JP2012198115A - Spectrum analyzer and frequency characteristic correction method thereof - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a spectrum analyzer capable of measuring accurate electric power when measuring a wide-band signal.SOLUTION: A frequency characteristic correction unit 14 includes an equalizer 30 and an equalization coefficient storage section 31 which stores an equalization coefficient of the equalizer 30. A detection unit 15 detects a digital signal having frequency characteristics corrected on condition that a reference signal for correction for correcting frequency characteristics unique to a frequency conversion unit 12 is input to the frequency conversion unit 12, and a control unit 17 stores the equalization coefficient based upon a detection result of the digital signal obtained by the detection unit 15 in the equalization storage section 31 for a center frequency of a frequency band to be measured.

Description

  The present invention relates to a superheterodyne spectrum analyzer and a frequency characteristic correction method thereof.

  As a conventional superheterodyne spectrum analyzer, a frequency conversion unit that extracts a signal in a measurement target frequency band from an input signal and converts it to a signal in a specific intermediate frequency band, and a control that controls the measurement target frequency band , An analog / digital converter that converts the signal frequency-converted by the frequency converter to a digital signal, a frequency characteristic correction unit that corrects the frequency characteristic of the digital signal, and a detection unit that detects the signal with the corrected frequency characteristic (For example, refer to Patent Document 1).

  This conventional spectrum analyzer calculates the correction amount so that the detection result when a sine wave signal with a known frequency is input to the frequency conversion unit is constant at each frequency, and the calculated correction amount is set for each frequency. By setting the characteristic correction unit, the frequency characteristic specific to the frequency conversion unit is corrected.

  The conventional spectrum analyzer calculates the correction amount of each frequency based on the power of the center frequency detected at each frequency. This is because the detection characteristic determined by the frequency characteristic peculiar to the frequency conversion part and the frequency characteristic of the measurement filter constituting the detection part has an accurate resolution bandwidth (hereinafter referred to as “RBW”). It is assumed that it has Gaussian characteristics.

  Here, when the sine wave signal is input to the frequency converter, the detection characteristic can be corrected with the power at the detected center frequency even if the detection characteristic is not an accurate Gaussian characteristic. However, when a wideband signal is input to the frequency converter, a result different from that when the detection characteristic is an accurate Gaussian characteristic may be obtained as described below.

  In general, the frequency characteristic of a measurement filter of a spectrum analyzer is approximated by a Gaussian curve (hereinafter, the measurement filter is also referred to as a “Gaussian filter”), and RBW is the passband width at the 3 dB attenuation point of the Gaussian filter. That means. Depending on the standard, the passband width at the 6 dB attenuation point of the Gaussian filter may be referred to as RBW.

  The accuracy with which a Gaussian filter can be approximated by a recent spectrum analyzer is said to be RBW ≦ 3 MHz. Some spectrum analyzers can realize RBW wider than 3 MHz, but even if RBW matches, the attenuation curve does not match the Gaussian curve. ) In many cases.

  This is a band-pass filter used in the internal circuit of the spectrum analyzer, in particular, a YIG (Yttrium Iron Garnet) tuned band-pass filter (YIG Tuned Filter) used in the input stage of the quasi-microwave band spectrum analyzer. This is because it is limited by the characteristic of “YTF”.

  In addition, in a conventional spectrum analyzer, since a measurement with higher frequency resolution can be performed with a narrower RBW, a Gaussian filter having a high accuracy with RBW> 3 MHz is rarely required.

  However, in recent years, a Gaussian filter with a wide RBW and high accuracy has been required for analysis of a broadband modulation signal and broadband noise. For example, in wideband communication called UWB (Ultra Wide Band), Non-Patent Document 1 specifies the technical conditions of an on-vehicle radar system, and the peak power value radiated from the system at 22 to 29 GHz is 0 dB / 50 MHz or less. Is required.

