JP2008107172A - Spectrum analyzer - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To solve such a problem that the sweep type spectrum analyzer cannot measure the correct spectrum at the low frequency region because of generating a large spurious signal at the vicinity of 0 Hz because if the frequency of input signal is low, the frequency of the local oscillation signal becomes near to the intermediate frequency. <P>SOLUTION: When the low frequency range is measured, the measurement signal is not input to the frequency converter part but the spectrum data is made to form from the output of the low pass filter while eliminating the high frequency component of the measurement signal by the low pass filter. The spurious signal in the vicinity of 0 Hz can be restricted because the output signal of the frequency converter part is not used for the spectrum formation in the low frequency region. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a spectrum analyzer capable of accurately measuring a spectrum down to a low frequency range by reducing the influence of a spurious signal near 0 Hz in a sweep type spectrum analyzer.

  A spectrum analyzer is a measuring device that accurately measures various frequency components contained in a signal. The configuration of the spectrum analyzer is described in Patent Document 1, for example. The outline of the sweep-type spectrum analyzer will be described below based on Patent Document 1.

  FIG. 3 is a configuration diagram of the spectrum analyzer disclosed in Patent Document 1. In FIG. In FIG. 3, the input signal is converted into a signal having a predetermined amplitude by the attenuator 10 and input to the frequency conversion unit 11. The frequency converter 11 includes mixers 11a and 11d, local oscillators 11b and 11e, and a bandpass filter 11c. The output of the attenuator is frequency-mixed with the output of the local oscillator 11b by the mixer 11a and converted to the first intermediate frequency. This first intermediate frequency signal is input to the bandpass filter 11c, and unnecessary frequency components are removed. Then, the first intermediate frequency signal is frequency-mixed with the output of the local oscillator 11e by the mixer 11d, and converted into a second intermediate frequency signal.

  The second intermediate frequency signal is converted into a digital value by the AD converter 12, and unnecessary frequency components are removed by the digital filter 13, and then stored in the waveform memory 14. The digital filter 13 is a band-pass filter having a bandwidth corresponding to the frequency resolution of the spectrum analyzer with a predetermined frequency as the center.

  The data stored in the waveform memory 14 is signal-processed by the DSP 15 and displayed on the display unit 16. The clock oscillator 17 supplies a clock to the AD converter 12, the digital filter 13, the waveform memory 14, and the DSP 15. The sweep control unit 18 sweeps the frequency of the local oscillator 11b in the frequency range specified by the main control unit 19.

  In the spectrum analyzer having such a configuration, when attempting to measure a spectrum in the vicinity of a frequency of 0 Hz, the output frequency of the local oscillator 11b is close to the first intermediate frequency, and a large spurious signal is generated in the vicinity of 0 Hz. For this reason, the subject that the spectrum of 0 Hz vicinity cannot be measured correctly occurred.

In order to solve this problem, Patent Document 2 divides the entire measurable frequency band into a plurality of bands, and sets a pass band that is not affected by the 0 Hz signal and obtains an appropriate sweep speed for each band. The input frequency is swept in each divided band, and the maximum level of the detection output and the frequency at that time are obtained for each band. Of the maximum levels, the frequency that gives the highest level is set as the center frequency, and the input signal is swept and displayed in a desired frequency span.
JP-A-9-257843 Japanese Patent Laid-Open No. 11-281684

  However, as described above, the spectrum analyzer described in Patent Document 1 generates a large spurious signal because the oscillation frequency of the local oscillator 11b is close to the first intermediate frequency when attempting to measure a low frequency range. There is a problem that measurement near 0 Hz cannot be performed accurately. Further, since the local oscillation signal leaked from the mixer 11b is directly input to the AD converter 12, overflow of the AD converter and cross modulation distortion occur, and there is also a problem that measurement near 0 Hz cannot be performed accurately.

  The invention of Patent Document 2 solves the above-mentioned problem, but when measuring over a wide frequency range, the frequency resolution bandwidth varies depending on the measurement frequency, so that the uniformity of measurement results cannot be obtained. There was a problem.

