Digital Frequency Measuring Device

This invention relates to a digital frequency measuring device for measuring an input signal frequency in the digital manner.

In ordinary digital frequency measuring devices, the signal to be measured is converted into pulses, and applied to a gate for counting the number of pulses passing therethrough for a certain period of time, and then the count value is displayed in a digital form. With such a method, however, since the signal to be measured and the gate signal are not synchronized with each other, an error appears in the neighborhood of each of the leading and trailing edges of the gate signal. Especially, as the gate time approaches the pulse interval of the signal to be measured, the error increases. For example, in the case of measuring a 1 Hz signal using a gate time of 1 second, an error of .+-.1 Hz appears in the measurement.

Then, the prior art employs the following method to provide for enhanced resolving power. That is, a gate signal synchronized with the signal to be measured is applied to two gate circuits to control them, by which the signal to be measured and the reference clock signal are respectively controlled to obtain gate outputs. The gate outputs are respectively counted by two counters. Then, an integral value of the frequency to be measured is obtained from the count value of the signal to be measured, and values corresponding to errors between the count value of the clock signal and clock pulses appearing at the leading and trailing edges of the gate signal are obtained. And then an operation is performed based on the abovesaid values to obtain a fraction value smaller than one period of the signal to be measured. Since this system performs a numerical operation, it is referred to as the computing counter system, and known to be capable of highly accurate frequency measurement. However, this system requires two counters and a divider, too, and hence has the disadvantage that the construction is relatively complicated as a whole.

One object of this invention is to provide a digital frequency measuring device which is relatively simple in construction and inexpensive but has substantially the same performance as the conventional computing counter system.

Another object of this invention is to provide a digital frequency measuring device which is capable of achieving highly accurate frequency measurement in a short time.

Another object of this invention is to provide a digital frequency measuring device which is capable of achieving highly accurate frequency measurement up to a high frequency region in a short time.

Still another object of this invention is to provide a digital frequency measuring device with which it is also possible to measure time intervals with high resolving power.

In a frequency measuring device in which a gate circuit is controlled by a gate signal, pulses of an input signal having passed through the gate circuit are counted and the count value is displayed in the form of a digital value, a fraction detector circuit is provided which is supplied with the input pulses and the gate signal to detect the time intervals or differences .DELTA.T.sub.1 and .DELTA.T.sub.2 between the leading and trailing edges of the gate signal and the input pulses arriving immediately after them, respectively, and the period T of the input pulse signal and in which an operation (.DELTA.T.sub.1 - .DELTA.T.sub.2)/T is performed. The result of operation is converted into the corresponding digital value, and displayed as a fraction of the abovesaid count value. In the fraction detector circuit, the operation (.DELTA.T.sub.1 - .DELTA.T.sub.2)/T can be achieved after the time differences .DELTA.T.sub.1 and .DELTA.T.sub.2 and the period T are converted into voltages. That is, certain values are integrated respectively for the periods of time .DELTA.T.sub.1, .DELTA. T.sub.2 and T to obtain voltages .DELTA.V.sub.1, .DELTA.V.sub.2 and V. By performing an operation (.DELTA.V.sub.1 - .DELTA.V.sub.2)/V, an accurate operation can be achieved relatively easily, even if the time intervals .DELTA.T.sub.1 and .DELTA.T.sub.2 are very short. Further, the voltages .DELTA.V.sub.1 - .DELTA.V.sub.2 and V are supplied to a digital voltmeter and the voltage value .DELTA.V.sub.1 - .DELTA.V.sub.2 is measured using the voltage V as the reference, by which the operation (.DELTA.V.sub.1 - .DELTA.V.sub.2)/V is performed in the form of a digital value. Thus, in the frequency measuring device of this invention, only one counter is employed and the operation involved is relatively simple. Accordingly, a measuring device of the same performance as the conventional computing counter system can be obtained at lower cost. By the additional provision of a pulse interval measuring circuit, the pulse interval, that is, time, can also be measured with high accuracy. Namely, by a gate signal generated from the pulse interval measuring circuit, the gate circuit is opened to pass therethrough clock pulses and the number of clock pulses passing through the gate circuit is counted and, at the same time, the gate signal and the clock pulses are applied to the fraction detector circuit.

FIG. 1 is a block diagram showing a conventional highly accurate digital frequency measuring device;

FIG. 2 is a waveform diagram explanatory of the operation of the device shown in FIG. 1;

FIG. 3 is a block diagram illustrating an embodiment of a digital frequency measuring device of this invention;

FIG. 4 is a waveform diagram explanatory of the operation of the device exemplified in FIG. 3;

FIG. 5 is a block diagram showing in detail an example of the principal part of the digital frequency measuring device of this invention; and

FIG. 6 is a waveform diagram explanatory of the operation of the part shown in FIG. 5.