  This can be verified by the fact that the power when detected with a Gaussian filter with RBW = 50 MHz is 0 dB or less, but as described above, there is a spectrum analyzer having accurate Gaussian filter characteristics with RBW = 50 MHz. Therefore, power equivalent to RBW = 50 MHz is calculated using a conversion formula (for example, see Non-Patent Document 2) using power measured by a Gaussian filter with RBW = 1 MHz or RBW = 3 MHz.

  However, since the power calculated using the conversion formula may cause a conversion error depending on the UWB modulation method, it is desired to directly measure RBW = 50 MHz. Note that such radiated power is usually measured at a specific center frequency with the spectrum analyzer set to the zero span mode (fixed measurement frequency).

  In order to accurately approximate the RBW = 50 MHz Gaussian filter to the 60 dB attenuation point, an analysis band of 224 MHz is required. In this case, it is necessary to widen the bandwidth of the frequency conversion unit to 224 MHz or more, and when the bandwidth of the frequency conversion unit becomes so wide, it is difficult to completely flatten the frequency characteristics in the band. Ripple is generated at a period of several tens of MHz.

  Since the pass characteristic of the spectrum analyzer is a convolution of the pass characteristic of the frequency converter and the pass characteristic of the detector, if the ripple described above occurs, the Gaussian characteristic can be realized with high accuracy in the detector. The detection characteristic deviates from the Gaussian characteristic.

That is, as shown in FIG. 5, in the RBW = 3 MHz Gaussian filter, the RBW becomes narrower with respect to the ripple period, and therefore the frequency characteristic | G (f) | of the frequency converter can be regarded as almost constant. . Therefore, if the power detected by the frequency characteristic | H (f) | of the measurement filter is corrected at the center frequency f ₀ , the frequency characteristic | G (f) | of the frequency converter is accurately corrected over each frequency. be able to.

On the other hand, in a Gaussian filter with RBW = 50 MHz, since the RBW becomes wider with respect to the ripple period, the frequency characteristic | G (f) | of the frequency converter cannot be regarded as constant. Therefore, even if the power detected by the frequency characteristic | H (f) | of the measurement filter is corrected at the center frequency f ₀ , the frequency characteristic | G (f) | of the frequency converter is accurately corrected over each frequency. Can not do.

JP-A-8-136593 (FIG. 3)

FFC rules (Part 15, Subpart F, Section 15.515) ITU-R recommendation SM. 1754

  As described above, the conventional spectrum analyzer cannot accurately correct the frequency characteristics of the frequency conversion unit when a wide RBW is set, so that it measures accurate power when measuring a wideband signal. There was a problem that could not be.

  The present invention has been made to solve such a problem, and an object of the present invention is to provide a spectrum analyzer capable of measuring accurate power when measuring a wideband signal and a method for correcting the frequency characteristic thereof. .

  The spectrum analyzer of the present invention extracts a signal in a frequency band to be measured from an input signal, converts the frequency to a signal in a specific intermediate frequency band, and controls to control the center frequency of the frequency band to be measured , An analog-to-digital converter that converts the signal frequency-converted by the frequency converter to a digital signal, a frequency characteristic correction unit that corrects the frequency characteristic of the digital signal, and a signal that has been corrected for the frequency characteristic In the spectrum analyzer including the detection unit, the frequency characteristic correction unit includes an equalizer and an equalization coefficient storage unit that stores an equalization coefficient of the equalizer, and the detection unit includes: The digital signal is converted on the condition that a correction reference signal for correcting a frequency characteristic specific to the frequency converter is input to the frequency converter. The control unit is configured to store an equalization coefficient based on a detection result of the digital signal by the detection unit in the equalization coefficient storage unit with respect to a center frequency of the measurement target frequency band. Has been.

  With this configuration, the spectrum analyzer of the present invention corrects the frequency characteristic specific to the frequency converter with respect to the center frequency of the frequency band to be measured by the equalizer, so that accurate power can be obtained when measuring a broadband signal. Can be measured.

  The control unit may store an equalization coefficient in which the frequency characteristic of the equalizer is an inverse characteristic of the transfer function of the frequency conversion unit in the equalization coefficient storage unit.

