Frequency Measuring Instrument

Abstract

To provide a technology that measures, by a simple structure, the frequency of a frequency signal with high accuracy.SOLUTION: In a frequency measurer 1 that measures the frequency of a frequency signal using a PLL circuit, a multiplication unit 12 multiplies a digital frequency signal to be measured and a digital frequency signal for comparison and outputs a phase difference signal that corresponds to a phase difference between these digital frequency signals, and a loop filter 14 integrates the phase difference signal and performs low-pass filtering. A DDS 16 generates the digital frequency signal for comparison on the basis of a frequency setting signals derived by adding together the output signal of the loop filter 14 and a frequency value signal that indicates the center frequency of the pullable frequency range of the PLL circuit. Meanwhile, a frequency conversion unit 18 converts the output signal of the loop filter 14 into the frequency of the digital frequency signal to be measured.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present invention relates to a frequency measuring instrument that measures the frequency of a frequency signal. [Background technology]

[0002] In a detection device that detects a sensing target contained in gas or liquid, frequency measurement is performed to detect a change in oscillation frequency that accompanies adsorption of the sensing target to a quartz crystal oscillator that serves as a sensing sensor (for example, Patent Document 1). When performing frequency measurement, it is required to perform highly accurate frequency measurement while suppressing equipment costs and power consumption using as simple a logic circuit as possible.

[0003] Patent Document 2 describes a technique for estimating the frequency of an input signal input to a Phase Locked Loop (PLL) circuit based on a control signal for controlling an oscillator in the PLL circuit. However, the PLL circuit described in Patent Document 2 is configured to generate and output a signal having the same frequency and phase as the input signal. For this reason, Patent Document 2 does not disclose a configuration suitable for a dedicated frequency measuring instrument that measures the frequency of a frequency signal input from the outside. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] JP 2008-249729 A [Patent Document 2] Japanese Patent Application Publication No. 06-58963 Summary of the Invention [Problem to be solved by the invention]

[0005] The present invention has been made under such circumstances, and provides a technique for measuring the frequency of a frequency signal with high accuracy using a simple configuration. [Means for solving the problem]

[0006] This frequency measuring instrument is a frequency measuring instrument that measures the frequency of a frequency signal, an analog-to-digital converter for digitizing the frequency signal to obtain a digital frequency signal of the measurement object; a multiplication unit that multiplies the digital frequency signal to be measured by a digital frequency signal for comparison and outputs a phase difference signal corresponding to a phase difference between these frequency signals; a loop filter that adds the phase difference signal acquired from the multiplication unit and an integral value of the phase difference signal acquired so far, and outputs a new integral value as an output signal; an adder that adds an output signal of the loop filter and a frequency value signal indicating a preset frequency and outputs the result as a frequency setting signal; a PLL (Phase Locked Loop) circuit including a DDS (Direct Digital Synthesizer) that generates the digital frequency signal for comparison based on the frequency setting signal; a frequency conversion unit that converts an output signal of the loop filter into a frequency of the digital frequency signal to be measured, The preset frequency is a frequency within a range of frequencies that can be locked by the PLL circuit.

[0007] The frequency measuring device may have the following features. (a) The preset frequency is the center frequency of a range of frequencies that can be entrained. (b) An averaging unit that performs cumulative averaging of the output signal of the loop filter, and the frequency conversion unit converts the output signal after averaging by the averaging unit into the frequency. (c) The digital frequency signal to be measured is a digital frequency signal digitized by the analog / digital converter and then input to the multiplication section after being multiplied or divided. Effect of the Invention

[0008] This frequency measuring instrument measures the frequency of a frequency signal input to a PLL circuit based on the output signal from a loop filter provided in the PLL circuit, and obtains a phase difference signal indicating the phase difference between these frequency signals using a multiplication unit that multiplies the digital frequency signals to be measured and for comparison. A multiplication unit that only multiplies digital frequency signals together has a simpler configuration than other types of phase comparators, so accurate frequency measurement can be performed without using complex logic circuits. [Brief description of the drawings]