Referring first to FIG. 1, a conventional computing counter system will be described. A signal Pa to be measured is supplied to a terminal 1 from a source 40 of the signal to be measured, and frequency divided by a frequency divider 2 down to 1/N, as shown in FIG. 2A, thereafter being applied to a gate circuit 3a. The frequency-divided output from the frequency divider 2 is also supplied to a gate signal forming circuit 4 to derive therefrom such a gate signal Pb as shown in FIG. 2B which is synchronized with the signal Pa to be measured and has a pulse width of, for example, about one second. The gate signal Pb is applied to the gate circuits 3a and 3b. The gate circuit 3a controls by gating the signal Pa which is supplied to a first counter 5a, while the gate circuit 3b controls by gating a clock pulse Pc which is applied to a second counter 5b. The clock pulse Pc is shown in FIG. 2C, and is derived from a clock pulse generator 6. Thus, the signal Pa and the clock pulse Pc are respectively counted by the first and second counters 5a and 5b for the duration of the gate signal Pb, and their count values L and M are supplied to an arithmetic circuit 7.

A fraction detector circuit 8 is provided, which is supplied with the clock pulse Pc and the gate signal Pb to detect a time interval or difference .DELTA.T.sub.1 between the leading edge of the gate pulse Pb and the clock pulse Pc immediately following it, and a time interval or difference .DELTA.T.sub.2 between the trailing edge of the gate pulse Pb and the clock pulse Pc immediately following it, as depicted in FIG. 2C. The time differences .DELTA.T.sub.1 and .DELTA.T.sub.2 thus detected are digitized by an A-D converter 9, and applied to the arithmetic circuit 7.

In the arithmetic circuit 7, only the integer part of the frequency F of the signal Pa to be measured is obtained by performing an operation L.multidot.N/(M - 1).multidot.T (where T is the pulse interval of the clock pulse Pc and N the dividing frequency of the frequency divider 2). But, by applying the time differences .DELTA.T.sub.1 and T - .DELTA.T.sub.2 to the arithmetic circuit 7 and performing an operation ##EQU1## an accurate frequency including a fractional value less than one period can be obtained, and is displayed by a digital display 10.

With the measuring device shown in FIG. 1, the signal Pa can be measured including the fractional value less than one period of the signal, so that highly accurate frequency measurement can be achieved. However, this system involves the two gate circuits 3a and 3b, the two counters 5a and 5b and the arithmetic circuit 7 for performing an operation including division as described above, and hence is complicated in the circuit construction as a whole and expensive.

FIG. 3 illustrates in block form an embodiment of a digital frequency measuring device according to this invention. In FIG. 3, parts corresponding to those in FIG. 1 are identified by the same reference numerals. The clock pulse from the clock pulse generator 6 is applied to the gate signal forming circuit 4 to provide the gate signal Pb of unit time width, for instance, one second (refer to FIG. 4B), by which the gate circuit 3a is controlled to be opened and closed. The gate circuit 3a is supplied with the signal Pa (FIG. 4A) from the signal source 40 through the 1/N frequency divider 2, and the gated output is counted by the counter 5a. The resulting count value is a value indicative of the unit time, for example, one second, so that the numerical value itself indicates the frequency. The count value thus obtained is applied to the digital display 10 to indicate, for instance, the measured numerical value above the decimal point.

On the other hand, the numerical value below the decimal point is obtained by converting the output from the fraction detector circuit 8 by the A-D converter 9 into a digital form. With the present invention, in the fraction detector circuit 8, there are produced voltages .DELTA.V.sub.1 and .DELTA.V.sub.2 (refer to FIG. 4D) which are respectively proportional to the time differences between integral multiples of the signal Pa to be measured and the gate signal Pb, that is, the time .DELTA.T.sub.1 between the leading edge of the gate signal Pb and the input pulse Pa immediately following it, and the time .DELTA.T.sub.2 between the trailing edge of the gate signal Pb and the input pulse Pa immediately following it, and there is produced the ratio (.DELTA.V.sub.1 - .DELTA.V.sub.2)/V, that is, the ratio of the voltage difference .DELTA.V.sub.1 - .DELTA.V.sub.2 between the voltages .DELTA.V.sub.1 and .DELTA.V.sub.2 to one period of the signal Pa. The voltage ratio is converted by the A-D converter 9 into a digital signal to obtain the fraction of the measured frequency below the decimal point.

