Browse Prior Art Database

HIGH ACCURACY FAST FREQUENCY COUNTER

IP.com Disclosure Number: IPCOM000006577D
Original Publication Date: 1992-Aug-01
Included in the Prior Art Database: 2002-Jan-16
Document File: 2 page(s) / 72K

Publishing Venue

Motorola

Related People

Kazimierz (Kai) Siwiak: AUTHOR

Abstract

The frequency of a periodic signal may be deter- mined by counting zero crossings of the signal in a spe- chic interval, or by measuring the time period between two zero crossings of the signal. The first method is preferred for high frequency signals, while the second method is preferred for very low frequencies. In practi- cal cases, the best accuracy can be obtained by using a hybrid of the two counting methods, especially for sig- nals in the presence. of noise.

This text was extracted from a PDF file.
At least one non-text object (such as an image or picture) has been suppressed.
This is the abbreviated version, containing approximately 57% of the total text.

Page 1 of 2

0 M

MOTOROLA INC. Technical Developments Volume 16 August 1992

HIGH ACCURACY FAST FREQUENCY COUNTER

by Kazimierz (Kai) Siwiak

  The frequency of a periodic signal may be deter- mined by counting zero crossings of the signal in a spe- chic interval, or by measuring the time period between two zero crossings of the signal. The first method is preferred for high frequency signals, while the second method is preferred for very low frequencies. In practi- cal cases, the best accuracy can be obtained by using a hybrid of the two counting methods, especially for sig- nals in the presence. of noise.

It can be shown that the average error E, of a fre- quency count is related to the signal to noise ratio rby:

E, = IfN-El e-r

where fN is the noise center of the band containing the signal and fc is the actual signal frequency. For example, if we require the average counting error E to be no more than 15 Hz, and the bandwidth of the system allows IfN-f=l to b e as much as 1700 Hz, then the miniium required signal to noise ratio is 7 dB. It can further be shown that the probability Pthat the actual counter error E and the average error ES,, differ by no more than m counts is described by:

        P{lE.& cm)=erf(m~n/& where erf@) is the error function and n is the prede- termined number zero crossings counted. Hence it is desirable to be able to design the desired counter events n based on the desired accuracy as described by the above formulas. With reference to Figure 1, a counter circ...