  With this configuration, the spectrum analyzer of the present invention can correct the frequency characteristics unique to the frequency converter to a fixed value for each measurement frequency.

  The spectrum analyzer of the present invention may include a correction reference signal generation unit that generates the correction reference signal.

  With this configuration, the spectrum analyzer of the present invention can correct the frequency characteristics unique to the frequency converter without obtaining a correction reference signal from an external device.

  The correction reference signal generation unit may generate an impulse as the correction reference signal.

  With this configuration, the spectrum analyzer of the present invention can apply impulses as correction reference signals.

  Further, the correction reference signal generation unit generates a signal obtained by modulating a pulse signal that is sufficiently shorter than 1 / RBW with a signal having a frequency near the center frequency of the measurement target frequency band within the measurement target frequency band. It may be generated as a correction reference signal.

  With this configuration, the spectrum analyzer of the present invention can apply, as a correction reference signal, a signal having a frequency near the center frequency of the frequency band to be measured and obtained by modulating a pulse signal sufficiently shorter than 1 / RBW. it can.

  The frequency characteristic correction method of the present invention includes a frequency conversion unit that extracts a signal in a frequency band to be measured from an input signal and converts the signal to a signal in a specific intermediate frequency band, and a center frequency in the frequency band to be measured. A control unit for controlling the frequency, an analog-digital converter for converting the signal frequency-converted by the frequency conversion unit into a digital signal, a frequency characteristic correction unit for correcting the frequency characteristic of the digital signal, and the frequency characteristic is corrected. In the frequency characteristic correction method of the spectrum analyzer, the frequency characteristic correction unit includes an equalizer, and an equalization coefficient storage unit that stores an equalization coefficient of the equalizer. Provided that a correction reference signal for correcting the frequency characteristic specific to the frequency converter is input to the frequency converter. Detecting the digital signal; and the control unit stores an equalization coefficient based on a detection result of the digital signal by the detection unit with respect to a center frequency of the frequency band to be measured. Storing in the unit.

  Therefore, the frequency characteristic correction method of the present invention corrects the frequency characteristic specific to the frequency converter with respect to the center frequency of the frequency band to be measured by the equalizer, so that accurate power can be obtained when measuring a wideband signal. Can be measured.

  The present invention can provide a spectrum analyzer capable of measuring accurate power when measuring a broadband signal, and a frequency characteristic correcting method thereof.

It is a block diagram of the spectrum analyzer which concerns on embodiment of this invention. It is a flowchart which shows the correction coefficient calculation operation | movement of the spectrum analyzer which concerns on embodiment of this invention. It is a conceptual diagram for demonstrating the calculation process of the response coefficient series in the correction coefficient calculation operation | movement of the spectrum analyzer which concerns on embodiment of this invention. It is a flowchart which shows the signal analysis operation | movement of the spectrum analyzer which concerns on embodiment of this invention. It is a conceptual diagram which shows the relationship between the frequency characteristic of a frequency conversion part when the wide RBW is set to the conventional spectrum analyzer, and the frequency characteristic of a measurement filter.

  Embodiments of the present invention will be described below with reference to the drawings.

  As shown in FIG. 1, the spectrum analyzer 1 according to the embodiment of the present invention includes a correction reference signal generation unit 10 that generates impulses, a switch 11, a frequency conversion unit 12 that down-converts high-frequency signals, and a down-converter. An analog-digital converter (hereinafter referred to as “ADC”) 13 that converts the converted analog intermediate frequency signal into a digital signal, and a frequency characteristic correction unit that corrects the digital signal according to the frequency characteristic of the frequency conversion unit 12. 14, a detection unit 15, a switch 16, a control unit 17 that controls each unit of the spectrum analyzer 1, a display unit 18 including a liquid crystal display, and an input unit 19 including a keyboard device and a pointing device. .

  The correction reference signal generation unit 10 is controlled by the control unit 17 to generate a pulse having a width sufficiently narrower than the reciprocal of the center frequency of the band to be measured (hereinafter simply referred to as “measurement center frequency”). .

