JPH1062461A - Signal spectrum measuring apparatus - Google Patents

Abstract

PROBLEM TO BE SOLVED: To shorten the measuring time of an entire band by setting the measuring time selecting the pass frequency of a BPF circuit so as to make the same shorter than the min. time necessary for the pass frequency and correcting a peak signal level in a direction increasing the same. SOLUTION: The measuring time of the pass band of a BPF circuit is set so as to be shorter than a necessary min. time and correction is applied to the peak hold signal outputted from a peak hold circuit corresponding to the difference between a set measuring time and a necessary min. time or a ratio of both of them to set an accurate signal level. For example, level conversion ratios correcting the peak hold signal levels of center frequencies f1-fn successively lowering in a half ratio are set to A1-An and, for example, the values thereof are set so as to successively redouble like An=2<n-1> . When a measuring time is set to T1 along with respective pass bands, a measuring time of a frequency round becomes Tt=nTi and, as the number of measuring bands increase from Tt=Σ2<n-1> NT1 in a usual case, a time can be shortened and the lowering of measuring accuracy can be also prevented by level correction.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a bandpass filter circuit capable of obtaining frequency spectrum components of a plurality of passbands in a time-division manner, and a peak of each passband of an output signal of the bandpass filter circuit. The present invention relates to a signal spectrum measuring apparatus having a peak hold circuit for detecting and holding a value, and more particularly to a technique for pursuing both a reduction in measurement time and an increase in accuracy.

[0002]

2. Description of the Related Art For example, there is a spectrum measuring and displaying apparatus as an apparatus for detecting and displaying individual signal levels of a plurality of discrete frequencies of an audio signal. As shown in FIG. 10, the input audio signal is taken into a band-pass filter circuit 1 whose passing frequency switches at high speed in a cyclic manner, and a peak hold circuit 2 detects and holds the peak value of the passing frequency signal. This is converted into a digital signal by the control / drive unit 3 and the liquid crystal display device 4 is driven to display the frequency spectrum of the audio signal.

The output signal of the band-pass filter circuit 1 is
Assuming that the center frequency fo of a certain pass band is fo = f1, an output signal at the frequency of f1 (shown by a solid line).
Is a time td from the input time, as shown in FIG.
An input signal (shown by a broken line) only when 1 has elapsed.
, And the peak hold circuit 2 holds the peak level of this signal (indicated by a dashed line). On the other hand, the center frequency fo of the pass band is fo = f1
/ 2, the output signal at that frequency is time td2 (=
Only when td1 × 2) has elapsed, the signal becomes the same level as the input signal, and the peak hold circuit 2 holds the peak level of this signal.

As described above, the output signal of the band-pass filter circuit 1 becomes the same level as the input signal after a lapse of a predetermined time, and a peak level signal corresponding to the input signal can be obtained by the peak hold circuit 2. However, the elapsed time differs depending on the center frequency of the pass band, and the elapsed time increases as the center frequency decreases. This elapsed time is the minimum required measurement time to obtain a predetermined measurement accuracy, and is determined by the driving capability of the elements (capacitors, equivalent resistances, etc.) in the bandpass filter circuit 1 and the internal active elements. The time is proportional. This measurement time is
As described above, if the center frequency of the pass band is halved,
Approximately double. That is, assuming that the frequency fn of the lowest pass band of the center frequency is fn = f1 / 2 ^{n− 1} , this measurement time is about n−about the measurement time in the case of the pass band of the highest center frequency f1. Reaches 1x.

[0005]

As described above, the minimum measurement time required for measuring an accurate peak level of a signal in a specific pass band becomes longer as the frequency becomes lower. Frequencies f1, f2, f3,.
.., Fn, where f1>f2>f3>...> Fn, the corresponding minimum required measurement time T1, T2, T
, Tn are T1 <T2 <T3 <... <Tn
Need to be At this time, the relationship between the center frequency and the measurement time is as shown in the following equation (1). f1 · T1 ≒ f2 · T2 ≒ f3 · T3 ≒ ········· fn · Tn ··· (1)

Therefore, for example, the center frequency of each pass band is defined as f2 = f1 / 2, f3 = f1 / 4, f4 = f1
/ 8,..., Fn = f1 / 2 ^{n− 1} , the minimum required measurement time in each pass band is T2 = 2T1, T3 = 4T1, ...
.., Tn = 2 ^{n− 1 As in} the case of T1, the length becomes longer successively. Therefore, the total time Tt of one cycle for measuring the entire band is as shown by the following equation (2), and is very long. Tt ≒ T1 + 2T1 + 4T1 +... +2 ^{n- 1} · T1 = Σ2 ^{n- 1} · T1 (2)

As described above, in the conventional signal spectrum measuring apparatus, since the measuring time especially in the low frequency band becomes long, there is a problem that the contents are unnatural when displaying the spectrum in real time. If an attempt is made to shorten the measurement time, another problem arises in that a low-frequency signal cannot be measured accurately.