That is, if the count value of the counter 5a is taken as L, the frequency F of the signal Pa is given as follows: ##EQU2## It is difficult to accurately measure the very short time intervals .DELTA.T.sub.1 and .DELTA.T.sub.2, so that, in the present invention, the time intervals .DELTA.T.sub.1 and .DELTA.T.sub.2 and the pulse interval T of the clock pulse Pc are respectively integrated by integrators and converted into the voltage values .DELTA.V.sub.1, .DELTA.V.sub.2 and V for the arithmetic operation. Accordingly, the frequency F becomes as follows: ##EQU3## Thus, the small numerical value below the decimal point is also counted and this numerical value can be displayed as a fraction by the digital display 10.

With reference to FIG. 5, a detailed description will be given of the principal part of the digital frequency measuring device according to this invention.

The signal Pa to be measured (FIG. 6A), supplied to the input terminal 1, is applied through the 1/N frequency divider 2 and the gate circuit 3a to the counter 5a, and its count value is displayed by the digital display 10 in the same manner as described above. Now, a specific construction of the fraction detector circuit 8 will be described. In the illustrated example, the fraction detector circuit 8 comprises one D type flip-flop circuit 11, two set-reset type flip-flop circuits 12 and 13, integrators 14a, 14b and 14c respectively connected to the output sides of the flip-flop circuits 11, 12 and 13, and a differential amplifier 15 for obtaining the difference between the integrated outputs from the integrators 14b and 14c.

A clock terminal C of the D type flip-flop circuit 11 and reset terminals R of the flip-flop circuits 12 and 13 are supplied with the signal Pa, and a data terminal D of the D type flip-flop circuit 11 is supplied with, for example, a measurement start control signal Sb (FIG. 6B) from a control circuit 16. When the signal Pa arrives at the D type flip-flop circuit 11 while the measurement start control signal Sb has a logic "1" to provide the logic "1" at the terminal D of the flip-flop circuit 11, the output at its output terminal Q becomes of the logic "1". The logic "1" output is supplied to the integrator 14a and the control circuit 16. When supplied with the logic "1", the control circuit 16 inverts the control signal to the terminal D of the flip-flop circuit 11 to a logic "0". Upon arrival of the next input pulse Pa, the logic "0" at the terminal D is read in the flip-flop circuit 11 to return its output terminal Q to the logic "0". Accordingly, at the start of measurement, the integrator 14a is supplied with a pulse Sc (FIG. 6C) whose duration corresponds to one period of the signal Pa and, for the time T, a certain current is integrated by the integrator 14a to derive at its output the integrated voltage V corresponding to one period of the signal Pa.

On the other hand, the set terminal S of the flip-flop circuit 12 is supplied with the gate signal Pb (FIG. 6D) from the gate signal forming circuit 4, and set by rising of the leading edge of the gate signal Pb to put the output from the flip-flop circuit 12 to the logic "1", and reset by the subsequent input pulse Pa. Consequently, the integrator 14b is supplied with a pulse Se (FIG. 6E) whose duration corresponds to the time difference .DELTA.T.sub.1 between the leading edge of the gate signal Pb and the immediately subsequent input pulse Pa, deriving at the output of the integrator 14b the integrated voltage .DELTA.V.sub.1 proportional to the time difference .DELTA.T.sub.1. The gate signal Pb is polarity inverted by an inverter 17, and applied to the set terminal S of the flip-flop circuit 13. The flip-flop circuit 13 is set from the trailing edge of the gate signal Pb, and reset upon arrival of the next input pulse Pa. Accordingly, the integrator 14c is supplied with a pulse (FIG. 6F) whose duration corresponds to the time difference .DELTA.T.sub.2 between the trailing edge of the gate signal Pb and the next subsequent input pulse Pa and, as a result of this, the integrated voltage proportional to the time difference .DELTA.T.sub.2 is obtained at the output of the integrator 14c. The outputs .DELTA.V.sub.1 and .DELTA.V.sub.2 from the integrators 14b and 14c are applied to the differential amplifier 15 to derive at its output the difference voltage .DELTA.V.sub.1 - .DELTA.V.sub.2. The ratio (.DELTA.V.sub.1 - .DELTA.V.sub.2)/V of the voltage .DELTA.V.sub.1 - .DELTA.V.sub.2 to the voltage V from the integrator 14a is converted by the A-D converter 9 into the corresponding digital value. The A-D converter 9 may be, for example, a digital voltmeter, by which the value of the difference voltage .DELTA.V.sub.1 - .DELTA.V.sub.2 is measured using, as the reference value, the voltage V corresponding to one period of the signal Pa, thereby obtaining the digital value of the voltage ratio (.DELTA.V.sub.1 - .DELTA.V.sub.2)/V. This digital value is the fraction of the count value derived from the counter 5a, and may be displayed, for instance, on the portion after the decimal point of the display 10.