  The spectrum analyzer 1 takes two modes: a correction coefficient calculation mode for calculating a correction coefficient for correcting the frequency characteristic unique to the frequency converter 12 and a signal analysis mode for signal analysis of the signal under test.

  The switch 11 has a terminal S1 connected to the correction reference signal generation unit 10, a terminal S2 connected to the input terminal T to which the signal under test is input, and a terminal S3 connected to the frequency conversion unit 12. is doing.

  The switch 11 is controlled by the control unit 17, and when the spectrum analyzer 1 takes the correction coefficient calculation mode, the terminal S1 and the terminal S3 are connected. When the spectrum analyzer 1 takes the signal analysis mode, the terminal S2 is connected. Are connected to the terminal S3.

  The frequency converter 12 includes an attenuator 20, a pre-selector 21, an oscillator 22 that generates a sine wave signal having a variable frequency, and a mixer 23 that mixes the signal generated by the oscillator 22 with the signal that has passed through the pre-selector 21. A low-pass filter (hereinafter referred to as “LPF”) 24, an oscillator 25 that generates a sine wave signal having a constant frequency, a mixer 26, and an LPF 27.

  The pre-selector 21 is composed of YTF, and allows a band signal designated by the control unit 17 to pass therethrough. The oscillator 22 generates a sine wave signal having a frequency specified by the control unit 17.

  The mixer 23 mixes the signal that has passed through the pre-selector 21 and the sine wave signal generated by the oscillator 22. Further, the LPF 24 removes a component on the high frequency side as a result of mixing by the mixer 23.

  Here, the control unit 17 changes the band that is passed through the pre-selector 21, while the sine wave that is generated by the oscillator 22 so that the center frequency of the mixing result from which the high frequency side component is removed by the LPF 24 is constant. The frequency of the signal is controlled. For this reason, the signal that has passed through the LPF 24 becomes an intermediate frequency signal having a constant center frequency.

  That is, the oscillator 22, the mixer 23, and the LPF 24 constitute a first down converter that down-converts a high frequency signal into a first intermediate frequency signal having a constant center frequency.

  The mixer 26 mixes the signal that has passed through the LPF 24 and the sine wave signal generated by the oscillator 25. Further, the LPF 27 removes a component on the high frequency side as a result of mixing by the mixer 26.

  That is, the oscillator 25, the mixer 26, and the LPF 27 constitute a second down converter that down-converts the first intermediate frequency signal into the second intermediate frequency signal.

  The frequency characteristic correction unit 14 includes an equalizer 30 and an equalization coefficient storage unit 31 that stores the equalization coefficient of the equalizer 30. The equalizer 30 is configured by a digital filter, and in the present embodiment, is configured by an FIR (Finite Impulse Response) filter including a plurality of taps.

  The equalization coefficient storage unit 31 is configured by a storage medium such as a RAM (Random Access Memory) or a flash memory. The equalization coefficient storage unit 31 stores the coefficient of each tap of the FIR filter constituting the equalizer 30 for each center frequency.

  The detection unit 15 is configured by a Gaussian filter. In the present embodiment, the Gaussian filter is configured by an FIR filter. In the present embodiment, the Gaussian filter constituting the detection unit 15 is configured to approximate a Gaussian filter of RBW = 50 MHz to at least a 60 dB attenuation point.

  The switch 16 has a terminal S 1 connected to the ADC 13, a terminal S 2 connected to the equalizer 30 of the frequency characteristic correction unit 14, and a terminal S 3 connected to the detection unit 15. The terminals of the switch 16 are connected in the same manner as the terminals of the switch 11.

  That is, the switch 16 is controlled by the control unit 17, and when the spectrum analyzer 1 takes the correction coefficient calculation mode, the terminal S1 and the terminal S3 are connected, and when the spectrum analyzer 1 takes the signal analysis mode, Terminals S2 and S3 are connected.

