattering particle moving through this overlap volume scatters light from the scattering beam in all directions and, in particular, in the direction of the reference beam which is assumed to be normally incident on the surface of the photomultiplier 13. The photomultiplier thus sees two waves, the reference beam at the laser frequency ν o , and the scattered wave at a frequency ν o  +ν D . The Doppler shift ν D  is given by Eq. (2) where θ is the scattering angle (and also the angle subtended by the two beams). Thus the photocurrent, which is proportional to the incident intensity, is modulated at the beat frequency ν D  (between the two waves) for a time corresponding to the time of passage of a scattering particle through the overlap volume. Typical output patterns in this operating mode are shown in FIGS. 6a-6c. 
     In either case, if one can determine ν D , the modulation frequency within the burst, one can measure (to the same accuracy) the component of the velocity of the scattering particle that is in the plane of the two beams and perpendicular to their bisector (perpendicular to the fringe planes). This can result in a complete vector velocity measurement since, to measure a different component, one can simply rotate the beams as required. By this means one actually measures the velocity of the fluid in the common case where the particles move with the local fluid velocity. This can be achieved by processing as many input channels as necessary to achieve a simultaneous measurement of more than one component of the velocity at one or more locations in the flow. 
     OBJECTS AND SUMMARY OF THE INVENTION 
     An object of this invention is to provide a signal responsive burst period measuring system. 
     Another object of the invention is to qualify cycles of an oscillatory signal burst as to amplitude and time information which together afford sufficient pattern recognition capabilities so as to distinguish the burst cycles from any noise present. 
     Yet another object of this invention is to provide accurate and reliable means of starting and stopping the measurement of the frequency (period) of an input signal burst of an unknown number of cycles. 
     These and other objects of the invention are achieved by testing every oscillation of an input signal as to its amplitude in both its positive and negative excursions with respect to a reference and, having qualified a cycle as to its amplitude in both polarities, testing for an immediately following reference crossing. The reference may be set to circuit ground or to some voltage level of either polarity with respect to circuit ground. A burst period measurement is then made from one qualified reference crossing to the next. Separate voltage comparing means are provided for each amplitude test. 
     The circuitry defines two voltage levels symmetrically chosen about a third voltage level (V O ) which serves as the reference level. This results in a total of three levels placed at V O  +V L , V O , V O  -V L . The amplitudes V O  and V L  as well as the polarity of V O  can be chosen independently of each other and as required by the input signal characteristics. The qualifying logic requires that the signal excursions about V O  exceed the levels V O  -V L  and V O  +V L . Exceeding the V O  -V L  level causes one comparing means to emit a signal when the V O  -V L  level is crossed. When (and if) the V O  +V L  is subsequently crossed, the other comparing means initiates a signal indicating that both test levels have been exceeded in the proper sequence. A reference-crossing detection means then detects when the analog input signal crosses the reference level from a polarity of the second level. When a qualified reference crossing is detected, the signal indicating that all three qualifying test levels have been crossed is terminated, thus signaling the end of a cycle. The sense and polarity of all the level testing logic can readily be inverted, as by using an inverting or non-inverting signal amplifier at the input, as required. The first time a reference crossing is thus detected and signaled, a gating means is enabled to allow a main counter to begin counting clock pulses. Each subsequent reference crossing, qualified in accordance with the three level logic sequence described above, is tested to determine whether or not it occurs within a preselected time interval as measured from the next preceding qualified reference crossing. If it does, and generates a synchronous pulse, that pulse transfers the contents of the main counter, without stopping it, to a register. A second counter is provided to count the number of these transfer pulses. In this fashion, at any one time within each signal burst, the register contains the total time corresponding to the integral number of periods of cycles that have been qualified, while the second counter contains the number of periods that have been counted thus far. These data can be made available either during the signal burst, if cycle-by-cycle monitoring is required, or at the end of each signal burst if it is sufficient to compute the average period of the qualified cycles. A third counter measures time from some reference point in time. Whenever the gating means is enabled, the content of this third counter is read out to provide a basis for positioning of the corresponding burst in real time. 
     The novel features that are considered characteristic of this invention are set forth with particularity in the appended claims. The invention will best be understood from the following description when read in connection with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 illustrates a conventional frequency counter principle. 
     FIG. 2 illustrates the problem of using a conventional frequency counter for an oscillatory signal burst. 
     FIG. 3 illustrates schematically a dual-scatter laser Doppler velocimeter. 
     FIGS. 4a-4c illustrate three typical signal patterns of the velocimeter of FIG. 3. 
