JPH10170568A - Spectrum analyzer - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a spectrum analyzer that is small in floating amount of oscillating frequency of the first local oscillator, and high in the degree of accuracy of the frequency indication position of a frequency spectrum indicated. SOLUTION: With a phase lock group PLL1 and a reference oscillator 8, the oscillation frequency of the first oscillator L01 is set exactly at a specific frequency at every initiation of frequency sweep. In this set state, a sample hold circuit 6 provided to the phase lock group PLL1 is switched to a hold mode. By impressing lamp voltage to the hold voltage, the oscillation frequency of the first oscillator is frequency-swept. Besides, the oscillation frequency of the first oscillator is converted from frequency into voltage and the difference of the converted voltage signal and the lamp voltage is obtained. The difference is integrated and given to the first oscillator L01 in the constitution added by a control loop.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a spectrum analyzer used for analyzing, for example, a frequency component contained in a signal under measurement and displaying its frequency spectrum.

[0002]

2. Description of the Related Art FIG. 5 shows a schematic configuration of a spectrum analyzer. In the example of FIG. 5, the frequency of the signal under test input to the input terminal 1 is reduced by the first, second, and third frequency mixers MX1, MX2, and MX3 having a three-stage configuration. The signal is amplitude detected by an amplitude detector DT, the amplitude detection output is input to a Y-axis input terminal TY of a digital processing type display DSP, and a lamp voltage VL output from a lamp voltage generator 2 is input to an X-axis input terminal. TX
, The X-axis sweep is performed by inputting the frequency analysis result, and the frequency analysis result with the horizontal axis as a frequency scale is displayed.

[0003] FL1, FL2 and FL3 denote filters for extracting signals of the difference frequency from the frequency mixing outputs of the respective frequency mixers MX1, MX2 and MX3. Also LO
1, LO2 and LO3 are frequency mixers MX1 and M, respectively.
First, second, and third giving local oscillation signals to X2 and MX3
3 shows a local oscillator. Here, the second and third local oscillators LO
2 and LO3, a fixed oscillator having a stable frequency such as a crystal oscillator is used. However, the first local oscillator LO1 is composed of, for example, a voltage-controlled oscillator, and is supplied with a lamp voltage VL from a ramp voltage generator 2 to generate an oscillation frequency. Is swept. By this frequency sweep operation, the frequency of the signal under test input to the input terminal 1 is analyzed.

FIG. 6 shows an example of a control circuit for sweeping the oscillation frequency of the first local oscillator LO1. First
The oscillation frequency of the local oscillator LO1 is mainly controlled by the ramp voltage VL output from the ramp voltage generator 2, but every time the frequency sweep starts, the phase lock loop PLL1 accurately sets the center frequency of the frequency sweep bandwidth. It is working.

In other words, the frequency sweep operation of the first local oscillator LO1 is, for example, 5 GHz ± 50 kHz as shown in FIG.
When the frequency sweep starts, the oscillation frequency of the first local oscillator LO1 is controlled to 5 GHz by the phase lock loop PLL1. For this purpose, for example, a reference oscillator (an oscillator whose frequency is accurate like a crystal oscillator) 8
Oscillation frequency is set to 200 MHz,
Is obtained and input to the phase comparator 4.

On the other hand, the oscillation signal of the first local oscillator LO 1 is divided by the variable frequency divider 3 to, for example, a frequency of 1/500 and input to the phase comparator 4, and a signal having a reference frequency given from the reference oscillator 8 is output. Compare the phases. The phase comparison output of the phase comparator 4 is smoothed by the low-pass filter 5 and supplied to the control voltage input terminal of the first local oscillator LO1 through the sample hold circuit 6 and the analog adder 7. When the oscillation frequency of the first local oscillator LO1 is set to a set value, in this example, 5 GHz, the sample hold circuit 6 is controlled to the sample mode. Accordingly, when a change occurs in the smoothed output voltage of the low-pass filter 5, the change is immediately input to the first local oscillator LO1 through the sample hold circuit 6 and the analog adder 7. In this state, the output voltage of the lamp voltage generator 2 is maintained at 0 volt. Therefore, only the smoothed output voltage of the low-pass filter 5 is applied to the control voltage input terminal of the first local oscillator LO1.

Since the frequency division ratio of the frequency divider 3 is 1/500 and the frequency of the reference signal input from the frequency divider 9 to the phase comparator 4 is 10 MHz as in the above-described example, 3
The frequency of the signal input to the phase comparator 4 is 10 MHz
In order for the oscillation frequency of the first local oscillator LO1 to be 5
GHz. Therefore, when the sample and hold circuit 6 inserted in the phase lock loop PLL1 is controlled to the sample mode, the first local oscillator LO
The oscillation frequency of 1 is controlled to a state of 5 GHz.

