JP3294759B2 - Spectrum analyzer - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a spectrum analyzer for measuring a frequency characteristic of a signal under test.

[0002]

2. Description of the Related Art Signals used in mobile communication systems such as automobile telephones and mobile telephones are modulated by various methods.
Further, in order to effectively use a communication line, a TDMA (time division multiple access) system is adopted as a communication system.
The frequency of a carrier that carries a signal used in such a mobile communication system is as high as several hundred MHz to several GHz.

Generally, a spectrum analyzer is used to accurately measure various frequency components contained in such a signal. FIG. 5 is a block diagram showing a schematic configuration of a spectrum analyzer for measuring the frequency characteristics of the high-frequency signal under measurement as described above.

A high-frequency signal under test a input through an input terminal 1 is adjusted to a predetermined level by an attenuator 2 and input to a frequency converter 3. The high frequency signal under test a input to the frequency converter 3 is mixed with the local oscillation signal b from the local oscillator 5 by the signal mixer 4 and converted into an intermediate frequency signal having an intermediate frequency. Then intermediate frequency signals band-limited by BPF 6, again, the output from the local oscillation signal b ₁ and were mixed the frequency converter 3 as the final intermediate-frequency signal c from the local oscillator 8 in a different signal mixer 7 Is done.

The oscillation frequency of the local oscillator 5 of the frequency converter 3 is swept by a sweep controller 9 over a predetermined frequency range. As a result, the frequency f _I of the intermediate frequency signal c output from the frequency converter 3 also changes in synchronization with the sweep operation.

The intermediate frequency signal c whose frequency has been reduced and output from the frequency converter 3 is input to the next RBW filter 10. The RBW filter 10 is formed of, for example, a band-pass filter having the frequency characteristics shown in FIG. 6, and selects only a necessary intermediate frequency signal except for unnecessary frequency components. From the peak level at the pass center frequency f _C of the frequency characteristic of this bandpass filter, 3
The bandwidth (RBW) at the time of the decrease in dB indicates the frequency resolution in the spectrum analyzer.

Since the frequency f _I of the intermediate frequency signal c output from the frequency conversion unit 3 changes in synchronization with the sweep operation, the output output from the RBW filter 10 over time within one sweep period (sweep cycle). The signal has a time-series waveform in each frequency component of the signal under test converted into the intermediate frequency signal c by the sweep reception.

The output signal from the RBW filter 10 is gain-adjusted by the amplifier 11 and then logarithmically converted by the LOG converter 12. The output signal whose signal level has been converted into the unit of dB is detected by the next detector 13. As a result, the signal detected during the sweep period indicates the magnitude of the time-series waveform at the swept frequency. Therefore, the horizontal axis is frequency,
If the vertical axis is the amplitude, the waveform is a frequency spectrum waveform.

A signal representing the frequency spectrum waveform output from the detector 13 is input to the next VBW filter 14. The VBW filter 14 removes a high-frequency component (noise component) of a frequency spectrum waveform 18 finally displayed on a display 17 mounted on a front panel 19 of a spectrum analyzer shown in FIG.
(Low-pass filter).

The analog frequency spectrum waveform output from the VBW filter 14 has a peak value at each time axis position detected by a peak detector 15 to obtain a final frequency spectrum waveform 18 in an envelope detection state. Can be The signal indicating the final frequency spectrum waveform is converted into digital data by the next A / D converter 16. The frequency spectrum waveform converted into the digital data is displayed on the display 17 of the front panel 19 as described above.

Therefore, as shown in FIG.
Is output on the display 17 of the front panel 19. By changing the sweep frequency range and the display range of the frequency on the display 17, it is possible to measure the frequency spectrum over a wide frequency range and within an arbitrary frequency range.

Further, by changing the bandwidth (RBW) of the RBW filter 10, the frequency resolution of the spectrum analyzer can be changed to an arbitrary value. In general, increasing the sweep frequency range will increase the bandwidth (RBW)
To reduce the frequency resolution. That is, RB
The bandwidth (RBW) of the W filter 10 is changed according to the change of the sweep frequency range.