     FIG. 5 illustrates schematically a reference-scatter laser Doppler velocimeter. 
     FIGS. 6a-6c illustrate three typical signal patterns of the velocimeter of FIG. 5. 
     FIG. 7 is a block diagram of a system embodying the present invention. 
     FIG. 8 is a timing diagram illustrating one mode of operation (v o  =signal zero) of an analog input processor in the system of FIG. 7. 
     FIG. 9 is an exemplary implementation of the analog input processor. 
     FIG. 10a is a timing diagram illustrating the operation of the front end of a digital input processor in the system of FIG. 7, and FIG. 10b illustrates an exemplary implementation. 
     FIG. 11 is an exemplary organization of the digital input processor of FIG. 7. 
     FIG. 12a is a read-out timing diagram for the digital input processor of FIG. 11. 
     FIG. 12b illustrates the format of data read out of the digital input processor. 
     FIG. 13 is an exemplary organization of the timer in the system of FIG. 7. 
     FIG. 14 illustrates a modification of the system of FIG. 7 for reading out time history data on a cycle-by-cycle basis. 
     FIG. 15 is a timing diagram illustrating an alternate mode of operation (v o  &gt;v L  &gt;0) of the analog input processor in the system of FIG. 7. 
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
     In the description of a preferred embodiment that follows, it will be assumed that the input signal burst to be measured undergoes quasi-periodic and quasi-symmetric excursions about zero as shown in FIG. 2. The case of asymmetric bursts (e.g., FIG. 4) will be described later on. It will also be assumed that the signal has been amplified sufficiently by an appropriate bandpass amplifier 19 (shown in FIG. 3) to result in a sufficient peak-to-peak amplitude to clear the ±V L  settings as required. 
     The architecture of the signal processing burst period counter and timer is presented schematically in FIG. 7. An analog input processor 20 receives an analog input signal AIN shown in FIG. 8 from the amplifier 19 (FIG. 3) and generates a digital input signal DIN transmitted to a digital input processor 30. The latter signal is a TTL (transistor-to-transistor logic) compatible pulse train whose negative slopes are coincident with the negative slope zero crossings of the analog input. The user can specify through front panel settings 20a an amplitude level V L , to construct two test levels (-V L  AND +V L , indicated by dashed lines in FIG. 8) which must be exceeded by the analog input AIN (in both the positive and negative excursions) and followed by a zero crossing in the proper sense (direction) for a DIN pulse to be generated. it should be noted that the signal DIN remains at the bit 0 level until the positive slope of the analog input signal AIN exceeds the level +V L . The signal DIN then goes to the bit+1 level and remains there until a zero crossing occurs in a following negative slope of the signal AIN. The sequence of positive and negative slopes of the DIN signal described above is repeated with every qualified cycle of the analog input signal AIN. Should a zero crossing occur without the -V L  and +V L  levels being exceeded in amplitude, and in that order it, 
     i. is ignored if no DIN pulse has been generated yet from the burst, or it 
     ii. lowers a burst end signal (BE) to signify the end of the burst, but only if at least one DIN pulse has been generated. (The assumption has been made that the envelope of a valid input signal burst will rise monotonically to reach a maximum and then fall monotonically to zero.) 
     While the BE signal is low the analog input processor 20 is disabled and will ignore any input signal burst. In that manner DIN pulses are produced from a valid input signal burst, until the analog input signal AIN fails to cross the -V L , +V L  and zero levels in that order, whereupon the zero crossing sets the signal BE low to terminate operation of the digital input processor 30 in respect to counting DIN pulses from that burst. The signal BE is reset by the negative slope of a GO signal (shown in FIG. 8) from the digital input processor 30 which signifies that the digital input processor has been disabled and is transmitting the burst count to recording and display devices (not shown). 
     The analog input processor 20 remains disabled during the time the GO signal is low and is only enabled when the GO signal goes high (output transmission complete, system reset and ready). The GO signal then enables the analog input processor for it to again respond to a burst, but only if the signal AIN exceeds the levels -V L  and +V L  and in that order. An exemplary circuit for such an analog input processor is shown in FIG. 9. It has been tested with signal bursts of frequencies from a few hundred Hertz to 15 Megahertz. 
     Operation of the circuit shown in FIG. 9 is as follows. A high-pass filter 21 is provided with a switch, SW, on the front panel 20a to offset the zero reference, such as to process signals resulting from the dual-scatter mode as shown in FIG. 4a, as will be more fully described with reference to FIG. 15. For the reference-scatter mode, no offset is required and the switch is set in the position shown. The two levels -V L  and +V L  are thus symmetrically chosen about zero (circuit ground). For a valid zero crossing, it is required that the input waveform exceed both of these levels -v L  and +v L , and in order just prior to a zero crossing. 