The digital frequency measuring device described above is also applicable to measurement of the time width or the pulse interval. To this end, a pulse interval measuring circuit 18 is added, whose input terminals 18a and 18b, respectively supplied with a start signal and a stop signal, are connected to the set terminal S and the reset terminal R of the flip-flop circuit 19, respectively. In place of the output from the gate signal forming circuit 4, the output from a flip-flop circuit 19 is applied to the gate circuit 3a and the set terminal S of each of the flip-flop circuits 12 and 13. Further, in place of the output from the 1/N frequency divider 2, the clock pulse from the clock pulse generator 6 is supplied to the gate circuit 3a through a switch 20 and, at the same time, applied to the clock terminal C of the D type flip-flop circuit 11 and the reset terminal R of each of the flip-flop circuits 12 and 13. In the measurement, the switch 20 is turned on to apply the start and the stop pulse to the input terminals 18a and 18b to open the gate circuit 3a for a period of time corresponding to the pulse interval to be measured, and the clock pulses having passed through the gate circuit 3a in the above period of time are counted by the counter 5a. Further, its function value is detected by the fraction detector circuit 8, and displayed by the display 10. Accordingly, with the present invention, the pulse interval can also be measured with high accuracy.

As has been described in the foregoing, according to the digital frequency measuring device of this invention, the fraction value, which is smaller than one period of the signal to be measured, can also be obtained in addition to the count value of the counter 5a, so that highly accurate frequency or pulse interval measurement can be achieved. With this invention, since one gate circuit, one counter and one arithmetic circuit can be omitted from the measurement system shown in FIG. 1, the circuit construction of this invention can be simplified. Further, for the purpose of enhancing the accuracy of measurement, the measuring device of this invention is adapted to obtain a fraction value of two to three digits from the time difference between the leading and trailing edges of the gate signal and pulses of the input signal to be measured, so that resolving power can be improved to 1/10.sup.2 to 1/10.sup.3. And, even if the frequency of the signal to be measured is divided down to 1/N by the 1/N frequency divider 2 provided at the stage preceding the gate circuit 3a, the gate time of the gate circuit 3a need not be extended and highly accurate frequency measurement can be performed with a short gate time. Moreover, even if the frequency of the signal to be measured is divided down to a certain frequency at the preceding stage of the gate circuit 3a, since the measuring time need not be extended, high frequencies can also be measured with high accuracy.

In other words, in the frequency measurement by opening the gate circuit for the unit time and counting the number of pulses of signal to be measured which pass through the gate circuit, where the frequency of the signal to be measured is divided down to 1/N by a frequency divider provided at the stage preceding the gate circuit, if the frequency is to be measured with the same accuracy as in the case where the frequency division does not take place, it is necessary to extend the gate time N times and increase the time for measurement. In general, a decimal counter is employed as the counter for display, and if it is formed with a flip-flop circuit, a feedback circuit and some other circuits are needed, resulting in reduced response speed. In the case of measuring a frequency to which this counter cannot respond, the frequency to be measured is usually divided down to 1/N to which the counter can respond, and then applied to the gate circuit. In this case, the gate time must be extended N times and the time for measurement becomes long. With the digital frequency measuring device of this invention, however, the accuracy of measurement can be raised more than two to three digits by the detection of fraction. Accordingly, if it is required merely to measure the frequency of a signal with the same accuracy as in the case of measuring with the counter only, the gate time can be shortened to 1/10.sup.2 to 1/10.sup.3 of that in the latter case.

Further, since the frequency of the signal to be measured, which is applied to the gate circuit 3a, can be reduced, the gate circuit and the counter 5a may be inexpensive ones. Accordingly, this invention has a merit of providing an inexpensive but highly accurate frequency measuring device which requires less circuit elements.

In the foregoing, the time differences between the leading and trailing edges of the gate signal and the input pulses arriving immediately after them are taken as .DELTA.T.sub.1 and .DELTA.T.sub.2, respectively. But it is also possible that the time differences between the leading and trailing edges of the gate signal and the time until arrival of a predetermined number of input pulse signals, for example, two, after the leading and trailing edges of the gate signal, respectively, are used as .DELTA.T.sub.1 and .DELTA.T.sub.2. In such a case, the voltage corresponding to the period T is also integrated for a period of time 2T and an operation 2(.DELTA.T.sub.1 - .DELTA.T.sub.2)/2T is performed. It will reasily be understood that this also provides exactly the same results as obtainable with the foregoing example. Further, the frequency divider may also be left out.

It will be seen that many modifications and variations may be effected without departing from the scope of novel concepts of this invention.