Abstract:
Spurious pulses are eliminated in the output of a comparator performing analog to digital conversion by addition of logic which eliminates pulses having a width less than a selected width from the output of the comparator.

Description:
This invention was made with support under Prime Contract #F04701-95-C-0017 awarded by the U.S. Department of the Air Force, HQ Space and Missile System Center (AFMC). The Government of the United States of America has certain rights in this invention. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The subject invention relates generally to analog to digital signal conversion and more particularly to a method and apparatus operable on the output of a comparator to provide increased accuracy in the face of noise. 
     2. Description of Related Art 
     As illustrated in FIG. 1, conversion of an analog signal to a digital pulse train normally employs a comparator  11  and various analog signal processing such as analog pre-filtering  13  combined with hysterisis  15 . Noise may be introduced at the input to the comparator  11 , as symbolically represented at adder  17 , or after analog pre-filtering, as symbolically represented at adder  19 . Errors in the output pulse train result when such noise-induced error exceeds any hysterisis processing and filtering contained in the comparator  11 . In cases where an extremely high pulse rate is required, such noise compromises the bandwidth and phase of the analog to digital conversion performed by the comparator  11 . 
     SUMMARY OF THE INVENTION 
     According to the invention, a pulse filter is provided which prevents a pulse having a width less than a specified time duration from propagating into the pulse train at the output of the comparator. The invention operates on the output of any comparator circuit and provides a minimum pulse width rejection of any pulse which may result from noise that sets or resets the comparator. This pulse width rejection can be programmable or designed directly into the hardware/software/firmware such that a pulse less than a specified time duration will not be allowed to propagate into the pulse train. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     An illustrative embodiment of the just summarized invention will now be described in detail in conjunction with the drawings of which: 
     FIG. 1 is a block diagram illustrating prior art comparator circuitry; 
     FIG. 2 is a circuit schematic of circuitry according to the preferred embodiment of the invention; 
     FIG. 3 is a waveform diagram usefull in illustrating operation of the preferred embodiment. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Logic according to the preferred embodiment is illustrated in FIG.  2 . While such logic is illustrated in the form of digital logic componentry, those skilled in the art will appreciate that such logic may be readily implemented in other forms such as software or firmware. 
     According to the diagram of FIG. 2, the output signal S 1  of a comparator  21  provides an input to an inverter amplifier  23 , a first delay element Q 1 , and an AND gate  25 . The AND gate  25  receives second and third inputs from the output of the first delay element Q 1  and the output of a second delay element Q 2 , respectively. 
     The output of the first delay element Q 1  and is also input to the second delay element Q 2 . Thus, two stages of several delay are provided in the embodiment of FIG.  2 . 
     A second AND gate  27  receives the output of the inverter  23  as a first input, and the respective inverted outputs of the delay elements Q 1  and Q 2  as second and third inputs. The output of the first AND gate  25  is supplied to the J input of JK flip-flop F/F, while the output of the second AND gate  27  is fed to the K input of the flip-flop F/F. The Q output of the flip-flop F/F forms the output of the circuit and is shown being supplied to a synchronous counter  33 . The delay elements Q 1  and Q 2 ; JK flip-flop F/F and the synchronous counter  33  each receive a clock signal input provided on signal line  34  from a clock generator  35 . 
     With respect to operation of the logic of FIG. 2, the comparator  21  is asynchronous and may change state at anytime. The outputs of each of the delay elements Q 1  and Q 2  are a function of the state of their input at the clock edge and generate one (Q 1 ) and two (Q 2 ) clock delay synchronous copies of the output of the comparator  21 . The outputs of the delay elements Q 1  and Q 2  can only change state at the clock edge. As a result, the output of the comparator  21  is clocked into the first delay element Q 1  at each clock interval. This manner of operation synchronizes the comparator timing and forces the Q 1  output waveform to always be an integral number of sample clocks. 
     As shown in FIG. 2, an “And” function of the three inputs S 1 , Q 1  and Q 2  is formed and provides the J input to the flip/flop F/F, which then operates as a valid pulse generating circuit. The output F/F logic functions of the flip-flop F/F are as follows: 
     
       
           J=Q   1  &amp;  Q   2  &amp; Input 
       
     
     
       
           K=Q   1 * &amp;  Q   2 * &amp; Input 
       
     
     In the above equations, Q 1 * and Q 2 * are the inverted binary values of Q 1  and Q 2 , respectively. 
     If the comparator  21  resets as a result of a narrow pulse noise spike before the next clock, the delay element Q 1  will be set to False and the delay element Q 2  will be set to True. Therefore, in the case where Q 1  has been set False and Q 2  has been set True, the “J” logic equation is not satisfied. Thus, the Output F/F remains in the reset state and the noise pulse does not appear on the Q output of the JK flip-flop F/F. 
     In the case that the comparator  21  receives an input pulse which is wider than 2 clock periods, the delay element Q 1  sets True on the first clock, the delay element Q 2  sets True on the second clock, and Q 1  and the Input remain True at the second clock. As a result, according to the J logic function above, the Q output of the JK flip-flop F/F will set to True. 
     FIG. 3 illustrates the results of the introduction of noise to the comparator  21  for the mechanization shown in FIG.  2 . In particular, FIG. 3 graphs the clock signal on signal line  34  on a first horizontal axis  51 ; the output signal S 1  of the comparator  21  on a second horizontal axis  53 ; the Q 1  and Q 2  outputs on respective axes  55 ,  57 ; and the J and K logic functions and Q output of the flip flop F/F on respective axes  59 ,  61 ,  63 . Spurious noise pulses  65  in the comparator output S 1  are also shown, as well as their propagation through and elimination by the logic of the preferred embodiment. From FIG. 3 it can be seen that the valid comparator pulses which are propagated through the filter are delayed by two clock periods. 
     The synchronous pulse count accumulated by the counter  33  of FIG. 2 is shown above the bottom line  67  of FIG.  3 . The solid line  69  is the pulse count without the filter logic of the preferred embodiment, and dashed line  71  is the pulse count obtained with the filter. These results shows that the unfiltered count  69  is in error by 6 counts, while the filtered output  71  has rejected all of the noise pulses and only propagated the actual valid counts. 
     The just-described pulse filter approach ignores any pulses that are present on the output of the comparator  21  which are less than the number of synchronous clocks. The filter shown in FIG. 2 will reject any error pulse which is less than 1 clock period and may reject pulses that are as wide as (2-ε), depending on the relationship of the asynchronous rise and fall time of the comparator to the synchronous clock. The period of a rejected pulse is adjustable by use of a clock generator or programmable clock source which varies the frequency of the synchronous clock. As the frequency varies, the time period of the clock pulse width also varies, thus changing the noise pulse rejection period. As an example, using a 1 MHz clock, a noise pulse width of ε to 1 μsec would always be rejected, and noise pulses from &gt;1 to 2 usec would also be rejected, depending on the timing of the asynchronous rise and fall time to the clock. If the clock frequency were changed to 2 MHz the noise pulse width rejection would be reduced by a factor of 2. 
     Logic according to the invention can be expanded to add Q 3  to Q(n) delay elements and to add n states to the input of the AND function. With this expansion of states, rejection of pulse widths from 2 to (n−1) pulse widths can be achieved. The entire filter mechanization is compatible with real time pulse width rejection modification by providing a programmable clock generator or programmable counter and appropriate AND logic states. 
     From the above description, those skilled in the art will appreciate that various adaptations and modifications of the just-described preferred embodiments can be configured without departing from the scope and spirit of the invention. Therefore, it is to be understood that within the scope of the appended claims, the invention may be practiced other than as specifically described herein.