The oscillation frequency of the first local oscillator LO1 is 5G
The sample / hold circuit 6 is switched to the hold mode while the frequency is controlled to Hz. When the sample hold circuit 6 is switched to the hold mode, the phase lock loop P
LL1 is released and the first local oscillator LO1 enters a free-running oscillation state by the voltage held by the sample hold circuit 6.

In this free-running oscillation state, the ramp voltage VL is supplied from the ramp voltage generator 2 to increase the oscillation frequency of the first local oscillator LO1 from 5 GHz-50 kHz to 5 GHz + 50 kHz.
The frequency range up to is swept. As described above, in the related art, while the center of the sweep frequency of the first local oscillator LO1 is set accurately, the lamp voltage VL output by the lamp voltage generator 2 sweeps a predetermined frequency range around the center frequency by the ramp voltage VL. I have.

[0010]

According to the circuit configuration shown in FIG. 6, the phase locked loop PLL1 forms a closed loop when the sample and hold circuit 6 is controlled to the sample mode. The oscillation frequency is accurately set to, for example, 5 GHz. On the other hand, when the sample and hold circuit 6 is switched to the hold mode, the phase lock loop PLL1 is opened and the control operation is released, so that the oscillation frequency of the first local oscillator LO1 depends on the hold voltage of the sample and hold circuit 6. ,
If the hold voltage of the sample and hold circuit 6 drifts, the oscillation frequency of the first local oscillator LO1 drifts in accordance with the drift.

FIG. 8 shows the influence of the drift of the hold voltage of the sample and hold circuit 6. Curve A shown in FIG. 8 shows the frequency change of the normal frequency sweep signal. Curve B shows the frequency change of the frequency sweep signal when the hold voltage drifts. That is, since the hold voltage of the sample and hold circuit 6 generally uses the charge voltage of the capacitor, the voltage tends to decrease with time. Therefore, even if the lamp voltage VL changes linearly, the frequency of the frequency sweep signal does not reach the target maximum frequency FMAX (drift amount DF) due to the gradual decrease of the hold voltage, and the frequency sweep is performed. There is a disadvantage that the bandwidth becomes narrower than the intended range.

In addition, since the change in the sweep frequency becomes non-linear, the displayed frequency position of the frequency spectrum displayed on the display unit does not match the frequency scale, and the accuracy of the frequency display becomes poor. SUMMARY OF THE INVENTION An object of the present invention is to propose a configuration of a spectrum analyzer which can eliminate such disadvantages, improve the frequency stability, and improve the frequency display accuracy.

[0013]

According to the first aspect of the present invention, the oscillation frequency of the first local oscillator is set to the center frequency of the frequency sweep bandwidth by the phase lock loop. At the same time, in a state where the sample hold circuit inserted in the phase lock loop is switched to the hold mode, the hold voltage and the ramp voltage of the sample hold circuit are added by the first analog adder, and the addition result is the first local oscillator. A frequency-voltage converter for converting a change in the oscillation frequency of the first local oscillator into a voltage signal in a frequency sweep oscillator of a spectrum analyzer configured to sweep the oscillation frequency of the first local oscillator. A second analog adder for obtaining an added value of the voltage conversion output signal of the frequency-to-voltage converter and the lamp voltage, a comparison between the added value of the second analog adder and a reference voltage, and integration of the deviation voltage. A configuration in which an integrator for applying the integrated voltage to the first analog adder as the ramp voltage is added. Those were.

According to the second invention of this application, the first local oscillator sets the oscillation frequency to the center frequency of the frequency sweep bandwidth each time the frequency sweep operation is started, and the first local oscillator includes:
A frequency-voltage converter for converting a change in the oscillation frequency of the local oscillator into a voltage change, an analog adder for calculating a deviation between a voltage signal output from the frequency-voltage converter and a lamp voltage output from the lamp voltage generator, A frequency control loop constituted by an integrator for controlling the oscillation frequency of the first local oscillator is added to a state in which the deviation voltage output from the analog adder is always equal to the voltage of the reference voltage source.

In any of the first and second inventions of this application, a frequency control loop is added to the first local oscillator during the frequency sweep period, so that the oscillation frequency during the frequency sweep period is a ramp voltage. , And changes exactly following the change of the frequency, and the drift amount of the frequency can be controlled to be small. Therefore, according to the present invention, the frequency drift is small,
It is possible to provide a spectrum analyzer with high accuracy of the frequency display position of the displayed frequency spectrum.