  Accordingly, an object of the present invention is to accurately measure a signal having a low frequency around 0 Hz by generating spectrum data from the output of the low-pass filter without inputting a measurement signal to the frequency converter when measuring in a low frequency range. It is to provide a spectrum analyzer capable of

In order to solve such a problem, the invention according to claim 1 of the present invention,
A frequency converter that converts the frequency of the input analog signal to an intermediate frequency;
A low-pass filter that removes high-frequency components of the input analog signal;
A first switch that outputs a measurement signal to the low-pass filter when a frequency range to be measured is low, and outputs a measurement signal to the frequency converter when the frequency range is high;
An AD converter that converts an input analog signal into a digital value;
A second switch that selects the output of the low-pass filter when the frequency range to be measured is low, selects the output of the frequency converter when the frequency range is high, and outputs the selected output to the AD converter When,
A signal processing unit that receives the output of the AD conversion unit and generates spectral data;
Spectral data generated by the signal processing unit is input, and a display unit that displays the spectral data;
Is provided. A spurious signal near 0 Hz can be suppressed.

The invention according to claim 2
A frequency converter that converts the frequency of the input analog signal to an intermediate frequency;
A low-pass filter that removes high-frequency components of the input analog signal;
A switch that outputs a measurement signal to the low-pass filter when the frequency range to be measured is low, and outputs a measurement signal to the frequency converter when the frequency range is high,
A first AD converter that receives the output of the low-pass filter and converts the input signal into a digital value;
A second AD converter that receives the output of the frequency converter and converts the input signal into a digital value;
When the outputs of the first and second AD converters are input and the frequency range to be measured is low, spectrum data is generated based on the output of the first AD converter, and when the frequency range to be measured is high A signal processing unit for generating spectrum data based on an output of the second AD converter;
Spectral data generated by the signal processing unit is input, and a display unit that displays the spectral data;
Is provided. A spurious signal near 0 Hz can be suppressed.

The invention according to claim 3 is the invention according to claim 1 or claim 2,
When the frequency range to be measured extends from the low frequency region to the high frequency region, the low frequency region generates spectrum data from the output of the low pass filter, and the high frequency region generates spectrum data from the output of the frequency converter. Then, these spectrum data are synthesized and displayed. A spurious signal can be suppressed and a spectrum in a wide frequency range can be obtained.

The invention according to claim 4 is the invention according to any one of claims 1 to 3,
The signal processing unit band-limits the input digital value with a band-limiting filter, and generates spectrum data by detecting and log-converting the band-limited value. A general spectral data generation method can be used.

The invention according to claim 5 is the invention according to any one of claims 1 to 4,
The signal processing unit is configured to generate spectrum data by performing FFT analysis on input data when a frequency range to be measured is low. Data processing is simplified.

The invention according to claim 6 is the invention according to any one of claims 1 to 5,
The DC offset of the low-pass filter is corrected based on the output value of the low-pass filter when no signal is input. Spurious signals can be further suppressed.

The invention according to claim 7 is the invention according to any one of claims 1 to 6,
The DC offset of the AD converter is corrected based on the DC component of the AD converter to which the output of the frequency converter is input. Spurious signals can be further suppressed.

As is apparent from the above description, the present invention has the following effects.
According to the first, second, third, fourth, fifth, sixth and seventh aspects of the invention, when the frequency range to be measured is low, spectrum data is generated from a signal obtained by removing the high frequency component by inputting the measurement signal to the low pass filter. When the frequency range is high, the frequency conversion unit converts it to an intermediate frequency, and generates and displays spectrum data from the converted signal. Also, when the frequency range to be measured extends from the low frequency region to the high frequency region, the spectrum data generated from the output of the low-pass filter and the spectrum data generated from the output of the frequency converter are combined and displayed. did.