[0009] [Figure 1] FIG. 1 is a block diagram of a frequency measuring instrument according to an embodiment. [Diagram 2] 4 is a graph showing the results of frequency measurement performed using the frequency measuring device. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0010] 1 shows an example of the configuration of a frequency measuring instrument 1 according to the present embodiment. The frequency measuring instrument 1 of this example measures the frequency of a frequency signal to be measured using an output signal from a loop filter 14 provided in a PLL circuit. The PLL circuit includes an analog / digital converter (ADC) 11, a multiplier 12, an LPF 13, a loop filter 14, an adder 15, and a direct digital synthesizer (DDS) 16. As described above, when measuring the frequency of a frequency signal output from a detection device using a quartz crystal oscillator, the frequency fluctuation range is predetermined according to the range in which the detection device can measure the concentration of the object to be sensed. Thus, the pull-in frequency range of the PLL circuit can be set to coincide with the above-mentioned frequency fluctuation range or to include the frequency fluctuation range.

[0011] The ADC 11 converts an analog frequency signal (e.g., a sine wave signal) to be measured, which is input from the measured signal input unit 101, into a digital frequency signal. The ADC 11 may have a function of multiplying the frequency signal by upsampling, or dividing the frequency by downsampling. Alternatively, a multiplier or divider may be provided in the subsequent stage of the ADC 11 to multiply / divide the digital frequency signal.

[0012] The multiplier 12 multiplies the digital frequency signal to be measured, which is input from the ADC 11 side, by a digital frequency signal for comparison, which is input from the DDS 16 side, and outputs the result as a phase difference signal. When the digital frequency signal to be measured is expressed as sin(ω1t+α) and the frequency signal of the comparison digital frequency signal is expressed as sin(ω2t+β), the multiplied value is given by the following equation (1). sin(ω1t+α)×sin(ω2t+β) =cos((ω2-ω1)t+β-α)-cos((ω2+ω1)t+β+α) …(1) When the frequency difference between the two frequency signals is sufficiently small and the angular frequency can be expressed as ω2≒ω1≒ω, the above formula (1) can be simplified to the following formula (1)'. cos(β-α)-cos(2ωt+β+α) …(1)'

[0013] From equation (1)', when the PLL circuit is locked, the measurement target digital frequency signal and the comparison digital frequency signal are in an orthogonal state with a frequency match and a phase difference of 90°, and the DC component of the multiplied value is zero. On the other hand, if there is a frequency difference and the phases are not orthogonal, a DC component (cos(β-α)) corresponding to the phase difference is output. The PLL circuit of this example uses the above-mentioned DC component as the phase difference signal. The LPF (Low Pass Filter) 13 attenuates the AC component of the double wave in the second term of the equation (1)′, and outputs the phase difference signal, which is the DC component described above, to the loop filter 14.

[0014] The loop filter 14 performs low-pass filtering by adding the phase difference signal acquired from the multiplication unit 12 to the integral value of the phase difference signal acquired so far, and outputting the new integral value as an output signal. As described later, when the PLL circuit is locked and the average value of the phase difference signal becomes zero, the loop filter 14 outputs a digital signal having an average value corresponding to the frequency of the digital frequency signal to be measured. Various setting items of the loop filter 14 are adjusted so as to adjust the loop band of the PLL circuit and damping related to the transient response according to the fluctuation range of the frequency signal to be measured.

[0015] The adder 15 adds the output signal output from the loop filter 14 and a frequency value signal indicating a preset frequency input via the frequency value signal input unit 102. As the preset frequency, a frequency within a lockable frequency range, for example, a center frequency of the lockable frequency range, is set. In the frequency measuring instrument 1 of this example in which no frequency signal is output from the PLL circuit, the lockable frequency range is a frequency range in which the PLL circuit can be locked for the analog frequency signal to be measured input from the measured signal input unit 101. The adder 15 adds the output signal of the loop filter 14 and the above-mentioned frequency value signal, and outputs the result to the DDS 16 as a frequency setting signal.