The present invention has been made in view of the above points, and has as its object to reduce the time required for measuring the entire band,
Another object of the present invention is to provide a signal spectrum measuring device capable of performing accurate measurement even in a low frequency range.

[0009]

According to a first aspect of the present invention, there is provided a band pass filter circuit for switching a plurality of pass bands and outputting a frequency spectrum signal of each pass band in a time division manner. And a peak hold circuit for detecting and holding the peak value of the output signal of the pass band, wherein at least one pass frequency of the band pass filter circuit is measured for selecting the pass frequency. While setting the time shorter than the required minimum time required for the pass frequency,
A signal spectrum measuring apparatus is characterized in that a peak signal level obtained by the peak hold circuit is corrected to increase the level.

According to a second aspect of the present invention, in the first aspect, the correction is a signal spectrum measuring apparatus, wherein the correction is a correction corresponding to a time difference between the required minimum time and the measurement time.

In a third aspect based on the first or second aspect, the measurement time of each pass band is set to be substantially the same, and the correction is made to the peak signal level of each pass band in accordance with each measurement time. The signal spectrum measuring device is characterized in that it is added.

[0012]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS First, with respect to the same signal, a state in which the peak hold signal of a peak hold circuit 2 changes from the time when the signal is input to the bandpass filter circuit 1 is shown in FIG. The relationship between the time Ta until the peak hold signal reaches the same voltage level Vin as that of the input signal and the time Tb of the level Vin / 2, which is の of the voltage level Vin, is expressed as Tb = Ta / 2. is there. That is, when a shorter measurement time is set for the minimum required measurement time (approximately Ta), the level of the output signal of the peak hold circuit 2 must be smaller by the amount corresponding to the difference or ratio between the two times. I understand.

Therefore, in the present invention, the measurement time of at least one or more pass bands on the low frequency side of the band-pass filter circuit 1 is set to a time shorter than the required minimum time, and the measurement is performed for the frequency loop measurement. In addition to shortening the time, the level of the peak hold signal output from the peak hold circuit 2 is corrected (level conversion) in a subsequent circuit in accordance with the difference or ratio between the set measurement time and the required minimum time. ) To obtain an accurate signal level.

For example, assume that the level conversion ratios for correcting the peak hold signal levels of the center frequencies f1 to fn, in which the frequencies are successively reduced by half, are A1 to An, and the values are A1 = 1 and A2 = 2, A3 = 4, ...
..., setting so as to increase in a sequential bye relationship as An = 2 ^n-1, the measurement time for both the passband T1
The measurement time Tt for one cycle of the frequency is as follows: Tt = nT1 (3) When this equation (3) is compared with the above-mentioned equation (2), since nT1 <-2 ^{n− 1} · T1 (n> 1) (4), the signal measurement time of the entire pass band is measured. As the number increases, the time can be significantly reduced, and even if the measurement time is reduced in the low frequency range, the decrease in measurement accuracy can be eliminated by correcting the display level.

Hereinafter, embodiments of the present invention will be described in detail. FIG. 4 is a diagram showing a circuit configuration of the bandpass filter circuit 1. This bandpass filter circuit 1 is of a switched capacitor type. OP1 is an operational amplifier that functions as an integrator, and one of the capacitors C11 to C1n is selectively connected between the output terminal and the inverting input terminal. The choice of this capacitor is
A plurality of pairs of analog switches X11, X12,.
This is performed by turning on one of the paired switches of X1n under the control of the switch signals M1 to Mn.
The integration constants are determined by the capacitors C11 to C1n.

OP2 is a similar operational amplifier and has capacitors C21 to C21 between its output terminal and an inverted input terminal.
Any one of C2n is selectively connected. The selection of this capacitor is based on a plurality of pairs of analog switches X21, X21.
, X2n is turned on under the control of the switch signals M1 to Mn. Again, capacitors C21-C2
The integration constant is determined by n.

A circuit comprising analog switches S1 to S4 and a capacitor Cb, a circuit comprising analog switches S5 to S8 and a capacitor C3, and analog switches XS9 to S
14 and capacitors Ca and Ci are switched capacitor circuits, respectively, and function as equivalent resistances when driven by clocks φ1 and φ2. The clock φ1 is an odd-numbered switch (S1, S3, S5, S7, S9, S11, S1) of the switches S1 to S14.
3), and the clock φ2 is driven by the even-numbered switch (S
2, S4, S6, S8, S10, S12, S14).