[0013]

However, the spectrum analyzer shown in FIG. 5 still has the following problems to be solved. That is, the bandwidth (RBW) of the RBW filter 10 indicating the frequency resolution of the spectrum analyzer, the adjustment of the pass center frequency f _C , and the LO
It is necessary to carry out calibration work for the linearity of logarithmic conversion in the G converter 12, the deviation of the amplitude value due to the switching of the bandwidth (RBW) in the RBW filter 10, and the occurrence of the tuning deviation in the sweep frequency range of the bandwidth (RBW). .

In particular, the R of a conventional spectrum analyzer
In the BW filter 10, a steep characteristic is obtained by cascade-connecting a plurality of filters, so that more precision is required.

As described above, in a spectrum analyzer that performs a frequency analysis process on a high-frequency signal using analog electronic components, calibration before actually starting measurement is complicated, and the efficiency of measurement operation is greatly reduced. There were concerns.

Further, even if the adjustment and calibration are completely performed before the start of the measurement, there is a concern that the frequency characteristics of each component fluctuate according to the change of the measurement environment after the start of the measurement. There are concerns that cannot be obtained.

The present invention has been made in view of such circumstances, and digitizes the processing for the intermediate frequency signal output from the frequency conversion unit, thereby simplifying complicated adjustment work and calibration work at the start of measurement. Can be omitted,
It is an object of the present invention to provide a spectrum analyzer that can greatly reduce the work load on an operator and can maintain high measurement accuracy even if the measurement environment changes.

[0018]

SUMMARY OF THE INVENTION In order to solve the above-mentioned problems, the present invention provides a frequency converter for converting the frequency of an analog signal to be measured into an intermediate frequency, and changing a local oscillation frequency in the frequency converter. Control unit for sweeping the frequency of the intermediate frequency signal output from the frequency converter, an A / D converter for converting the intermediate frequency signal output from the frequency converter to a digital intermediate frequency signal, and an output from the A / D converter. An RBW digital filter that receives the digital intermediate frequency signal, selects and outputs a frequency component having a predetermined bandwidth that determines the frequency resolution, and a waveform memory that stores a time-series waveform of the signal output by the RBW digital filter.
Detection processing means for detecting the time-series waveform stored in the waveform memory, and LOG conversion processing means for logarithmically converting the signal level of the time-series waveform detected by the detection processing means to obtain a frequency spectrum waveform; The spectrum analyzer includes a VBW digital filter for removing a high frequency component of a frequency spectrum waveform obtained by a LOG conversion processing unit, and a display for displaying a frequency spectrum waveform from which the high frequency component has been removed by the VBW digital filter.

Then, the spectrum analyzer
The detection processing means, the LOG conversion processing means and the VBW
The digital filter is a detection processing program,
LOG conversion processing program and VBW digital filter
It is executed according to the processing program. Furthermore, with CPU
A storage unit for storing a processing program, wherein the CPU receives a digital intermediate frequency signal output from the RBW digital filter, and the CPU performs processing in accordance with the processing program.
P and a detection processing program stored in an external storage device.
And a main control unit for reading out a LOG conversion processing program and a VBW digital filter processing program , writing the processing program in a storage unit, and causing a DSP to execute processing in accordance with the processing program.

In the thus constructed spectrum analyzer, the input high-frequency signal to be measured, for example, is converted into an intermediate frequency signal whose frequency is controlled by the sweep controller in the frequency converter. The analog intermediate frequency signal output from the frequency converter is converted into a digital intermediate frequency signal by an A / D converter. Then, this digital intermediate frequency signal is input to the RBW digital filter.

The RBW digital filter has a function of passing a frequency component having a bandwidth corresponding to the frequency resolution of the spectrum analyzer, similarly to the analog RBW filter described above. Since the frequency of the intermediate frequency signal output from the frequency conversion unit changes in synchronization with the sweep operation, the output signal output from the RBW digital filter as time elapses within the sweep period is swept and converted to the intermediate frequency signal. It becomes a time series waveform at each frequency of the measured signal under test.