[0016] The DDS 16 generates a digital frequency signal (e.g., a sine wave signal) having a frequency corresponding to the frequency setting signal acquired from the adder 15. For example, when a frequency setting signal obtained by adding the upper limit of the variable range of the value of the output signal output from the loop filter 14 to the frequency value signal is input, the DDS 16 is set to generate a digital frequency signal having a frequency at the upper limit of the entrainable frequency range of the PLL circuit. When a frequency setting signal obtained by adding the lower limit of the variable range of the output signal of the loop filter 14 to the frequency value signal is input, the DDS 16 is set to generate a digital frequency signal having a frequency at the lower limit of the entrainable frequency range. When a frequency setting signal obtained by adding the median of the variable range of the output signal of the loop filter 14 to the frequency value signal is input, the DDS 16 is set to generate a digital frequency signal having a center frequency of the entrainable frequency range.

[0017] The frequency measuring instrument 1 of this example uses a PLL circuit having the configuration described above to measure the frequency of a frequency signal input from the measured signal input unit 101. As described above, the output signal of the loop filter 14 is a variable that determines the frequency of the comparison digital frequency signal output from the DDS 16. Therefore, the frequency measuring instrument 1 of this example specifies the frequency of the frequency signal to be measured that is input from the measured signal input unit 101 based on the output signal of the loop filter 14.

[0018] Therefore, as shown in the block diagram of FIG. 1, the output signal output from the loop filter 14 is also supplied to the frequency conversion unit 18 in parallel with the input to the adder unit 15. As described above, when the PLL circuit is locked, the frequency of the digital frequency signal to be measured and the digital frequency signal to be compared match. The output signal from the loop filter 14 is a variable that determines the frequency of the digital frequency signal to be compared. Therefore, if the correspondence between the frequency setting signal input from the adder unit 15 to the DDS 16 and the frequency of the digital frequency signal to be compared, the value of the frequency value signal input from the frequency value signal input unit 102, and if the digital frequency signal is multiplied or divided, the multiplication value or division value are known, the frequency of the frequency signal to be measured can be uniquely identified based on the output signal.

[0019] Using the above-mentioned relationship, the frequency conversion unit 18 performs a calculation to convert the output signal of the loop filter 14 into the frequency of the digital frequency signal to be measured, and outputs the converted value via the frequency measurement value output unit 103. The frequency of the digital frequency signal indicates the frequency of the analog frequency signal input from the measured signal input unit 101.

[0020] 1, an averaging unit 17 that performs cumulative averaging of the output signal of loop filter 14 is provided in the front stage of frequency measuring device 1 in this embodiment. For example, averaging unit 17 cumulatively adds the output signal a number of times corresponding to a preset clock count, and outputs the average value (moving average value) to frequency conversion unit 18. By converting the output signal averaged by averaging unit 17 into a frequency, it is possible to suppress the influence of fluctuations in the value of the output signal due to the occurrence of fluctuations in the PLL circuit after locking, and perform frequency measurement with higher accuracy. Incidentally, it is not essential to provide the averaging unit 17 in the frequency measuring instrument 1, and the output signal of the loop filter 14 may be directly converted into a frequency.

[0021] The frequency measuring instrument 1 according to this embodiment has the following advantages. When measuring the frequency of a frequency signal input to a PLL circuit based on an output signal from a loop filter 14 provided in the PLL circuit, a phase difference signal indicating the phase difference between these frequency signals is obtained using a multiplier 12 that multiplies the digital frequency signals to be measured and for comparison. The multiplier 12, which only multiplies digital frequency signals together, has a simpler configuration than other types of phase comparators such as a phase frequency type comparator, and therefore can perform accurate frequency measurement without using a complex logic circuit.