     The signal AIN from the filter 21 is applied directly to the inverting input terminal of a differential amplifier 22 for comparison with the level -V L  set in a potentiometer 23 as a reference level +V L  and inverted by an operational amplifier 24. When the level -V L  is exceeded in amplitude by a negative excursion of a burst, a flip-flop FF 1  is set. It remains set until reset by the output of a differential amplifier 25 which compares the analog input AIN with the positive reference +V L . The trailing edge of a pulse thus produced clocks a J-K flip-flop FF 2 . Its Q output is inverted by a NAND gate G 1  to initiate a pulse in the digital input signal DIN. The pulse will persist until the flip-flop FF 2  is reset by a zero-crossing detector comprised of a differential amplifier 26. Clamping diodes D 1  and D 2  provide one sided hysteresis for the zero-crossing detector. 
     The output of the zero-crossing detector is a positive pulse inverted by a NOR gate G 2 . The leading edge of the resulting negative pulse from the gate G 2  clocks a flip-flop FF 3 . The Q output of that flip-flop then drops to a low level and clears the flip-flop FF 2 . It also clears the flip-flop FF 3  via NAND gates G 3  and G 4 . At the same time, the output of the flip-flop FF 3  clocks a flip-flop FF 4 , thus storing a bit 1 in the flip-flop FF 4 . Note that at least one input to a NAND gate G 5  is a bit 0, both before and after the flip-flop FF 2  is reset. Consequently, the output of the NAND gate G 5  remains at a bit 1 level. A NAND gate G 6  inverts that signal to a bit 0. Consequently, a flip-flop FF 5  will remain reset such that its Q output will continue to be high. That Q output is fed back as the signal BE to the flip-flop FF 2  to enable it to again be set when the levels -V L  and +V L  are again exceeded, and in that order. Subsequent pulses are thus produced in the output signal DIN, one pulse for each cycle of the analog input in which the levels -V L  and +V L  are exceeded. Each pulse terminates at the negative-slope zero crossing. Consequently, the periods between the trailing edges of DIN pulses correspond accurately to the periods of the burst cycles. 
     When a cycle of the burst fails to exceed the level and or the level +V L , and in that order the flip-flop FF 1  will not trigger the flip-flop FF 2  to initiate another output pulse in the signal DIN. Consequently, when the next zero crossing is detected, both input terminals of the NAND gate G 5  will be at the logic 1 level, so when the flip-flop FF 3  is clocked by the next output of the zero detector, the flip-flop FF 5  is set, thus driving its Q output (the signal BE) low. The flip-flop FF 5  remains set to disable the circuit until a signal GO clears the flip-flop FF 5 . Note that the signal GO is high to enable the NAND gate G 4  until the digital input processor drives the signal GO low. The negative slope of the signal GO then resets the flip-flop FF 5  to drive the signal BE high. The signal BE thus signifies that a burst has ended when it goes low. 
     The digital input (DIN) pulses are sent to the digital input processor 30 (FIG. 7). There the total time interval corresponding to a fixed number of valid zero crossings (i.e., zero crossings following -V L  and +V L  crossings, and in that order, and passing time tests described below), or the total time interval and total number of valid zero crossings within a single burst can be determined. Assuming that the operating mode is the latter of the two, the processing sequence is as follows. 
     The negative slope of the DIN pulses fires a one-shot TI shown in FIG. 10b. The period τ 1  of that one-shot is set on the digital input processor front panel 30a and sets a flip-flop to raise a gate signal G shown in FIG. 10a. That gates a 100 MHz crystal controlled clock from a source 31 into a main counter 32, shown in FIG. 11. The negative slope of each pulse from the one-shot TI fires a second one-shot T2 whose period τ 2  is also set on the front panel 30a. The next negative slope of DIN refires the one-shot TI, refires the one-shot T2 and generates a transfer pulse TRP, provided it occurs during the period τ 2 . The TRP pulse transfers the contents of the main counter 32 into a main buffer 33 and is also counted by an event counter 34. In that manner the number in the main buffer is equal to the total number of clock pulses that have occurred in between the time the gate signal G went high and the positive slope of the first TRP pulse, which is a measure of the duration of the first valid period of the signal burst. 