The time-series waveform output from the RBW digital filter is temporarily stored in a waveform memory and then input to a digital signal processor (hereinafter abbreviated as DSP). This DSP is a kind of ultra-small high-speed operation module in which a calculation program is written.

In the present invention, each program for realizing the detection processing means, the LOG conversion processing means, and the VBW digital filter is written in the DSP. D
The time-series waveform input into the SP is subjected to detection processing by the detection processing means to obtain an amplitude value, and the LOG conversion processing means performs logarithmic conversion of the amplitude value. As a result, a frequency spectrum waveform having the amplitude value of the output of the LOG conversion processing means on the vertical axis when the horizontal axis is the frequency of the sweep reception is obtained.
Further, after the high frequency component of the frequency spectrum waveform is removed by the VBW digital filter, the signal is output from the DSP.

The frequency spectrum waveform output from the DSP is displayed on a display. By digitizing the processing for the intermediate frequency signal output from the frequency conversion unit, complicated adjustment work and calibration work at the start of measurement can be omitted.

[0025]

DESCRIPTION OF THE PREFERRED EMBODIMENTS One embodiment of the present invention will be described below with reference to the drawings. FIG. 1 is a block diagram showing a schematic configuration of the spectrum analyzer of the embodiment. The same parts as those of the conventional spectrum analyzer shown in FIG. Therefore, the detailed description of the overlapping part is omitted.

The signal level of a high-frequency analog signal under test a of several hundred KHz to several GHz, for example, input through the input terminal 1 is adjusted to a specified level by the attenuator 2 and input to the frequency conversion unit 3. You. The high frequency signal under test a input to the frequency conversion unit 3 is combined with the local oscillation signal b from the local oscillator 5 by the signal mixer 4 and converted into an intermediate frequency signal having an intermediate frequency. This intermediate frequency signal is B
After band-limited by the PF6 @, is output again from the frequency converter 3 is synthesized with a local oscillation signal b ₁ from the local oscillator 8 by the signal mixer 7 as a final intermediate frequency signal c.

The oscillation frequency of the local oscillator 5 of the frequency converter 3 is swept by the sweep controller 21 over a predetermined frequency range. As a result, the frequency f _I of the intermediate frequency signal c output from the frequency converter 3 also changes in synchronization with the sweep operation.

The intermediate frequency signal c having the frequency f _I reduced to, for example, 6 MHz is sampled by the next A / D converter 22 with a sampling signal output from the clock oscillator 23 and having a sampling frequency f _{S of} , for example, 30 MHz. Thus, for example, a 12-bit digital intermediate frequency signal c ₁ is obtained. Digital intermediate frequency signal c
₁ is input to the next RBW digital filter 24.

The RBW digital filter 24 includes, for example,
It is composed of a decimation digital filter capable of realizing high-speed processing of executing digital filter processing while changing the sampling frequency. And this R
The BW digital filter 24 is the analog RB described above.
It has a frequency characteristic similar to that of the W filter 10, and as shown in the frequency characteristic of FIG. 6, a bandwidth (RBW) corresponding to the frequency resolution of this spectrum analyzer centered on a predetermined center frequency f _C. Has the function of passing the frequency component of

This bandwidth (RBW) is determined by the sweep controller 2
At 1 the value is changed in conjunction with the size of the sweep frequency range.
The frequency f _I of the intermediate frequency signal c ₂ output from the frequency converter 3 and converted to digital by the A / D converter 22 is
Unnecessary components are removed by the RBW digital filter 24, resulting in a time-series waveform output over time.

The time-series waveform output from the RBW digital filter 24 is temporarily stored in the waveform memory 25 and then input to the DSP 26. This DSP 26 is composed of a kind of ultra-compact high-speed operation module. In the DSP 26, a processing program stored in an external storage device 29 such as an HDD is read out and written in advance by a main control unit 28 such as a computer. The DSP 26 executes a plurality of types of arithmetic processing according to the written processing program.

