JPH0664095B2 - Spectrum analyzer - Google Patents

Description

TECHNICAL FIELD The present invention relates to an apparatus for measuring a frequency distribution of an input signal, and relates to a spectrum analyzer suitable for measuring a high frequency region with high resolution.

(Prior Art) Conventional FFT (Fas
Devices using t Fourier Transform) and devices using bandpass filters are known.

FIG. 6 shows a block diagram of an apparatus using the FFT analyzer. In FIG. 6, 1 is an input terminal, 2 is a low pass filter, 3 is an analog / digital converter, and 4 is an FFT processor. The low-pass filter 2 removes high frequency components from the input signal applied to the input terminal 1. The upper limit frequency of the low pass filter 2 is set to 1/2 or less of the sampling frequency of the analog / digital converter 3. The signal converted into the digital signal by the analog / digital converter 3 is subjected to Fourier transform calculation by the FFT processor 4, and the frequency spectrum is output. In such a device,
High-speed measurement is possible and frequency resolution can be set arbitrarily.

FIG. 7 shows a block diagram of an apparatus using a bandpass filter. In FIG. 7, 5 and 8 are mixers, 6 and 9 are oscillators, and 7
Is a bandpass filter, and 10 is a detection circuit. The input signal applied to the input terminal 1 is noise-removed by the low-pass filter 2 and then mixed with the output of the oscillator 6 by the mixer 5,
The frequency is shifted. The band-pass filter 7 extracts the necessary frequency component from the frequency-shifted signal, mixes it with the output of the oscillator 9 in the mixer 8, shifts the frequency again, and detects the amplitude in the detection circuit 10 to measure the amplitude. To obtain the frequency spectrum, the output frequency of the oscillator 6 is swept. This device is characterized in that it can measure up to a high frequency band.

(Problems to be Solved by the Invention) However, such a conventional technique has the following drawbacks. The device using the FFT analyzer of FIG. 6 is limited by the conversion speed of the analog-digital converter,
It is not possible to measure frequencies above 1/2 the sampling frequency. Therefore, it is not suitable for high frequency measurement. Further, in the device using the bandpass filter shown in FIG. 7, the frequency resolution is limited by the characteristics of the bandpass filter 7, so it is difficult to increase the resolution. Further, since the frequency of the oscillator 6 has to be swept, there is also a drawback that the measurement takes time.

(Object of the Invention) An object of the present invention is to provide a spectrum analyzer capable of measuring a frequency spectrum quickly and with high resolution even in a high frequency band.

(Means for Solving the Problems) In order to solve the above problems, the present invention relates to a first oscillator having a variable frequency, a first oscillator for mixing the output of the oscillator and a signal under measurement, and shifting to a constant frequency. 1 mixer and this 1st
A bandpass filter for band limiting the output of the mixer of
A second oscillator having a fixed frequency, a second mixer for inputting the output of the second oscillator and the output of the bandpass filter, and a Fourier transform for inputting the output of the mixer It is equipped with an analyzer.

(Embodiment) FIG. 1 is a block diagram showing an embodiment of a spectrum analyzer according to the present invention. In FIG. 1, reference numeral 1 is an input terminal to which a signal under measurement is applied. The low-pass filter 2 removes unnecessary high-frequency components from the signal under measurement applied to the input terminal 1, and inputs the signal to the first mixer 15. The output from the first oscillator 16 is also input to the first mixer 15. 17 is a bandpass filter, the first mixer
15 outputs are input. The output of the bandpass filter 17 and the output of the second oscillator 19 are input to the second mixer 18. The output of the second mixer 18 is input to the Fourier transform analyzer 20. The Fourier transform analyzer 20 includes a low-pass filter, an analog / digital converter, and a calculator.

FIG. 2 shows the power spectrum at each point in the block diagram of FIG. FIG. 2A shows the power spectrum of the output of the low-pass filter 2, and the components of the input signal with a frequency f _A or higher are limited. The output of the low pass filter 2 and the output of the first oscillator 16 (frequency f _B ) are mixed by the first mixer 15, and the power spectrum of the sum frequency and the power spectrum of the difference frequency are added. The spectrum is obtained. This is shown in FIG. 2b. S ₁ and S ₂ are the power spectra of the sum and difference frequencies, respectively, and the output of the first mixer 15 shows these S
The power spectrum of the sum S of ₁ and S ₂ is obtained. Second
FIG. 7C shows the power spectrum of the output of the bandpass filter 17, and only the portion of the power spectrum S between fc and fc + f _F passes. Note that f _F is set in the band of the Fourier transform analyzer 20, and fc> f _A. The output of the bandpass filter 17 and the output of the second oscillator 19 are the second mixer.
Mixed by 18. If the output frequency of the second oscillator 19 is set to fc (the lower limit of the band of the bandpass filter 17), the power spectrum of FIG. 2c is the portion of frequencies 0 to f _F as shown in FIG. 2d. And 2 fc to 2 fc + f _F part 2
Be divided. The part of 2fc to 2fc + f _F is removed by the low-pass filter in the Fourier transform analyzer 20, and 0 to f _F
Is converted into a digital signal by the analog / digital converter, and Fourier calculation is performed by the calculator to obtain the power spectrum. That is, input signal (fc-f _B ) ~
_{(Fc-f B + f F} ) portion is converted into a frequency band of (0 to F _F) is processed by Fourier transform analyzer 20, the power spectrum is measured. Therefore Changing stepwise at f _F intervals until the output frequency f _B of the first oscillator 16 exceeds fc-f _A from fc, the power spectrum between band 0 to F _A of the input signal is obtained. This is shown in FIG.
In FIG. 3, the shaded area is the area where the power spectrum is obtained at each step. Steps 1 to n (= f _A
By executing up to the integer part of −f _F + 1), the power spectrum between 0 and f _A is obtained.