[0016]

FIG. 1 shows an embodiment of a spectrum analyzer according to the present invention. Parts corresponding to those in FIG. 6 are denoted by the same reference numerals. That is, the PLL 1 indicates a first phase-locked loop for setting the oscillation frequency of the first local oscillator LO1 to the center frequency of the frequency sweep bandwidth with the accuracy of the oscillation frequency of the reference oscillator 8. PLL2 denotes a second phase lock loop provided so that the frequency of the reference signal given from the reference oscillator 8 to the first phase lock loop PLL1 can be changed. The second phase lock loop PLL2 includes a voltage controlled oscillator LO4, a variable frequency divider 11, a phase comparator 12, and a low-pass filter 13. The frequency reference signal output from the reference oscillator 8 is supplied to the phase comparator 12 through the variable frequency divider 14, and the oscillation frequency fLO
4 is controlled to fLO4 = Q · fREF. (Q is the frequency division number of the variable frequency divider 11, fREF is the frequency of the reference signal given to the phase comparator 12 from the variable frequency divider 14) If fREF is fREF = 1 MHz, and Q = 10, the voltage control oscillator LO4 The oscillation frequency fLO4 oscillates at fLO4 = 10 MHz.

The oscillation frequency of the first local oscillator LO1 is, for example, 5 GHz, and the frequency division ratio 1 / N of the variable frequency divider 3 is 1 / N = 1.
If it is set to / 500, the frequency of the signal given to the phase comparator 4 from the variable frequency divider 3 and the second phase lock loop PLL2 is 10 MHz. In the first phase lock loop PLL1, the first local oscillator LO1 oscillates at 5 GHz with the frequency of the signal supplied to both input terminals of the phase comparator 4 being 10 MHz, and this oscillation frequency is at the center of the frequency sweep bandwidth. Frequency.

When the oscillation frequency of the first local oscillator LO1 is set to the center frequency of the frequency sweep bandwidth, the switch S
W controls so as not to contact any of the contacts SL and FA. The lamp voltage generator 2 outputs 0 volt (the center voltage of the lamp voltage). By operating the sample and hold circuit 6 in the sampling mode in this state, the first local oscillator LO1 is connected to the phase lock loop PL.
The center frequency of the frequency sweep bandwidth is obtained by the control of L1. In this example, it oscillates at a frequency of 5 GHz. The sample-and-hold circuit 6 has a first local oscillator LO1 of 5 GHz.
A voltage for oscillating z is accumulated.

In this state, the sample-and-hold circuit 6 is switched to the hold mode, and the first local oscillator LO1 is set to the free-running oscillation state according to the hold voltage stored in the sample-and-hold circuit 6. Switch S for fast frequency sweep
W is switched to the contact FA side. When the switch SW is switched to the contact FA side, the hold voltage HV and the ramp voltage VL are simply supplied to the first analog addition circuit 7. The ramp voltage VL generates a ramp voltage starting from a negative voltage, and when the ramp voltage VL passes 0 volts, the oscillation frequency of the first local oscillator LO1 passes the center frequency of the frequency sweep bandwidth, in this example, 5 GHz.

When the oscillation frequency of the first local oscillator LO1 reaches the highest frequency of the frequency sweep bandwidth, the switch SW is switched to the center again, the sample and hold circuit 6 is switched to the sampling mode, and the phase lock loop PLL1 is switched to the sampling mode. A closed loop is formed again, and the sample hold circuit 6 takes in a voltage for controlling the oscillation frequency of the first local oscillator LO1 to 5 GHz again. High-speed frequency sweep is performed by repeating this.

The switching of the switch SW is automatically controlled by a control circuit (not shown). That is, the control circuit for switching and controlling the switch SW is provided with setting means for the high-speed sweep mode and the low-speed sweep mode, and the switching control of the switch SW is executed in accordance with the setting mode. When the low-speed sweep mode is set, the switch SW is switched to the contact SL in a state where the oscillation frequency of the first local oscillator LO1 matches the center frequency (5 GHz) of the frequency sweep bandwidth. When the switch SW is switched to the contact SL side, a frequency control loop including a frequency-voltage converter 17, a second analog adder 18, and an integrator 20 is connected to the first local oscillator LO1.

