JPH0980094A - Intermodulation distortion measurement method with spectrum analyzer - Google Patents

Abstract

(57) [Abstract] [Problem] It is possible to accurately measure even low-level third-order distortion. SOLUTION: The input attenuation amount is set to 0 dB (S ₂ ), the input attenuation amount is increased in 10 dB steps, and the third-order distortion level L ₃ at that time is measured (S ₃ ). Next, let δ
The ATT = 1 dB is changed to measure the third-order distortion level L ₃ ′ and the fundamental wave level L _1, and the change amount δD _is = L ₃ −L ₃ ′ of the third-order distortion level at that time _is obtained (S ₄ ). If δD _is is not within δATT ± ε, the process returns to step S ₃ and the attenuation amount is further increased by 10 dB, and the same operation is performed.
If it is within T ± ε, the third-order distortion L ₃ ′ at that time is taken as that of the input, and L ₁ −L ₃ ′ = ΔL is the level difference between the fundamental wave and its third-order distortion.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention uses a spectrum analyzer and is included in a signal input to the spectrum analyzer.
The present invention relates to a method for measuring intermodulation distortion such as second-order distortion and fifth-order distortion.

[0002]

2. Description of the Related Art Various electronic equipments, that is, communication equipments, video equipments, signal transmission equipments, broadcasting equipments, etc., when two signals (fundamental waves) are input, the non-linear characteristics of these equipments cause
These fundamental waves may be mixed in frequency, and intermodulation distortion such as third-order distortion and fifth-order distortion may occur. This intermodulation distortion has a great influence on the characteristics of these electronic devices.

When intermodulation distortion is measured using a spectrum analyzer, a test signal having the same amplitude of frequencies f ₁ and f ₂ from the signal generators 12 and 13 is applied to the object 11 as shown in FIG. The signals are combined and supplied by 14, and the output signal of the object 11 at that time is input to the spectrum analyzer 15 as an input signal to measure the intermodulation distortion generated from the object 11. In the spectrum analyzer 15, the input signal is supplied to the frequency mixer 17 through the input variable attenuation amount 16, is frequency-mixed with the local signal from the frequency sweep oscillator 18, and the mixed output is supplied to the bandpass filter 19 and the filter 19 is supplied. The output is amplified by the amplifier 21, the amplified output is frequency mixed with the local signal from the local oscillator 23 by the frequency mixer 22, the intermediate frequency signal is taken out by the band pass filter 24, and the output of the filter 24 is detected by the detector. The signal is detected at 26, and the detected output is passed through the low pass filter 27, then converted into a digital signal by the A / D converter 28 and stored in the buffer memory 29. The control unit 31 is a so-called CPU, and the parameter setting means 3
The sweep frequency oscillator 18 by setting the attenuation amount of the attenuator 16 according to the parameter set by 2 and controlling the ramp voltage generator 34 through the timing controller 33.
Control of the sweep frequency band, filter 19,
24 bandwidth RBW setting, filter 27 bandwidth VB
The setting of W, the sampling period of the A / D converter 28, and the like are performed, and display control of the data stored in the buffer memory 29 to the display 35 is performed.

Conventionally, the measurement of intermodulation distortion has been performed by the following procedure. 1. The center frequency is set to one of the input signals, that is, two fundamental waves that are the basis of the intermodulation distortion, that is, one of the signals of frequency f _{1 and} the signal of frequency f ₂ . 2. Press the span button to manually determine the input signal frequency range, that is, the display frequency range.

3. Press the bandwidth button to set the resolution bandwidth, that is, the bandwidth R of each of the band pass filters 19 and 24.
Resolution bandwidth RB by changing the setting of BW from automatic to manual
Set W. This is because when the resolution bandwidth RBW is automatically set, the intermodulation distortion level is small, or the spectrum of the intermodulation distortion is hidden at the tail of the input signal due to the influence of the bandpass filters 19 and 24. This is because the distortion spectrum may not be observed in some cases.

4. Press the TOI button to measure the third-order distortion. 5. Until four distinct peaks appear on the display surface, that is, four input signal (fundamental wave) spectra and two third-order distortion spectra generated by intermodulation of these two input signals appear, 1 Operations 3 to 3 are repeated while changing the setting of the resolution bandwidth RBW.