These clocks φ1 and φ2 are two-phase clocks having the same frequency and inverted in phase with each other, and are set to a frequency of, for example, 50 KHz to 100 KHz which is twice or more of the input audio signal, as shown in FIG. The duty is set such that when one is active ("H" turns on the switch), the other is completely inactive ("L" turns off the switch). That is, the clocks φ1 and φ2 do not become active at the same time.

In this band pass filter circuit 1, FIG.
The timing of the switch signals M1 to Mn is set as shown in FIG. As a result, when the switch signal M1 becomes active, the capacitors C11 and C21 are selectively connected at the same time. Next, when the switch signal M2 becomes active, the capacitors C12 and C22 are selectively connected at the same time. ... Next, when the switch signal Mn becomes active, the capacitors C1n and C2n are selectively connected at the same time, and when the switch signal M1 becomes active, the capacitors C11 and C21 are selectively connected at the same time. As described above, the integration capacitors are sequentially and cyclically switched, and the operational amplifiers OP1 and OP1 are switched.
Switching of the integration constant of OP2 is performed, and the passing frequency is switched. That is, a bandpass filter circuit in which a plurality of pass bands are set in a unit time in a time-division manner is realized. The center frequency fo of the pass band of the band-pass filter circuit 1 is determined by the following equation (5). fck
Are the frequencies of the clocks φ1 and φ2. fo = (fck / 2π) · {(Ca · Cb) / (C1n · C2n)} 1/2 ···· (5)

FIG. 7 is a diagram showing a circuit configuration of the peak hold circuit 2, in which an operational amplifier OP3 and a pMOS transistor MP1 connected to its output side, resistors R1, R2,
It comprises a holding capacitor Cp, a reset nMOS transistor MN1, and an operational amplifier OP4 functioning as a voltage follower. The combination of the operational amplifier OP3 and the transistor MP1 also constitutes a voltage follower.

In the peak hold circuit 2, the transistor MN1 is temporarily turned off by a reset signal (RST) generated at the pass band switching timing (switching timing of the switch signals M1 to Mn) of the band pass filter circuit 1 shown in FIG. , The electric charge of the capacitor Cp accumulated up to that point is discharged. Thereafter, when the output signal of the band-pass filter circuit 1 is input and the voltage of the inverting input terminal (IN) of the operational amplifier PO3 increases, the output voltage decreases corresponding to the voltage, and the transistor MP3
1 is increased, and the charge on the capacitor Cp is accelerated, so that a voltage corresponding to the voltage applied to the inverting input terminal is generated on the capacitor Cp. Thereafter, when the voltage level of the capacitor Cp becomes higher than the level of the voltage applied to its inverting input terminal, the output voltage of the operational amplifier OP3 changes in a direction to increase, so that the conductivity of the transistor MP1 decreases and the capacitor Cp The charging current to the battery decreases. Thus, the capacitor Cp is charged with the voltage corresponding to the peak value of the voltage input to the inverting input terminal, and is held until reset. The resistors R1 and R2 are for preventing overshoot of the charging current to the capacitor Cp.

FIG. 8 is a block diagram showing the internal configuration of the control / drive unit 3. As shown in FIG. An A / D converter 31 converts a peak hold signal output from the peak hold circuit 2 into a digital signal. A control unit 32 includes a microcomputer. The control unit 32 takes in a digital signal obtained by the A / D converter 31 and converts the digital signal into data for display, and a control signal M1 of the bandpass filter circuit 1 described above.
, Mn, clocks φ1 and φ2, a reset signal RST of the peak hold circuit 2, and the like. Reference numeral 33 denotes a display driver for driving the liquid crystal display device 4.

Next, the operation will be described. Here, for simplicity, when the number of passbands is 5, that is, when the center frequencies are f1 to f5, f2 = f1 / 2 and f3 = f1 /
4, a case where f4 = f1 / 8 and f5 = f1 / 16 will be described. In the band-pass filter circuit 1,
The minimum measurement time required to measure the center frequency f1 is T
1. The minimum measurement time required for measuring the center frequency f2 is T2 (= 2T1), the minimum measurement time necessary for measuring the center frequency f3 is T3 (= 4T1), and the minimum necessary for measuring the center frequency f4. The measurement time is T4 (= 8T1), and the minimum measurement time required to measure the center frequency f5 is T5 (= 8T1).
16T1).

