1. Technical Field
The present invention relates to measurement of frequencies, and more particularly, to a frequency measurement device capable of detecting a very small change in frequency.
2. Related Art
Frequency measurement methods are known such as a direct count method (for example, see JP-A-2001-119291) wherein the number of pulses passing within a prescribed gate period (gate time) is measured, a reciprocal method (for example, see JP-A-5-172861) wherein an accurate pulse period is measured and a frequency is calculated from the reciprocal of time, and a method (for example, see U.S. Pat. No. 7,230,458) wherein frequencies are calculated by obtaining a ΔΣ modulation signal. Although the direct count method can be realized with a relatively small-scale circuit, it is necessary to increase the gate time in order to increase frequency resolution (for example, the gate time needed for obtaining a resolution of 0.1 Hz is 10 seconds). Although the reciprocal method can solve this problem, the circuit for measuring pulse intervals accurately becomes larger in scale as compared to that of the direct count method. The method wherein frequencies are calculated by obtaining a ΔΣ modulation signal has a limitation in obtaining a dynamic range and has to trade off dynamic range against measurement accuracy. Although the measurement accuracy can be improved by obtaining a high-order ΔΣ modulation signal or using a PLL, there is a problem in that an additional analog circuit is needed, thus increasing the circuit size.
The present applicant has proposed a frequency measurement device based on a short gate time count method (also referred to as a short gate count method or a short gate method) as a new method substituting the above-described methods (for example, see JP-A-2009-250807). The short gate count method is configured to perform counting (sampling) of a measurement signal repeatedly without being interrupted at a prescribed short gate time to remove (filter out) high-frequency components from a series of obtained count values. In this way, the short gate count method improves both time resolution and frequency resolution as compared to the method of the related art. Since a frequency measurement device using this method is configured by a counter circuit and a small-scale computation circuit, it is possible to easily realize a multi-channel implementation while suppressing an increase in the circuit size. Moreover, as the gate time decreases, namely as the sampling frequency increases, it is possible to improve both dynamic range and resolution.
However, when the short gate count method is used, pattern noise occurs under certain conditions. The pattern noise increases as an operating point parameter which is obtained by the ratio of a measurement frequency (the frequency of a measurement signal) to a sampling frequency (the frequency of a gate signal) approaches a simple advantageous value. Therefore, the present applicant has proposed a method of decreasing the level of pattern noise by selecting a measurement frequency and a sampling frequency based on the distribution characteristics of an operating point parameter and a noise level (see JP-A-2010-085286).
However, in the method of the related art, it is difficult to reduce the level of pattern noise when the amount of change in the measurement frequency is large.