[0007] That is, for example, 3 of the signals of the frequency f _1, f ₂ on the display surface of the display unit 35 as shown in FIG. 4A and (fundamental signal) 41 and 42, the frequency (2f ₁ -f ₂₎ distortion Four
3. Make the four peaks of the third-order distortion 44 of the frequency (2f ₂ −f ₁ ) clearly appear. As shown in FIG. 4B, if the level of the fundamental waves 41, 42 on the input side of the input attenuator 16 is L, and the level difference between the third-order distortions 43, 44 with respect to the fundamental waves 41, 42 is ΔL, the third-order distortion 43. , 44, the distortion amount (level) is L-ΔL. Assuming that the attenuation amount in the input attenuator 16 is ATT, as shown in FIG.
The level of 1,42 becomes L-ATT, and the third-order distortion 43,4
The level of 4 is L- (ΔL + ATT). At the place, in the spectrum analyzer 15, the mixer 17 has X dB.
It is specified as a standard that the third-order distortion of X-YdBm occurs when m fundamental wave inputs are present. From the characteristic of third-order distortion generation, when the input fundamental wave level becomes X + ΔX, the third-order distortion level becomes Y + ΔY and ΔY = 3ΔX. That is, the third-order distortion generated in the mixer 17 increases the input fundamental wave level ΔX.
3 times 3ΔX.

Therefore, if the input attenuator 16 attenuates the level of the fundamental wave by ΔATT, the third-order distortions 43 and 44 also become ΔAT.
Although T is small, the third-order distortion generated in the mixer 17 is 3ΔA.
TT decreases. From such a relationship, when the attenuation amount of the input attenuator 16 is increased, the third-order distortion generated by the mixer 17 is gradually reduced and can be ignored, and as shown in FIG. 4D, the input of the mixer 17 is reduced. Output characteristics are fundamental wave 41, 4
2 becomes a straight line 45, the level of the fundamental waves 41 and 42 is small, and the input-output characteristic of the mixer 17 of the third-order distortion 43 and 44 is the fundamental wave 41 when the level of the third-order distortion generated in the mixer 17 is negligible. , 42 with respect to the characteristic straight line 45, the level is Δ
The straight line 46 is smaller by L, but the input fundamental waves 41, 42
When the level of is large to some extent, the third-order distortion generated from the mixer 17 cannot be ignored, and the characteristic of the third-order distortion of this mixer is a straight line 47. The third-order distortion appearing at the output of the mixer 17 is the sum of the input third-order distortion and the third-order distortion of the mixer.

[0009]

[Equation 1] Becomes Since it is the sum of exponential functions,
When the input level of the fundamental waves 41, 42 is lower than the intersection 48 of the and 47, the level of the input third-order distortion 43, 44 becomes dominant, and when the input level of the fundamental waves 41, 42 becomes higher than the intersection 48, The third-order distortion of the mixer becomes dominant.

Therefore, a state in which the amount of attenuation of the input attenuator 16 is large and there is no influence of the third-order distortion generated in the frequency mixer 17 in the spectrum analyzer, and this fundamental wave level,
By measuring the third-order distortion, the level of the third-order distortion generated in the object 11 with respect to the fundamental wave level can be known. From the above relationship, the intercept point can be calculated as follows. Assuming that the gain of the object 11 is 1, the input output level characteristic for the fundamental wave is as shown in FIG.
Line 45a, on the other hand, for that fundamental wave level,
The characteristic of the third-order distortion level generated in the object 11 is the straight line 46a.
Becomes When the fundamental wave input level is I _P and the output level is I ₀₁ , the straight line 45a is I ₀₁ = I _P , and when the third-order distortion level is I _O3 , the straight line 46a can be expressed as I _O3 = 3I _P + a, and the spectrum measuring the fundamental wave level I _P = I ₀₁ therein intermodulation negligible range analyzer 15, and to measure the third-order distortion level I _O3 of input, these I _P, I
The value of a can be obtained by substituting _O3 into the equation of the straight line 46a. Since the intercept point 49 is an intersection of the straight lines 45a and 46a, the straight lines 45a and 46a are equal to each other.
_The intercept point can be obtained as ₀ , I _P point (I _P + ΔL / 2, I _P + ΔL / 2). Where ΔL
= Is the I ₀₁ -I _O3.