  The control unit 17 is configured by a processor such as a CPU (Central Processing Unit) and the like. The mode setting of the spectrum analyzer 1 according to the designation by the input unit 19 etc., the setting of the measurement center frequency designated by the input unit 19 etc. Each part of the spectrum analyzer 1 is controlled such as calculation of the equalization coefficient of the equalizer 30, setting of the equalization coefficient of the equalizer 30, display of the detection result by the detection unit 15, and the like.

  The operation of the spectrum analyzer 1 configured as described above will be described with reference to the drawings. FIG. 2 shows a correction coefficient calculation operation in the correction coefficient calculation mode of the spectrum analyzer 1.

  First, the pass band of the pre-selector 21 and the frequency of the signal generated by the oscillator 22 are set by the control unit 17 according to the measurement center frequency specified by the input unit 19 or the like (step S1), and the control unit 17 An impulse is generated by the correction reference signal generation unit 10 controlled by (step S2).

  Next, the impulse response sequence x (k) output from the ADC 13 is acquired by the control unit 17 (step S3). Here, k = {0, 1, 2,..., M−1}, and in this embodiment, M = 1024.

Next, the control unit 17 performs a discrete Fourier transform (also referred to as “DFT”) of the impulse response sequence x (k), and calculates a transfer function G (f) of the frequency conversion unit 12 ( Step S4). Here, the amplitude characteristic of G (f) is | G (f) |, the group delay characteristic is τ (f), and the amplitude characteristic of the inverse characteristic G ⁻¹ (f) of G (f) is 1 / | G. (F) | and the group delay characteristic is −τ (f), and as shown in FIG. 3, the control unit 17 converts an inverse discrete Fourier transform (Inverse) into | G ⁻¹ (f) | and −τ (f). By applying Discrete Fourier Transform (also referred to as “IDFT”), a response coefficient sequence y (k) is calculated (step S5).

  The response coefficient sequence y (k) calculated in this way becomes the equalization coefficient of the equalizer 30, and the response coefficient sequence y (k) is multiplied by a window function as necessary by the control unit 17. Is cut out (step S6). In the present embodiment, the control unit 17 cuts 1024 equalization coefficients into 100.

  Next, the response coefficient series y (k) is stored as an equalization coefficient in the equalization coefficient storage unit 31 by the control unit 17 (step S7), and the correction coefficient calculation operation ends. When the equalization coefficients for a plurality of center frequencies are stored in the equalization coefficient storage unit 31, the measurement center frequency is changed by the control unit 17, and the correction coefficient calculation operation is executed again.

  FIG. 4 shows a signal analysis operation in the signal analysis mode of the spectrum analyzer 1.

  First, the pass band of the pre-selector 21 and the frequency of the signal generated by the oscillator 22 are set by the control unit 17 in accordance with the measurement center frequency specified by the input unit 19 or the like (step S11).

  Further, the equalization coefficient corresponding to the measurement center frequency is set in the equalizer 30 by the control unit 17 from the equalization coefficients stored in the equalization coefficient storage unit 31 (step S12). Here, when the equalization coefficient corresponding to the measurement center frequency is not stored in the equalization coefficient storage unit 31, a correction coefficient calculation operation may be executed.

  In this state, when a signal under test is input from the input terminal T, the signal under test is frequency converted by the frequency converter 12 (step S13), converted to a digital signal by the ADC 13 (step S14), and frequency characteristic correction is performed. The frequency characteristic is corrected by the unit 14 (step S15) and detected by the detection unit 15 (step S16).

  Then, the detection result is displayed on the display unit 18 (step S17), and the signal analysis operation ends. When analysis is performed on a plurality of center frequencies, the control center 17 changes the measurement center frequency, and the signal analysis operation is executed again.

  As described above, the spectrum analyzer 1 according to the embodiment of the present invention corrects the frequency characteristic specific to the frequency converter 12 with the equalizer 30 with respect to the center frequency of the frequency band to be measured. Accurate power can be measured when measuring the signal.