  When performing measurement in a low frequency range, the measurement signal is not input to the frequency converter, but the output signal of the low-pass filter is used, so that the local oscillation signal leaking from the mixer is not input to the AD converter. Therefore, spurious signals near 0 Hz can be suppressed, and overflow and intermodulation distortion do not occur. Therefore, there is an effect that an accurate spectrum can be measured even in a low frequency range.

  In addition, it is possible to further suppress spurious signals by correcting the DC offset of the low-pass filter and AD converter using the low-pass filter output during the period when no signal is input or the DC component of the frequency converter output. There is also an effect.

  Hereinafter, the present invention will be described in detail with reference to the drawings. FIG. 1 is a block diagram showing an embodiment of a spectrum analyzer according to the present invention. In addition, the same code | symbol is attached | subjected to the same element as FIG. 3, and description is abbreviate | omitted. In FIG. 1, reference numeral 30 denotes a switch, which receives an output signal from the attenuator 10 and selects this signal for output to the frequency converter 11 and the low-pass filter 31. The low-pass filter 31 is a filter that removes the high-frequency component of the input signal and passes only the low-frequency component.

  Reference numeral 32 denotes a switch that selects the output of the low-pass filter 31 and the output of the frequency converter 11 and outputs it to the AD converter 12. Reference numeral 33 denotes a signal processing unit to which the output of the AD converter 12 is input. The signal processing unit 33 corresponds to the digital filter 13, the waveform memory 14, and the DSP 15 in FIG. A control unit 34 controls the switches 30 and 32.

  Next, the operation of this embodiment will be described. The attenuator 10 converts the input signal (measurement signal) into a signal of a predetermined level and outputs it. When the frequency range to be measured is high, the control unit 34 controls the switches 30 and 32, inputs the output signal of the attenuator 10 to the frequency conversion unit 11, and inputs the output of the frequency conversion unit 11 to the AD conversion unit 12. Like that.

  Therefore, this spectrum analyzer operates in the same manner as the spectrum analyzer of FIG. That is, the output signal of the attenuator 10 is frequency mixed with the output of the local oscillator 11b by the mixer 11a. Since the output signal of the local oscillator 11b is swept over a predetermined frequency range, each frequency component of the input signal is converted into a first intermediate frequency. This first intermediate frequency signal is band-limited by the bandpass filter 11c, mixed with the output of the local oscillator 11e by the mixer 11d, and converted to a second intermediate frequency.

  The second intermediate frequency signal is converted into a digital signal by the AD converter 12 and input to the signal processing unit 33. The signal processing unit 33 limits the band of the input signal with a band limiting filter that passes only a signal having a bandwidth corresponding to the frequency resolution, performs processing such as detection and log conversion, and the horizontal axis indicates the frequency and the vertical axis indicates the frequency. Creates signal level spectral data and outputs it to the display unit 16. At this time, synchronization with the frequency sweep of the local oscillator 11b is performed. The display unit 16 displays the input spectrum data.

  When the frequency range to be measured is low (for example, less than ½ of the pass width of the band limiting filter built in the signal processing unit 33), the control unit 34 controls the switches 30 and 32 to output the output of the attenuator 10. The signal is input to the low-pass filter 31 and the output of the low-pass filter 31 is input to the AD converter 12.

  The low pass filter 31 removes high frequency components included in the input signal. The output of the low-pass filter 31 is converted into a digital value by the AD converter 12 and input to the signal processing unit 33. The signal processing unit 33 performs FFT (Fast Fourier Transform) analysis on the input digital value, generates spectrum data, and outputs the spectrum data to the display unit 16. The display unit 16 displays this spectrum data.

  When the frequency range to be measured extends from the vicinity of 0 Hz to the high frequency region, the switches 30 and 32 are switched to obtain the spectrum data by alternately measuring the high frequency region and the low frequency region and combining these spectral data. And displayed on the display unit 16.