[0011]

As described above, in the prior art, the attenuation amount of the input attenuator is manually changed to judge whether or not there is an influence of the frequency mixer in the spectrum analyzer. However, it was not possible to automatically measure the intermodulation distortion. That is, the input attenuator 16 is for setting the input signal in a desired level range,
A typical spectrum analyzer has an attenuation of 10 dB.
It is configured to make a step change, and such a step change is sufficient. Conventionally, if the level of the third-order distortion does not change even if the input attenuator 16 is changed by 10 dB, the level is changed to the input third-order distortion 43. , 44 levels,
The difference from the input fundamental wave level at that time is the input fundamental wave and input 3
It was determined that the level difference from the next distortion was ΔL. In this way, when changing in 10 dB steps, there are only one or two measurement points that are not affected by the internal third-order distortion, and these points really affect the effect of the internal third-order distortion (mixer third-order distortion). In some cases, it was difficult to confirm whether it was not received, and it was not possible to accurately measure the third-order strain level.

Further, conventionally, the resolution bandwidth, that is, the change of the bandwidth of the band pass filters 19 and 24 is manually set, and the setting and the setting of the input attenuation amount are performed by trial and error. It is difficult to set an appropriate resolution bandwidth RBW because they influence each other on the spectrum display.

[0013]

According to the present invention, the input attenuation amount is increased by a predetermined value, the intermodulation distortion amount at that time is measured in the first step, and the increased input attenuation amount is sufficiently smaller than the predetermined value. A small value is changed, the intermodulation distortion amount at that time is measured in the second step, and the change amount of the intermodulation distortion amount measured in the second step with respect to the intermodulation distortion amount measured in the first step is obtained in the third step. The first step and the second step are repeated in the fourth step until the amount of change becomes less than or equal to a predetermined value, and the measured intermodulation distortion amount when the amount of change becomes less than or equal to the predetermined value is input in the fifth step. Intermodulation distortion amount of.

Further, the fundamental wave level in the intermodulation distortion amount obtained in the fifth step is measured in the sixth step, passes through the intermodulation distortion amount obtained in the fifth step, and the intermodulation based on the change of the fundamental wave level. An intersection between the distortion amount change characteristic straight line and the fundamental wave output level change characteristic straight line with respect to the fundamental wave level change passing through the fundamental wave level obtained in the sixth step is obtained as an intercept point in the seventh step.

[0015]

DETAILED DESCRIPTION OF THE INVENTION Hardware for carrying out the method of the present invention.
The air structure may be the same as that shown in FIG. this
In the embodiment of the invention, as shown in FIG.
Fix the level of the signal input to the riser to, for example, Lv
(S₁). The attenuation value of the input attenuator 16 is an initial value, generally
0 dB (no attenuation) and the third-order distortion amount L at that time_ThreeMeasure
Do (S₂). Next, the input attenuator 16 is set to a predetermined step described later.
Step amount δATT, which is sufficiently smaller than the
If it is changed by 1 dB, the third-order distortion amount L at that time is changed._Three′ Is measured,
Step S₂Measured amount L at_ThreeChange amount, that is, 3
Change in next strain δD_is= L _Three-L_Three′ Is calculated (S_Three).
As the input attenuator 16, change the attenuator by 1 dB.
Configure to enable

Change amount δD of this third-order distortion_isIs δATT-ε
And δATT + ε are checked (S_Four) (Ε
Should be within this range, for example, about 0.2 to 0.3 dB)
If step S_FiveAnd change the attenuation of the input attenuator 16 to
Increase the constant step amount, for example, 10 dB, then increase the third order
Distortion amount L_ThreeAnd measure step S_ThreeReturn to and perform the same process.
Yes, step S_FourAt δD_isIs within δATT ± ε
For example, this third-order distortion amount L_Three, L _Three′ Is the third-order distortion 4 in the input signal
It is recognized that 3,44 are in the dominant state. Therefore
Level L of the fundamental waves 41, 42 when₁′ Is measured and this
And the third-order distortion amount L at this time_Three′ And the level difference ΔL = L₁'-
L_Three′ Is calculated (S₆).