The frequency-to-voltage converter 17 is, for example, a saturating amplifier 17A for shaping the waveform of a signal having the oscillation frequency of the first local oscillator LO1 into a rectangular wave, and the signal of the signal converted into a rectangular wave by the saturating amplifier 17A. It can be constituted by a one-shot multivibrator 17B that outputs a pulse having a constant pulse width at the rising timing, and an integrator 17C that integrates the pulse output from the one-shot multivibrator 17B.

In this example, the signal supplied to the frequency-to-voltage converter 17 is such that the frequency conversion means converts the oscillation signal of the first local oscillator LO1 to a low frequency by the frequency conversion means and provides the signal. In the example shown in FIG. 1, the frequency conversion means includes a frequency mixer 15 and a low-pass filter 16 for extracting a signal having a difference frequency generated on the output side of the frequency mixer 15.

Also in the low-speed sweep mode, the switch SW is switched to the neutral position and the lamp voltage generator 2 operates at 0 volt under the control of the phase lock loop PLL1 so that the first local oscillator LO1 is controlled to oscillate at 5 GHz. Is output. The integrator 20 is connected to a voltage source 19 for providing an offset voltage VOSE. At the same time, the second phase lock loop PLL2 switches the switch S
When W is converted to the contact SL, one of the variable frequency dividers 11 and 14 is controlled to change the oscillation frequency of the voltage controlled oscillator LO4.

The frequency displacement is calculated by the second analog adder 1
8 is a displacement amount at which the deviation voltage output from the reference voltage source 8 becomes equal to the offset voltage VOSE output from the reference voltage source 19. FIG. 2A shows the relationship between the lamp voltage VL and the frequency-voltage conversion output voltage VF output from the frequency-voltage converter 17. As shown in the figure, the frequency-voltage conversion output voltage VF has an offset voltage VOS in the negative direction with respect to a change in the lamp voltage VL.
The output is displaced by the same voltage as E. Therefore, when the ramp voltage VL and the frequency-voltage conversion output voltage VF are added in an analog manner, the added output of the second analog adder 18 becomes as shown in FIG.
As shown in B, the voltage has a constant negative voltage VS. By applying this negative voltage VS to the integrator 20, the integrator 20 integrates the deviation between the voltage VS and the offset voltage VOSE, and outputs an integrated voltage VO shown in FIG. 2C. When the ramp voltage VL passes through 0 volts, the integrated voltage VO also passes through 0 volts. At this time, the first local oscillator LO1 is supplied with the hold voltage HV from the sample and hold circuit 6. Therefore, the first local oscillator LO1
Oscillates at the center frequency fS of the frequency sweep bandwidth, and when the ramp voltage VL changes from the minimum value to the maximum value, the oscillation frequency of the first local oscillator LO1 is swept from fMIN to fMAX.

Here, when the hold voltage HV stored in the sample hold circuit 6 drifts, the drift causes the oscillation frequency of the first local oscillator LO1 to change. This frequency change is caused by the frequency-voltage converter 17
This occurs as a voltage change at the output side of the. The hold voltage HV of the sample and hold circuit 6 generally drifts in a decreasing direction. Therefore, the oscillation frequency of the first local oscillator LO1 fluctuates in a direction to become lower than the normal frequency.
When the frequency fluctuates in the lower direction, the converted output voltage VF of the frequency-voltage converter 17 fluctuates in the negative direction, and as a result, the deviation voltage input to the integrator 20 also changes in the negative direction. Therefore, the integrated voltage VO output from the integrator 20 changes in the direction of increasing in the positive direction, and the drift of the hold voltage HV is corrected by the correction voltage ΔV that changes in the positive direction. Therefore, the sweep frequency of the first local oscillator LO1 is swept while maintaining the correct frequency.

FIG. 3 shows a modified embodiment of the present invention. In this embodiment, the frequency divider 2 is provided before the frequency-voltage generator 17.
1 is provided to divide the frequency of the signal output from the low-pass filter 16 and convert the frequency of the signal input to the frequency-voltage converter 17 to a lower frequency. By converting the frequency of the signal input to the frequency-to-voltage converter 17 to a low frequency in this way, the circuit elements that make up the frequency-to-voltage converter 17 are not particularly required to operate at high speed, and therefore are configured with inexpensive circuit elements. The benefits can be obtained.

FIG. 4 shows an embodiment of the second invention of this application. This embodiment shows a case where the oscillation frequency of the first local oscillator LO1 is set to the center frequency of the frequency sweep bandwidth without using the phase lock loop. That is, in this embodiment, as a control loop of the oscillation frequency of the first local oscillator LO1, the frequency divider 22 for dividing the oscillation frequency of the first local oscillator LO1, the frequency-voltage converter 17, and the second analog adder 18 And the integrator 20. The integrator 20 has a reference voltage source 1
9 is connected.