As shown in FIG. 2, the DSP 26 has an input interface (I / F) serving as an input / output interface.
34 and an output I / F 35, as shown in FIG.
When the PU 32 executes the processing program written in the RAM 33, the detection processing unit 50 and the LO
A G conversion processing unit 51, a VBW filter processing unit 52, a peak detection processing unit 53, and a display editing processing unit 54 are provided.

Next, the operation of each unit 50 to 54 will be described step by step. The input I / F 34 sequentially captures a time-series waveform for one sweep period (sweep cycle) stored in the waveform memory 25 and sends it to the next detection processing unit 50. Detection processing unit 50
Is the next LOG conversion processing unit 5 which removes the negative component of the input time-series waveform and generates a detected waveform indicating the amplitude of the time-series waveform.
Send to 1.

The LOG conversion processing section 51 is constituted by a conversion table in which the logarithmically converted values are stored in advance for each data value expected to be input. Is read out and output. Therefore, this LOG
The time-series waveform output from the conversion processing unit 51 is a frequency spectrum waveform with the time axis being the frequency axis and the vertical axis being shown in units of decibels.

The frequency spectrum waveform output from the LOG conversion processing section 51 is input to the next VBW filter processing section 52. The VBW filter processing unit 52 is configured by, for example, an infinite impulse response filter (IIR filter), which is a type of linear digital filter. Functionally, the VBW filter processing unit 52 is configured by an LPF (low-pass filter) that removes a high-frequency component of a frequency spectrum waveform, similarly to the analog VBW filter 14 illustrated in FIG.

The frequency spectrum waveform output from the VBW filter processing section 52 is sent to a peak detection processing section 53. The peak detection processing unit 53 detects a peak value at each time axis position of the frequency spectrum waveform,
A final frequency spectrum waveform in a state where the envelope detection is performed is generated. The frequency spectrum waveform output from the peak detection processing unit 53 is sent to the next display editing processing unit 54.

The display editing processor 54 develops the input frequency spectrum waveform into an image to be displayed on the display screen of the display 27 and sends it to the display 27 via the output I / F 35. For example, a display device 27 including a CRT display device or a liquid crystal display device displays and outputs the received frequency spectrum waveform.

The main control unit 28 sends a command for setting a sweep range to the sweep control unit 21 in addition to a function of writing a processing program to the DSP 26. The sweep control unit 21 sweeps the local oscillation signal b of the frequency conversion unit 3 in the frequency range specified by the main control unit 28. Further, the main control unit 28 sets the bandwidth (RBW) of the RBW digital filter 24 to a value corresponding to the sweep range. Further, the sweep control unit 21 displays the display 2
7 is set.

Accordingly, it is possible to measure the frequency spectrum of the signal under test a over a wide frequency range and within an arbitrary frequency range. Next, the DSP of the main control unit 28
The setting procedure of the processing program for the H.26 will be described.

FIG. 3 is a block diagram showing a schematic configuration of the DSP 26 and the main control unit 28. For a system bus 31 in the DSP 26, a CPU 32, a RAM 33, an input I / O
F34, output I / F 35, internal address decoder 36,
An external address decoder 37 and the like are connected.
A bus switch 38 and a signal switch 39 are incorporated in P26.

In the RAM 33, the detection processing program corresponding to the detection processing section 50, the LOG conversion processing program corresponding to the LOG conversion processing section 51, and the peak detection processing section 53 written from the main control section 28 are stored. A corresponding peak detection processing program, a VBW filter processing program corresponding to the VBW filter processing section 52, and a display editing processing program corresponding to the display editing processing section 54 are stored.

A CPU 42, a main memory 43, a bus buffer 44, a reset circuit 45, an external storage device I / F 46, an address decoder 47, and the like are connected to a system bus 41 in the main control unit 28. The bus buffer 44 is connected to the external address decoder 37 on the DSP 26 side and the bus switch 38 via the connection bus 40.

The external storage device I / F of the main control unit 28
The external storage device 29 described above is connected to 46.
Various processing programs for writing to the DSP 26 are stored in the external storage device 29 in advance.