Note that these measurement times correspond to the center frequency f
1 is the ON duration of the switch signal M1, and the center frequency f
2 is the ON duration of the switch signal M2, and the center frequency f
3 is the ON duration time of the switch signal M3, and the center frequency f
4 is the ON duration of the switch signal M4, and the center frequency f
Reference numeral 5 denotes an ON duration of the switch signal M5, which is set.

Under such conditions, in the conventional method, as shown in FIG. 2A, each of the center frequencies f1 to f
Since the measurement times T1 to T5 are assigned to the 5 passbands, the total measurement time Tt in which the frequency scan of the band-pass filter circuit 1 makes one cycle is: Tt = T1 + T2 + T3 + T4 + T5 = 31T1 (6) Was.

Therefore, in the present embodiment, the center frequency f
As shown in FIG. 1, the measurement time is set to f1 → T1, f2 → T2, and f3 to f5 → T3 for the pass bands 1 to f5. Then, the level of the peak hold signal of the peak hold circuit 2 obtained at each measurement time is converted into a level by the control section 32 of the control / dry section 3 and then sent to the driver 33 as display data.
The conversion ratios are f1 to f3 → 1, f4 → 2, and f5 → 4. In other words, corresponding to the ratio of reduced measurement time,
The conversion is performed so that the level of the display data is increased.

Therefore, in the present embodiment, the total measurement time Tt in which the frequency scan of the band-pass filter circuit 1 makes one cycle is as follows: Tt = T1 + T2 + T3 + T3 + T3 = 15T1 (7) It is reduced to about half compared to Moreover, the displayed data is corrected and converted into a large value corresponding to the reduced measurement time, so that the accuracy does not deteriorate. FIG. 2B is a diagram for explaining this case.

[0028]

As described above, according to the present invention, the level of the peak hold signal is corrected in accordance with the reduction of the measurement time to obtain a spectrum signal. Therefore, the total measurement time of all passbands can be reduced by shortening the measurement time. And at the same time, the obtained spectrum signal is also an accurate signal. Therefore, it is possible to perform signal measurement in a short time without deteriorating the measurement accuracy even in a low frequency band, and this effect becomes more conspicuous as the number of switching bands increases, so that natural spectrum display of a large number of bands can be realized. Become.

Further, by making the measurement time common in each pass band, the pass band switching of the band-pass filter is simplified, so that the control of the switch signal for the switching is also simplified. is there.

[Brief description of the drawings]

FIG. 1 is an explanatory diagram of level conversion and measurement time reduction by a control / drive unit of a spectrum measurement device according to an embodiment of the present invention.

FIG. 2 is an explanatory diagram in which the total measurement time of one round of the entire pass band by the spectrum measuring apparatus of the embodiment is compared with the case of the conventional method.

FIG. 3 is an explanatory diagram of an output signal of a band-pass filter circuit and a peak hold signal of a peak hold circuit.

FIG. 4 is a circuit diagram showing a circuit configuration of a bandpass filter circuit.

FIG. 5 is a waveform diagram of a two-phase clock for controlling a band-pass filter circuit.

FIG. 6 is a waveform diagram of a switch signal for controlling a bandpass filter circuit.

FIG. 7 is a circuit diagram showing a circuit configuration of a peak hold circuit.

FIG. 8 is a block diagram showing a configuration of a control / dry unit.

FIG. 9 is a waveform chart for explaining a difference between frequencies of output signals of the band-pass filter circuit.

FIG. 10 is a block diagram illustrating a configuration of a spectrum measurement device.

[Explanation of symbols]

1: bandpass filter circuit, 2: peak hold circuit, 3: control / drive unit, 31: A / D converter, 3
2: control unit, 33: display driver, 4: liquid crystal display device.

Claims (3)

[Claims] 1. A bandpass filter circuit for switching a plurality of passbands and outputting a frequency spectrum signal of each passband in a time division manner, and detecting a peak value of an output signal of each passband of the bandpass filter circuit. A signal spectrum measuring apparatus having a peak hold circuit for holding the frequency as a minimum, and for at least one pass frequency of the band-pass filter circuit, a measurement time for selecting the pass frequency is a necessary minimum time required for the pass frequency. A signal spectrum measuring apparatus, wherein a shorter time is set and a correction is made to increase the peak signal level obtained by the peak hold circuit. 2. The signal spectrum measuring apparatus according to claim 1, wherein the correction is a correction corresponding to a time difference between the required minimum time and the measurement time. 3. The measurement time of each pass band is set to be substantially the same, and the correction is applied to the peak signal level of each pass band according to each measurement time.
Or the signal spectrum measuring device according to 2.