In order to set the oscillation frequency of the first local oscillator LO1 to the center frequency of the frequency sweep bandwidth, the voltage output from the ramp voltage generator 2 is set to 0 volt. The first local oscillator LO1 depends on the voltage VF supplied from the frequency-to-voltage converter 17.
Is set to the center frequency of the frequency sweep bandwidth.

After this setting, a ramp voltage VL is generated to sweep the oscillation frequency of the first local oscillator LO1. The circuit 23 shown in FIG. 4 is a span adjustment circuit for setting the frequency sweep bandwidth. Reference numeral 24 denotes an inverter circuit for inverting the polarity of the lamp voltage.
Also in this embodiment, the deviation between the converted output voltage of the frequency-to-voltage converter 17 and the lamp voltage VL is always the reference voltage source 19.
The control loop operates so as to be equal to the offset voltage VOSE, so that the oscillation frequency of the first local oscillator LO1 follows the ramp voltage VL and is not affected by the drift.

[0031]

As described above, according to the present invention, a control loop constituted by the frequency-voltage converter 17, the second analog adder 18 and the integrator 20 is added at the time of frequency sweep. The drift of the hold voltage of the sample and hold circuit 6 is removed by the control loop,
The drift of the oscillation frequency of the local oscillator LO1 can be suppressed. Further, since the sweep frequency of the first local oscillation frequency changes faithfully following the change of the lamp voltage of the lamp voltage generator 2, the accuracy of the frequency display position of the frequency spectrum displayed on the display can be improved. Benefits are also obtained. Therefore, according to the present invention, it is possible to provide a spectrum analyzer having high reliability of measurement results.

[Brief description of the drawings]

FIG. 1 is a block diagram showing one embodiment of the present invention.

FIG. 2 is a waveform chart for explaining the operation of the present invention.

FIG. 3 is a block diagram showing a modified embodiment of FIG. 1;

FIG. 4 is a block diagram showing an embodiment of the second invention of this application.

FIG. 5 is a block diagram schematically illustrating a spectrum analyzer.

FIG. 6 is a block diagram for explaining a conventional technique.

FIG. 7 is a graph for explaining the operation of FIG. 6;

FIG. 8 is a graph for explaining inconvenience of the conventional technique.

[Explanation of symbols]

 PLL1, PLL2 Phase lock loop LO1 First local oscillator 2 Ramp voltage generator 3 Variable frequency divider 4 Phase comparator 5 Low-pass filter 6 Sample hold circuit 7 First analog adder 8 Reference oscillator 15 Frequency mixer 16 Low band Pass filter 17 Frequency-voltage converter 18 Second analog adder 19 Reference voltage source 20 Integrator

Claims (2)

[Claims] A frequency analysis of a frequency component of a signal under test is performed by a frequency sweep signal output from a first local oscillator, and the oscillation frequency is set to a center frequency of a frequency sweep bandwidth by a phase lock loop every time a frequency sweep operation is started. The set state is stored in a sample and hold circuit inserted in the phase lock loop as a hold voltage, and the hold voltage is added to a ramp voltage in a first analog adder, and the analog added voltage is added to the first local oscillator. A voltage control terminal for sweeping the oscillation frequency of the first local oscillator in frequency. In a state where the first local oscillator performs a frequency sweep operation, a change in the oscillation frequency of the first local oscillator is measured by a voltage. A frequency-voltage converter for converting to change, and this frequency-voltage converter A second analog adder for obtaining a deviation voltage between the voltage signal output by the second analog adder and the lamp voltage; A spectrum analyzer characterized by adding a frequency control loop constituted by an integrator applied to a first analog adder. 2. A reference for frequency-analyzing a frequency component of a signal under measurement by a frequency sweep signal output from a first local oscillator, and setting the oscillation frequency to a center frequency of a frequency sweep bandwidth every time a frequency sweep operation is started. A voltage source;
A frequency-voltage converter for converting a change in the oscillation frequency of the local oscillator into a voltage change, an analog adder for calculating a deviation between a voltage signal output from the frequency-voltage converter and a lamp voltage output from the lamp voltage generator, A frequency control loop comprising an integrator for controlling the oscillation frequency of the first local oscillator is provided so that the deviation voltage output from the analog adder is always equal to the voltage of the reference voltage source. Spectrum analyzer.