The main control unit 28 and DSP having such a configuration
At 26, a procedure for writing each processing program stored in the external storage device 29 into the RAM 33 of the DSP 26 will be described.

The CPU 42 of the main control unit 28 drives the reset circuit 45 via the system bus 41 to
6 is sent a reset signal r. As a result, DS
The bus switch 38 and the signal switch 39 in the P26 are operated to connect the RAM 33 to the connection bus 40 as shown in FIG.
And the write control signal line of the external address decoder 37 is connected to the RAM 33, and the operation of the CPU 32 is reset.

In this state, the RAM of the DSP 26
33 enters the address space of the CPU 42 of the main control unit 28. Therefore, the CPU 42 of the main control unit 28
Read out each processing program stored in the external storage device 29 via the external storage device I / F 46, and read out the DSP via the system bus 41, the bus buffer 44 and the connection bus 40.
26 is directly written into the RAM 33.

When the CPU 42 of the main control unit 28 finishes writing all necessary processing programs to the RAM 33 of the DSP 26, it drives the reset circuit 45 via the system bus 41 to release the reset signal r for the DSP 26. I do.

As a result, the bus switch 38 and the signal switch 39 in the DSP 26 operate to connect the RAM 33 to the system bus 31 and to connect the write control signal line of the internal address decoder 36 to the RAM 33. At the same time, the CPU 32 shifts to the operating state.

The external storage device 29 described above
As an example, an IC card memory, an FDD, or the like can be adopted. The external storage device 29 may be constituted by a ROM or the like incorporated in the main control unit 28.

Therefore, the CPU 32 of the DSP 26
In accordance with each processing program written in the RAM 33, each of the above-described arithmetic processing steps 50 to 54 is sequentially executed on each data of the time-series waveform input from the waveform memory 25 via the / F34, and an execution result is output as I / Display 2 via F35
7 is output.

In the thus constructed spectrum analyzer, the input high-frequency analog signal under test a, for example, is converted by the sweep control section 21 into the frequency conversion section 3.
Converts the frequency f _I into an intermediate frequency signal c that is subjected to sweep control. The analog intermediate frequency signal c output from the frequency converter 3 is converted by the A / D converter 22 into a digital intermediate frequency signal c ₁ .

The digital intermediate frequency signal c
_{In 1} , the RBW digital filter 24 extracts a frequency component of a bandwidth (RBW) corresponding to the frequency resolution. The time-series waveform output from the RBW digital filter 24 is temporarily stored in the waveform memory 25, and then stored in the DSP.
26.

The DSP 26 applies the time series wave for one sweep period (sweep cycle) stored in the waveform memory 25 to the
By software c E A technique, detection processing, LOG conversion processing, V
The BW filter processing, the peak detection processing, and the display editing processing are executed, and the finally obtained frequency spectrum waveform is displayed on the display 27.

As described above, by digitizing the processing for the intermediate frequency signal c output from the frequency converter 3, analog electronic components are eliminated. Therefore, complicated adjustment work and calibration work at the time of fixed start which are performed on each analog electronic component in the conventional apparatus shown in FIG. 5 can be omitted, so that the workload of the operator is greatly simplified, and the measurement work is simplified. Efficiency can be greatly improved.

The RBW digital filter 24, the waveform memory 25, and the DPS 26 have very little error in the calculation result even if the measurement environment such as temperature fluctuates slightly. Can be greatly improved.

The RBW digital filter 24 and DS
The pass frequency bandwidth, pass center frequency f _{C, and the} like of the VBW filter incorporated in P26 in software can be easily changed in software. Therefore, the RBW digital filter 2
4 can be easily changed to a value corresponding to the sweep range. Therefore, it is possible to measure the frequency spectrum of the signal under test a over a wide frequency range and within an arbitrary frequency range.

Further, the DSP 26 for performing the arithmetic processing exclusively
Since the processing speed of digital data is increased by adopting, the frequency spectrum analysis of the high-frequency signal under test a can be performed in almost real time.