  When measuring a low frequency range, the measurement signal is not input to the frequency converter 11 and the output of the low-pass filter 31 is input to the AD converter 12 to be converted into a digital value, and spectrum data is generated from this data. , A large spurious signal does not occur in the vicinity of 0 Hz, and the AD converter 12 overflows and no cross modulation distortion occurs. Therefore, the spectrum can be measured accurately even in the vicinity of 0 Hz.

  In this embodiment, when the switch 30 is on the frequency conversion unit 11 side, no signal is input to the low-pass filter 31, so that the output is only a DC offset. Therefore, by setting the switch 30 to the frequency conversion unit 11 side and the switch 32 to the low-pass filter 31 side and storing the output of the AD converter 12 at that time, the low-pass filter 31 and the AD are measured when measuring a low frequency range. The DC offset of the converter 12 can be corrected. In addition, since the DC component of the signal input to the AD converter 12 is zero during high frequency measurement, the DC offset of the AD converter 12 can be corrected by measuring the DC offset of the AD converter 12. it can. In this way, spurious signals near 0 Hz can be further suppressed.

  FIG. 2 shows another embodiment of the present invention. In addition, the same code | symbol is attached | subjected to the same element as FIG. 1, and description is abbreviate | omitted. In this embodiment, the switch 32 is omitted and two AD converters are provided.

  In FIG. 2, 40 is an AD converter, which receives the output signal of the low-pass filter 31, and converts this signal into a digital value. Reference numeral 41 denotes an AD converter which receives an output signal from the frequency converter 11 and converts this signal into a digital value. A signal processing unit 42 receives the output values of the AD converters 40 and 41, and generates spectrum data based on these values. The spectrum data generated by the signal processing unit 42 is input to the display unit 16. A control unit 43 controls the switch 30.

  Next, the operation of this embodiment will be described. When the frequency range to be measured is high, the control unit 43 operates the switch 30 to input the output signal of the attenuator 10 to the frequency conversion unit 11. The input signal is converted to a second intermediate frequency by the frequency converter 11, and the second intermediate frequency signal is converted to a digital value by the AD converter 41 and input to the signal processing unit 42. The signal processing unit 42 applies spectrum limiting filtering, detection, log conversion processing, and the like to the input digital value to generate spectrum data. This spectrum data is displayed on the display unit 16.

  When the frequency range to be measured is low, the control unit 43 controls the switch 30 and inputs the output signal of the attenuator 10 to the low pass filter 31. This signal has a high-frequency component removed by the low-pass filter 31, converted to a digital value by the AD converter 40, and input to the signal processing unit 42. The signal processing unit 42 obtains spectrum data by performing FFT analysis on the digital value and outputs it to the display unit 16. The display unit 16 displays this spectrum data.

  When the frequency range to be measured extends from the low frequency region to the high frequency region, the control unit 43 operates the switch 30 to alternately output the output signal of the attenuator 10 to the frequency conversion unit 11 and the low-pass filter 31. The signal processing unit 42 performs FFT analysis on the output of the AD converter 40 when the frequency is low to obtain spectrum data, and performs band limiting filtering, detection, and log conversion processing on the output value of the AD converter 41 when the frequency is high. The spectrum data is obtained, and the spectrum data is combined and output to the display unit 16. The display unit 16 displays the combined spectrum data. In this embodiment, two AD converters are required, but the switch 32 can be omitted, so that the configuration can be simplified.

  Also in this embodiment, when the low frequency range is measured, the output signal of the low-pass filter 31 is input to the AD converter 40 and converted into a digital value to generate spectrum data. Therefore, generation of a large spurious signal, AD conversion, etc. The overflow of the device 40 and the occurrence of intermodulation distortion are eliminated. Therefore, the spectrum can be measured accurately even in the vicinity of 0 Hz.

  In this embodiment as well, the input of the low-pass filter 31 is 0 when the switch 30 is on the frequency conversion unit 11 side. Therefore, by storing the output value of the AD converter 40 at this time, the DC offset between the low-pass filter 31 and the AD converter 40 can be corrected when the low frequency range is measured. Further, when measuring a high frequency range, the DC component of the signal input to the AD converter 41 becomes 0, and this can be used to correct the DC offset of the AD converter 41. By performing this correction, it is possible to further suppress spurious signals in the vicinity of 0 Hz.