  In the case where both the frequency characteristic correction unit 14 and the detection unit 15 are configured by FIR filters as in the present embodiment, the frequency characteristic correction unit 14 is convoluted by a single FIR filter by convolving these coefficients. Further, the detection unit 15 may be configured.

  In this case, resources mounted on a circuit such as an FPGA (Field-Programmable Gate Array) or an ASIC (Application Specific Integrated Circuit) constituting the FIR filter can be reduced.

  Further, in the present embodiment, the correction reference signal generation unit 10 has been described as generating an impulse. However, in the present invention, a frequency near the center frequency of the measurement target frequency band within the measurement target frequency band. A signal obtained by modulating a pulse signal that is sufficiently shorter than 1 / RBW may be generated as a correction reference signal.

  In the present invention, the correction reference signal generation unit 10 may be removed from the spectrum analyzer 1 and the spectrum analyzer 1 may input the correction reference signal from the external input terminal.

DESCRIPTION OF SYMBOLS 1 Spectrum analyzer 10 Correction | amendment reference signal production | generation part 11, 16 Switch 12 Frequency conversion part 13 ADC
14 Frequency characteristic correction unit 15 Detection unit 17 Control unit 18 Display unit 19 Input unit 20 Attenuator 21 Preselector 22, 25 Oscillator 23, 26 Mixer 24, 27 LPF
30 Equalizer 31 Equalization coefficient storage

Claims (6)

A frequency conversion unit that extracts a signal of a frequency band to be measured from an input signal and converts the frequency into a signal of a specific intermediate frequency band;
A control unit for controlling the center frequency of the frequency band to be measured;
An analog-digital converter that converts the signal frequency-converted by the frequency converter into a digital signal;
A frequency characteristic correction unit for correcting the frequency characteristic of the digital signal;
In a spectrum analyzer comprising a detection unit for detecting a signal whose frequency characteristics are corrected,
The frequency characteristic correction unit includes an equalizer and an equalization coefficient storage unit that stores an equalization coefficient of the equalizer,
The detector detects the digital signal on the condition that a correction reference signal for correcting a frequency characteristic specific to the frequency converter is input to the frequency converter,
The control unit stores an equalization coefficient based on a detection result of the digital signal by the detection unit in the equalization coefficient storage unit with respect to a center frequency of the measurement target frequency band. analyzer.   The spectrum according to claim 1, wherein the control unit stores an equalization coefficient in which the frequency characteristic of the equalizer is an inverse characteristic of a transfer function of the frequency conversion unit in the equalization coefficient storage unit. analyzer.   The spectrum analyzer according to claim 1, further comprising a correction reference signal generation unit that generates the correction reference signal.   The spectrum analyzer according to claim 3, wherein the correction reference signal generation unit generates an impulse as the correction reference signal.   The correction reference signal generation unit is a signal having a frequency near the center frequency of the measurement target frequency band within the measurement target frequency band, and a signal obtained by modulating a pulse signal sufficiently shorter than 1 / RBW is corrected. The spectrum analyzer according to claim 3, wherein the spectrum analyzer is generated as a reference signal for use. A frequency conversion unit that extracts a signal of a frequency band to be measured from an input signal and converts the frequency into a signal of a specific intermediate frequency band;
A control unit for controlling the center frequency of the frequency band to be measured;
An analog-digital converter that converts the signal frequency-converted by the frequency converter into a digital signal;
A frequency characteristic correction unit for correcting the frequency characteristic of the digital signal;
In the frequency characteristic correction method of the spectrum analyzer, comprising: a detection unit that detects the signal whose frequency characteristic is corrected,
The frequency characteristic correction unit includes an equalizer and an equalization coefficient storage unit that stores an equalization coefficient of the equalizer;
The detection unit detects the digital signal on the condition that a correction reference signal for correcting a frequency characteristic unique to the frequency conversion unit is input to the frequency conversion unit;
The control unit has a step of storing, in the equalization coefficient storage unit, an equalization coefficient based on a detection result of the digital signal by the detection unit with respect to a center frequency of the frequency band to be measured. The frequency characteristic correction method characterized by this.

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