Further, as shown in FIG.
From the external storage device 29, the symbol synchronization detection unit 61, the bit demodulation unit 62, and the synchronization word
3. By reading out the respective processing programs respectively corresponding to the decoding unit 64 and the like, and writing and executing them in the RAM 33 of the DSP 26, the DSP 26 can also execute the demodulation processing of the digital modulation signal.

[0059]

As described above, in the spectrum analyzer of the present invention, the intermediate frequency signal output from the frequency converter is converted into a digital intermediate frequency signal by the A / D converter, and the intermediate frequency signal thereafter is converted. Various processes for obtaining a frequency spectrum waveform such as an RBW filter process and a LOG conversion process for a frequency signal are performed by digital components and software.

Therefore, complicated adjustment work and calibration work at the start of measurement can be omitted, and the work load on the operator can be greatly reduced, and high measurement accuracy can be maintained even if the measurement environment changes.

[Brief description of the drawings]

FIG. 1 is a block diagram showing a schematic configuration of a spectrum analyzer according to a first embodiment of the present invention.

Fig. 2 DS built into the spectrum analyzer
The figure which shows each processing part formed in P

Fig. 3 DS built into the spectrum analyzer
Block diagram showing a schematic configuration of P and a main control unit

FIG. 4 is a diagram for explaining a DSP in which a demodulation processing function for a digital modulation signal is incorporated.

FIG. 5 is a block diagram illustrating a schematic configuration of a conventional spectrum analyzer configured with analog electronic components.

FIG. 6 is a frequency characteristic diagram of a general RBW filter.

FIG. 7 is a diagram showing a general frequency spectrum waveform displayed on a display device.

[Explanation of symbols]

2 ... attenuator, 3 ... frequency converter, 21 ... sweep controller, 2
2: A / D converter, 23: Clock oscillator, 24: RB
W digital filter, 25: waveform memory, 26: DS
P, 27: display unit, 28: main control unit, 29: external storage device, 50: detection processing unit, 51: LOG conversion processing unit, 52
... VBW filter processing section, 53 ... Peak detection section, 54 ...
Display / edit processing unit

──────────────────────────────────────────────────続 き Continuing on the front page (72) Takashi Seike, Inventor Anritsu Co., Ltd., 5-10-27 Minamiazabu, Minato-ku, Tokyo Field (Int.Cl. ⁷ , DB name) G01R 23/00-23/20

Claims (1)

(57) [Claims] A frequency converter for converting a frequency of an input analog signal to be measured into an intermediate frequency; and a frequency converter for changing a local oscillation frequency in the frequency converter to output a frequency of the intermediate frequency signal. A sweep controller (21) for sweeping, an A / D converter (22) for converting an intermediate frequency signal output from the frequency converter into a digital intermediate frequency signal, and an A / D converter output from the A / D converter. An RBW digital filter (2) that receives a digital intermediate frequency signal, selects and outputs a frequency component having a predetermined bandwidth that determines the frequency resolution.
4), a waveform memory (25) for storing a time-series waveform of the signal output from the RBW digital filter, a detection processing means (50) for detecting the time-series waveform stored in the waveform memory, and LOG conversion processing means (51) for logarithmically converting the signal level of the time-series waveform detected by the processing means to obtain a frequency spectrum waveform; and high-frequency components of the frequency spectrum waveform obtained by the LOG conversion processing means VBW digital filter that removes noise
(52) and a display (27) for displaying a frequency spectrum waveform from which high-frequency components have been removed by the VBW digital filter, wherein the detection processing means, the LOG conversion processing means, and the V
Each of the BW digital filters is a detection processing program.
LOG conversion processing program and VBW digital file
Storage unit that is executed along with the CPU processing program and further stores the CPU (32) and the processing program.
(33), receiving the digital intermediate frequency signal output by the RBW digital filter, the CPU
A DSP (26) for performing processing according to a processing program, and a detection processing program stored in an external storage device (29).
RAM , a LOG conversion processing program and a VBW digital filter processing program , and write the processing program into the storage section (33).
The spectrum analyzer according to (26) , further comprising a main control unit (28) for executing a process according to the processing program.