     FIG. 10b illustrates an exemplary embodiment of front end logic 35 shown in FIG. 11 for generating the gate signal G and the TRP pulses shown in FIG. 10a. A flip-flop FF 6  is set by a signal SET GO from reset and output logic 40 (FIG. 11) at the end of an output hold period in response to a signal EXT. OUTPUT HOLD. Once the signal GO is high, a flip-flop FF 7  is enabled to be set by the negative slope (trailing edge) of DIN pulses. A delay element δ 1  couples the Q output of the flip-flop FF 7  to its clear (CLR) input so that it resets itself. The result is a short positive pulse (of a duration set by the delay element) transmitted from the Q output terminal that sets a flip-flop FF 8  to raise the signal G. 
     Each pulse from the flip-flop FF 7  triggers the one-shot T1. The one-shot T1 then triggers the one-shot T2 at the end of its period τ 1 . Once the one-shot T2 is triggered, it begins to time out its period τ 2 . The Q output of the one-shot T2 enables a flip-flop FF 9  to be set by a DIN pulse via flip-flop FF 7 , provided it occurs within the period τ 2 . A delay element δ 2  couples the Q output of the flip-flop FF 9  to its input CLR to reset it. The result is a TRP pulse as shown in FIG. 10a. 
     Each time the flip-flop FF 7  emits a pulse in response to a DIN pulse, the Q output of that flip-flop clears (restores to its stable state) the one-shot T2 via an AND gate G 8  which has both of its input terminals normally high. In that manner the leading edge of a TRP pulse occurs substantially in time coincidence with the trailing edge of a DIN pulse that coincides with the zero crossing of a cycle qualified by the analog input processor 20 (FIG. 7). 
     If another DIN pulse is not received before the one-shot T2 times out, reset logic generates a negative pulse on the line labeled RESET to reset the flip-flops FF 6  and FF 8 , thus lowering the signals G and GO, as shown in FIG. 10a, to reset the main and event counters. That reset logic essentially operates on another timer in reset and output logic 40 (FIG. 11) triggered by TRP pulses. If allowed to time out for lack of a following TRP pulse, the Q output of that timer sets a resetting flip-flop to generate at its Q output terminal a negative RESET pulse. That timer and flip-flop are shown in FIG. 10b as a one-shot T3 and a flip-flop FF 10  with a delay element δ 3  for the sake of completeness. In the exemplary embodiment of FIG. 11 this reset logic is contained in the reset and output logic 40 in order that mode control logic 38 be employed to sometimes generate the RESET pulse even before the last TRP pulse of a burst, as when only a predetermined number of cycles of a burst are to be timed. In that event the reset logic would be implemented differently, with the function of the reset logic shown in FIG. 10a provided in a different way for one of two alternative modes by the processor sensing when T2 goes low without T1 being triggered. In the second mode, mode logic 38 would determine when the specified number of TRP pulses have been counted by the event counter 34, and would then initiate the generation of the RESET pulse. 
     In this manner, an input waveform with an instantaneous period τ is only processed if 
     
         τ.sub.2 &lt;τ&lt;τ.sub.1 +τ.sub.2 
    
     The periods τ 1  and τ 2  can be conveniently set using control knobs indicated generally by block 36. 
     Each TRP pulse will update the contents of the main buffer 33 with the contents of the main counter 32 at the time, and increment the event counter. This process continues (unless a negative slope of DIN occurs outside the τ 2  interval) until the end of the pulse sequence. When the gate signal G is lowered, which in turn lowers the signal GO, the whole system is disabled until GO is again set high, as by a manually operated push button or by some automatic system, at which time the counters 32 and 34 are reset. In this exemplary embodiment, the signal GO is controlled by an external output hold signal that is low until data transfer to recording and display devices is complete, and is then set high. Upon being set high, the reset and output logic 40 resets the counters and emits a &#34;set GO&#34; signal that sets the GO signal high again. While the signal GO is low, it not only disables the gate G 4  (FIG. 9) but also disables the comparator 25 from resetting the flip-flop FF 1 . That is done via a NOR gate G 7  and transistor Q 1 . When the GO signal is set high again, it releases the comparator 25, presets the flip-flop FF 3  with its Q output high, and clears the flip-flops FF 4  and FF 5  as shown in FIG. 9. 
     Lowering the signal GO generates the readout commands (PC1, PC2, PC3) which flag the data transfer (words OUT 1, OUT 2 and OUT 3, respectively) to recording and display devices as shown in FIG. 12a. FIG. 12b shows the format of the output data, OUT 1, OUT 2 and OUT 3. The OUT 3 data is the content of a timer counter 51 loaded into the timer buffer 52 of the timer 50 (FIG. 13) by the positive slope (leading edge) of each TRP pulse. This permits the time of each cycle of a burst to be read out in a manner described hereinafter with reference to FIG. 14 in order to reconstruct the burst, or determine its frequency (period) on a cycle-by-cycle basis. 