FIG. 4 shows another embodiment of the present invention. 10 in FIG.
1,102 is an input terminal, 201,202 is a low pass filter, 151,152
Is a first mixer, 16 is a first oscillator, 171, 172 are bandpass filters, 181, 182 are second mixers, 19 is a second oscillator,
201 is a Fourier transform analyzer. Fourier transform analyzer 2
The 10 incorporates two sets of low-pass filters and analog-digital converters. Further, the arithmetic unit is configured to be able to perform Fourier analysis of two signals. In this embodiment, the low pass filter 201, the first mixer 151, the first oscillator
16, the band pass filter 171, the second mixer 181, the second oscillator 19, and the Fourier transform analyzer 210 constitute one set of spectrum analyzers, and the low pass filter 202, the first mixer 152, and the first oscillator 16, the bandpass filter 172, the second mixer 182, the second oscillator 19, and the Fourier transform analyzer 210 constitute another set of spectrum analyzers. In this embodiment, not only can the power spectra of two signals be measured simultaneously, but the phase difference between them can also be measured.

In the embodiment shown in FIG. 1, the output frequency of the first oscillator is fc− (n
-1) f _F (n is the number of steps), and the input frequency range of the Fourier transform analyzer was 0 to f _F. However, as shown in FIG. 5a, due to the characteristics of the bandpass filter 17 and the like, there are frequency components of f _F or more and 0 or less, and a signal symmetrical with respect to the frequency axis overlaps with each other. As shown, an error occurs near the frequency 0. In order to avoid this, the frequency of the second oscillator 19 may be set to fc−Δf instead of fc. In this way, as shown in FIG. 5c,
The frequency range of the input signal of the Fourier transform analyzer 20 is Δf
f _F + Δf, and there is no overlap around frequency 0. It should be noted that Δf is a portion where there is no overlapping portion and may be set to a value as small as possible.

(Effects of the Invention) As specifically described with the above embodiments, according to the present invention, the first mixer 15 and the first oscillator 16 shift the input signal to the high frequency side to pass the bandpass filter, Further, the second mixer 18 and the second oscillator 19 are shifted to the low frequency side, and the Fourier transform analyzer 20 determines the frequency spectrum. Therefore, the Fourier transform analyzer can be used even at high frequencies where the Fourier transform analyzer cannot be used, so that the frequency spectrum can be quickly obtained with high frequency resolution.

Further, even when measuring in a wide frequency range, the output frequency of the first oscillator 16 may be changed in steps of the band f _F of the Fourier transform analyzer 20, which is significantly larger than the conventional method using a bandpass filter. Since the number of steps can be reduced, the measurement can be performed quickly.

Furthermore, since the change of frequency decomposition farm can be set by the Fourier transform analyzer 20, the band of the bandpass filter 17 and the first
The step width of the output frequency of the oscillator may be fixed, and the band pass filter and the oscillator can be easily manufactured.

[Brief description of drawings]

FIG. 1 is a block diagram showing an embodiment of the spectrum analyzer according to the present invention, FIG. 2 is a characteristic curve diagram for explaining the operation of the embodiment of FIG. 1, and FIG. 3 is for measuring a wide band. FIG. 4 is a block diagram showing another embodiment of the present invention, and FIGS. 5, 6 and 7 are block diagrams showing a conventional example. 15,151,152 …… First mixer, 16 …… First oscillator, 1
7,171,172 …… Bandpass filter, 18,181,182 …… Second mixer, 19 …… Second oscillator, 20,201 …… Fourier transform analyzer, f _B …… First oscillator output frequency, fc ……
Lower limit frequency of band pass band, f _F …… Band of Fourier transform analyzer.

Claims (1)

[Claims] 1. _A low-pass filter for removing a component having a frequency f _A or more of an input signal, a first oscillator having a variable oscillation frequency f _B , and a first oscillator for mixing an output of the low-pass filter and an output of the first oscillator. 1 mixer, a bandpass filter having a frequency band of f _{C to} f _C + f _F and receiving the output of the first mixer, and an oscillation frequency of the lower limit value f _C of the frequency band of the bandpass filter. An equal second oscillator, a second mixer that mixes the output of the bandpass filter and the output of the second oscillator, a low-pass filter that receives the output of the second mixer, and an analog output of the low-pass filter. analog-to-digital converter for converting from the output of the analog-digital converter consists calculator for obtaining the power spectrum subjected to Fourier calculation, the frequency band that can be analyzed is 0 to F _F Comprising a Rie conversion analyzer, said with the lower limit f _C of the frequency band of the band-pass filter is set to be larger than the upper limit value f _A of the frequency band of the low-pass filter, the oscillation frequency f _B of the first oscillator f _C f _C -f _a is changed stepwise at f _F interval to over the spectrum analyzer, characterized by being configured so that the power spectrum of the input signal is obtained.