[0022] Furthermore, in the frequency measuring instrument 1 of this embodiment, the adder 15 adds the center frequency of the capture frequency range of the PLL circuit to the output signal, and uses this value as the frequency setting signal. As a result, even when the capture frequency range of the PLL circuit or the frequency range to be measured is changed by adjusting the values ​​of the setting items of the loop filter 14, the value of the frequency value signal input from the frequency value signal input unit 102 can be changed in response to these changes. As a result, the degree of freedom in changing the frequency measurement range is increased compared to when the output signal of the loop filter 14 is used as the frequency setting signal of the DDS 16 as it is.

[0023] The above-mentioned frequency measuring instrument 1 is not limited to use for measuring the frequency of a frequency signal output from a detection device that detects an object to be sensed using a quartz crystal oscillator. It goes without saying that the frequency measuring instrument 1 of this example may be used to measure the frequency of a frequency signal output from other types of devices. EXAMPLES

[0024] (experiment) Using the frequency measuring instrument 1 according to the embodiment, the frequency of a frequency signal to be measured was changed and the frequency was measured. A. Experimental Conditions A frequency signal whose frequency was varied within the range of 10.277777 MHz ±0.18 kHz was input to the frequency measuring instrument 1 having the configuration described with reference to FIG. 1, and the value output after 62.5 ms was taken as the measurement result.

[0025] B. Experimental Results The results of the above experiment are shown in Figure 2. The horizontal axis of Figure 2 shows the deviation from the center frequency (10.277777 MHz) of the frequency signal input to the frequency measuring instrument 1. The vertical axis of Figure 2 shows the difference between the frequency measured by the frequency measuring instrument 1 and the actual frequency. 2, a highly accurate frequency measurement result of ±0.06 Hz (333 ppb) was obtained over a frequency fluctuation range of 0.36 kHz. The frequency measurement accuracy can be further improved as necessary by improving the stability of the reference oscillator that supplies the clock that operates the frequency measuring instrument 1, extending the measurement time, and improving the resolution of the calculation processing in the multiplication unit 12, LPF 13, loop filter 14, etc. [Explanation of symbols]

[0026] 1 Frequency measuring instrument 11 ADC 12 Multiplication section 14 Loop Filter 15 Addition section 16 DDS 18 Frequency conversion section

Claims

1. In a frequency measuring instrument for measuring the frequency of a frequency signal, an analog-to-digital converter for digitizing the frequency signal to obtain a digital frequency signal of the object to be measured; a multiplication unit that multiplies the digital frequency signal to be measured by a digital frequency signal for comparison and outputs a phase difference signal corresponding to a phase difference between these frequency signals; a loop filter that adds the phase difference signal acquired from the multiplication unit and an integral value of the phase difference signal acquired so far, and outputs a new integral value as an output signal; an adder that adds an output signal of the loop filter and a frequency value signal indicating a preset frequency and outputs the result as a frequency setting signal; a PLL (Phase Locked Loop) circuit including a DDS (Direct Digital Synthesizer) that generates the digital frequency signal for comparison based on the frequency setting signal; a frequency conversion unit that converts an output signal of the loop filter into a frequency of the digital frequency signal to be measured, 2. A frequency measuring instrument according to claim 1, wherein the preset frequency is within a range of frequencies that can be pulled in by the PLL circuit.

2. 2. The frequency measuring instrument according to claim 1, wherein the preset frequency is a center frequency of a frequency range that can be pulled in.

3. 3. The frequency measuring instrument according to claim 1, further comprising an averaging unit that performs cumulative averaging of an output signal of the loop filter, and the frequency conversion unit converts the output signal after averaging by the averaging unit into the frequency.

4. 4. The frequency measuring instrument according to claim 1, wherein the digital frequency signal to be measured is a digital frequency signal digitized by the analog / digital converter and then input to the multiplication section after being multiplied or divided.

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