Frequency measurement method and associated apparatus

A method for selecting a plurality of sampling rates f.sub.i for determining the frequency of a periodic signal, wherein said method comprises the steps of:

• selecting a first sampling rate f.sub.smax, wherein said first sampling rate f.sub.smax is the largest of said plurality of sampling rates f.sub.i ;
• determining the maximum estimatable frequency of said periodic signal;
• determining a frequency sample separation increment .DELTA.f, wherein said frequency sample separation increment .DELTA.f is based on the value of said maximum estimatable frequency, said first sampling rate f.sub.smax, and the minimum desired estimatable frequency;
• and
• determining a plurality of sampling rates fi,
• wherein the sampling rates are determined as follows:
• f.sub.i =f.sub.smax -(i*.DELTA.f)
• where f.sub.smax =first sampling rate, and
• i=(1, 2, 3, . . . ).

A frequency measurement apparatus for estimating the frequency of a periodic signal, wherein said apparatus comprises:

• an analog to digital converter for sampling the periodic signal at a plurality of sampling rates to obtain a plurality of time-domain sample sets of said periodic signal;
• a digital signal processor positioned to receive said plurality of time-domain sample sets from said analog to digital converter, wherein said digital signal processor comprises a converter for converting said plurality of time-domain sample sets to a respective plurality of frequency-domain sample sets;
• first determining means for receiving said plurality of frequency-domain sample sets from said digital signal processor, wherein said first determining means determines, for each of said plurality of frequency-domain sample sets, an alias frequency associated with the respective frequency-domain sample set;
• selecting means for selecting a subset of said plurality of alias frequencies;
• and
• second determining means for receiving said subset of alias frequencies, wherein said second determining means determines said frequency of said periodic signal based upon said selected subset of alias frequencies;
• wherein said selecting means further comprises means for selecting the subset of said plurality of alias frequencies so as to reduce errors in the subsequent determination of the frequency of said periodic signal such that the frequency of the periodic signal can be determined even though the frequency of the periodic signal exceeds the largest nyquist frequency established by said plurality of sampling rates.