Signal amplitude distribution analyzer

Abstract

A signal amplitude distribution analyzer measures the amplitude probability density function of electrical signals, and in particular, the amplitude probability distribution of noise signals. Such measurements may be used to determine the "Gaussianicity" of noise signals, that is, a measurement of how closely the amplitude distribution of noise signals corresponds to theoretical values derived from the Gaussian probability distribution density function. This theoretical density function represents the relative percentage of time that a noise signal is at a given amplitude. The invention gives an approximation of this function by measuring the amount of time a noise signal is between a window of two adjustable voltage levels. This is accomplished by producing an output voltage proportional to the amount of time a noise signal amplitude falls within the window of values defined by the two adjustable voltage levels. The center point of this window is then plotted versus the invention's output voltage, giving the amplitude distribution density. This is then compared to the theoretical density function, plotted on the same graph, to determine signal "Gaussianicity." The invention is calibrated to give an output that is Gaussian when analyzing a known Gaussian input. A continuous resolution measurement of the signal amplitude probability density function is possible, permitting accurate analysis of noise statistics in terms of skewness, clipping, etc. The cumulative amplitude distribution of noise signals can also be measured by the analyzer of the invention.

What is claimed is: 
     
       1. An apparatus for analyzing signals comprising: a first comparator for comparing voltage of said signals to a preselected lower voltage and for providing an output signal when the voltage of said signals is above said preselected lower voltage;   a second comparator for comparing voltage of said signals to a preselected higher voltage and for providing an output signal when the voltage of said signals is below said preselected higher voltage; and   an AND circuit coupled to said first and second comparators for providing a pulse upon simultaneously receiving said output signals from said comparators, said pulse having a pulse length corresponding to the elapsed time said signals are within a voltage window bounded by said lower and higher voltages.   
     
     
       2. An apparatus according to claim 1 further including: an integrator coupled to said AND circuit for integrating said pulse provided by said AND circuit to provide a voltage proportional to said elapsed time said signals are within said voltage window.   
     
     
       3. An apparatus according to claim 2 in which said integrator is a voltmeter. 
     
     
       4. An apparatus according to claim 1 in which said apparatus is calibrated by analyzing a known input signal. 
     
     
       5. An apparatus according to claim 4 in which said known input signal is a Gaussian signal. 
     
     
       6. A method for approximating the amplitude probability density distribution of signals comprising the steps of: comparing voltage of said signals to a preselected lower voltage in a first comparator;   providing an output signal from said first comparator when the voltage of said signals is above said preselected lower voltage;   comparing voltage of said signals to a preselected higher voltage in a second comparator;   providing an output signal from said second comparator when the voltage of said signals is below said preselected higher voltage;   providing a pulse from an AND circuit operably coupled to said first and second comparators upon said AND circuit simultaneously receiving said output signals from said comparators, said pulse having a pulse length corresponding to the elapsed time said signals are within a voltage window bounded by said lower and higher voltages; and   integrating said pulse provided by said AND circuit in an integrator to provide an output voltage proportional to said elapsed time said signals are within said voltage window.   
     
     
       7. A method according to claim 6 further including a step of analyzing a known input signal as a calibration of said comparators and said integrator.   
     
     
       8. A method according to claim 7 in which said known input signal is a Gaussian signal. 
     
     
       9. A method according to claim 8 further including steps of: analyzing unknown input signals by shifting said voltage window over a desired voltage range, each voltage window having a middle voltage point; and   plotting said output voltage versus said middle voltage point for each voltage window examined.   
     
     
       10. A method according to claim 9 further including the step of: comparing said plot to a theroretically derived probability density distribution to determine the magnitude of deviation of said plot from said theoretically derived probability density function.