  Further, in both of FIG. 1 and FIG. 2 embodiment, when the frequency range to be measured is low, the value obtained by digitally converting the output of the low pass filter 31 is subjected to FFT analysis to obtain spectrum data. In addition, frequency conversion processing by digital processing may be performed to convert to an intermediate frequency, and band limitation filtering, detection, and log conversion processing may be performed to obtain spectrum data.

  In both the embodiment shown in FIGS. 1 and 2, when the frequency range to be measured is low, the spectrum data is obtained by performing FFT analysis on the digitally converted value of the output of the low-pass filter 31, but the frequency range to be measured is AD converted. If the sampling frequency is less than half of the sampling frequency, spectrum data may be obtained by performing FFT analysis on the digitally converted value regardless of whether the frequency range is high or low.

It is a block diagram which shows one Example of this invention. It is a block diagram which shows the other Example of this invention. It is a block diagram of the conventional spectrum analyzer.

Explanation of symbols

DESCRIPTION OF SYMBOLS 10 Attenuator 11 Frequency conversion part 11a, 11d Mixer 11b, 11e Local oscillator 11c Band pass filter 12, 40, 41 AD converter 16 Display part 30, 32 Switch 31 Low pass filter 33, 42 Signal processing part 34, 43 Control part

Claims (7)

A frequency converter that converts the frequency of the input analog signal to an intermediate frequency;
A low-pass filter that removes high-frequency components of the input analog signal;
A first switch that outputs a measurement signal to the low-pass filter when a frequency range to be measured is low, and outputs a measurement signal to the frequency converter when the frequency range is high;
An AD converter that converts an input analog signal into a digital value;
A second switch that selects the output of the low-pass filter when the frequency range to be measured is low, selects the output of the frequency converter when the frequency range is high, and outputs the selected output to the AD converter When,
A signal processing unit that receives the output of the AD conversion unit and generates spectral data;
Spectral data generated by the signal processing unit is input, and a display unit that displays the spectral data;
A spectrum analyzer characterized by comprising: A frequency converter that converts the frequency of the input analog signal to an intermediate frequency;
A low-pass filter that removes high-frequency components of the input analog signal;
A switch that outputs a measurement signal to the low-pass filter when the frequency range to be measured is low, and outputs a measurement signal to the frequency converter when the frequency range is high,
A first AD converter that receives the output of the low-pass filter and converts the input signal into a digital value;
A second AD converter that receives the output of the frequency converter and converts the input signal into a digital value;
When the outputs of the first and second AD converters are input and the frequency range to be measured is low, spectrum data is generated based on the output of the first AD converter, and when the frequency range to be measured is high A signal processing unit for generating spectrum data based on an output of the second AD converter;
Spectral data generated by the signal processing unit is input, and a display unit that displays the spectral data;
A spectrum analyzer characterized by comprising:   When the frequency range to be measured extends from the low frequency region to the high frequency region, the low frequency region generates spectrum data from the output of the low pass filter, and the high frequency region generates spectrum data from the output of the frequency converter. The spectrum analyzer according to claim 1 or 2, wherein the spectrum data is synthesized and displayed.   The signal processing unit band-limits an input digital value with a band-limiting filter, and generates spectrum data by detecting and log-converting the band-limited value. The spectrum analyzer according to claim 3.   The spectrum analyzer according to any one of claims 1 to 4, wherein the signal processing unit generates spectrum data by performing FFT analysis on input data.   6. The spectrum analyzer according to claim 1, wherein a direct current offset of the low-pass filter is corrected based on an output value of the low-pass filter when no signal is input.   The DC offset of the AD converter is corrected based on the DC component of the AD converter to which the output of the frequency converter is input. Spectrum analyzer.

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