     In alternative modes of operation, a specified number of zero crossings can be set on the front panel 30a (FIG. 7) through knobs indicated generally in FIG. 11 by block 37, and the burst data will only be transmitted to the display and recording devices if a burst contains a number of cycles with valid zero crossings. 
     i. less than or equal to the specified number 
     ii. equal to the specified number, or 
     iii. greater than or equal to the specified number as determined by mode logic 38. The choice between the three is also made on the front panel 30a. 
     The digital input processor shown in FIG. 11 provides a 10 MHz square wave (100 MHz ÷ 10) via a divider 39a and the inverse of G (G) via an inverter 39b. There are used by the timer 50 (FIG. 13) to record the real time of the first valid zero crossing. Since the processor does not know in advance that the subsequent burst characteristics will pass all the imposed tests, the real time of the first valid zero crossing is always transferred from a timer counter 51 to a timer buffer 52. However, it is only transmitted to the recording devices using the output command PC3 shown in FIG. 12a. In this manner both the real time in buffer 52 and the total time interval of a succession of zero crossings in the main buffer 33 is recorded, and it is possible to reconstruct from that data the time history of the average valid zero crossing periods. If a time history on a cycle-by-cycle basis is desired, the content of the main buffer 33 could be read out in response to a PC1 command after each TRP pulse generated as shown in FIG. 14. 
     Referring now to FIG. 14, simple AND gates G 10  and G 11  may be employed with some suitable pulse delay means, such as a one-shot (monostable multivibrator) T4, to provide for reading out the main buffer after every TRP pulse. A mode control signal on a line 60 enables the AND gate G 10  to transmit each TRP pulse for this mode of operation. The negative slope (trailing edge) of each TRP pulse thus gated triggers the one-shot 60 to emit a pulst TRPDT which is essentially the TRP pulse delayed and inverted. Both the normal PC1 line shown in FIG. 14 as PC1* and the TRPDT line are normally high so that when either goes low a PC1 command is transmitted from the gate G 11 , which is when the normal PC1* line goes low at the end of a burst, and each time a TRP pulse triggers the one-shot T4 while the mode control line 60 is high. When not in this mode, i.e., when the mode control signal on the line 60 is low, only the normal print command signal PC1 is transmitted by the gate G 11 . 
     It should be noted that the 10 MHz clock is divided by a variable divider 53 set by switches in a front panel 54. In that manner the unit of time counted by the timer counter 51 can be selected for the particular application and operating environment. The time counter is reset to zero by INITIAL RESET when the system is initialized so that the real time counted and recorded is relative to the last time the system was initialized. It is with reference to that time that real time is measured. 
     To process one-sided signal excursions, like the output from the laser Doppler velocimeter operated in the dual scatter mode as described with reference to FIG. 3, the zero of the analog input processor from a bandpass amplifier 19 can be offset by an amount V o  through setting the switch SW shown in FIG. 9 in its alternate position. The logic of the amplitude discrimination is then the same, except that the level comparisons are now done on V O  -V L  and V O  +V L  as shown in FIG. 15. 
     From the foregoing it is evident that a signal responsive burst period counter and timer is made possible through the operation of the analog input processor in lowering the signal BE at the end of a burst, and in recognizing the beginning and end of each burst. Exemplary modes of use have been described. Other modes will occur to those skilled in the art. Applications and operating environments other than in laser Doppler velocimeters will also occur to those skilled in the art. It is therefore intended that the claims be interpreted to cover such other modes, and such other applications and operating environments. For instance, in some operating environments, the levels +V L  (or V O  -V L ) and -V L  (or V O  -V L ) may be tested in that order, and the zero (or reference) crossing detection that follows would be on the positive slope of the analog input signal. The output DIN of the analog input processor may then be used by a digital input processor in a variety of ways as described since each pulse of the output DIN occurs with its negative (positive) slope at its end in what is effectively the precise real time of a negative (positive) reference crossing of a qualified cycle in a burst, i.e., in a cycle which satisfies amplitude criteria. Any time delay between the actual reference crossing of a cycle and the end of the DIN pulse produced will be quite small and constant from one reference crossing to the next. Consequently, the periods between the ends of successive DIN pulses will correspond accurately to the periods of corresponding cycles in the burst. How these pulses are then used will depend upon the users requirement and operating environment.