Next, the fundamental wave level L₁'And the level difference ΔL
And the intercept point is calculated from
(S₇). (L₁′ + (ΔL / 2), L₁′ + (ΔL / 2)) Next, various parameters for automatically measuring this intermodulation distortion
The setting of the meter will be described. Input signal fundamental wave 41,
You may know the frequency of 42 in advance, in that case
These frequencies f₁, F₂Set manually. Yeah
If these frequencies are known, they are actually input
What is done may deviate from the known value,
In those cases, the input signal is once captured in the buffer memory 29.
Look for the two with the highest level in that data string,
The frequency that gives the maximum level is f ₁, F₂And (f
₁<F₂). From these frequencies, the center frequency f_c=
(F₁+ F₂) / 2 as the measurement center frequency,
The data is stored in the data storage unit 36.

Next, the frequency span is determined. In the case of the third-order distortion, both third-order distortions 43 and 44 need to be within the display screen. Therefore, if it is three times the difference f ₂ -f ₁ between the fundamental frequencies. Good. However, in reality, it is preferable that all the tails of the third-order distortions 43 and 44 are displayed on the screen, so that the frequency span is f ₂ −f.
_Set to 5 times ₁ This frequency span is also stored in the parameter storage unit 36.

Next, the pass band width RBW of the band pass filters 19 and 24 is determined. Fundamental wave 41, 42, third-order distortion 4
3 and 44 spectrums, for example, the tails of the signal 41 are the adjacent spectrums, in the above example, the signals 43 and 4
RBW is determined so as not to affect the peak of 2. This is uniquely determined from FIG. 2A showing the relationship characteristic between the dynamic range and the frequency interval (f ₂ −f ₁ ) in the adjacent carrier. That is, the vertical axis of FIG. 2A represents the noise level when the peak value is 0 dB, and therefore the absolute value of this vertical axis is the dynamic range, and the parameter of the characteristic curve indicates the settable bandwidth of the filters 19 and 24. The left side of a certain curve represents a portion where the peak of the adjacent spectrum is not applied to the bottom of the spectrum due to the influence of the filters 19 and 24 having the bandwidth indicated by the curve. Therefore, given the dynamic range and the frequency interval (f ₂ −f ₁ ), one having a smaller bandwidth RBW of the characteristic curve near this point may be selected.
For example, when the noise level is -100 dBc / Hz, that is, the dynamic range is 100 dB and the frequency interval is 40 kHz, the point is point A in FIG. 2A, and the bandwidth 3 kHz of the characteristic curve on the left side of this point A in the figure is RBW. If RBW is smaller than 3 kHz, the problem of spectrum overlap does not occur, but since the sweep time (measurement time) becomes long, the bandwidth of the curve on the left side as close as possible to point A is set.

Further, the dynamic range is determined as follows. When the dynamic range is increased, that is, when the noise level is decreased, distortion and spurious are generated due to the influence of the frequency mixers 17 and 22 inside the spectrum analyzer. Usually, the limit of the maximum value (minimum value of noise level) of the dynamic range that does not cause such distortion and spurious is previously obtained by measurement, and a margin is added to the limit value and stored in the spectrum analyzer. This is given, for example, by the dashed curve in Figure 2A. The measurable range is the range above this dashed curve in FIG. 2A. For example, if the frequency interval is 40 kHz, 4
The dynamic range is smaller than the value (point B) 113 dB on the broken line curve at 0 kHz, that is, the noise level is 11
It is necessary to set it to 3 dBc / Hz or more.

When the RBW is large, the noise level becomes large and the dynamic range becomes small. When the noise generated in the spectrum analyzer is N _s and the attenuation amount of the input attenuator 16 is ATT, the noise level NL can be expressed by the following equation. NL = N _s +10 log (RBW) + ATT When SL is the maximum signal level that can be input, the measurement limit is SL-NL. This limit line is shown by a dotted line in the noise level-frequency interval characteristic diagram of FIG. 2B. If the absolute value of the dynamic range is larger than this, the signal will be distorted or the peak will protrude from the display point.

The dynamic range DR thus becomes _{DR <SL-N s -10 log} (RBW) -ATT. From this, the set value of RBW and the settable range of the dynamic range (however, the vertical axis represents the noise level value which is the inverted value of the dynamic range) are shown in FIG. 2B. As the internally generated noise level N _s increases, the lower limit value increases, so the dotted line of the noise level rises upward, and when N _s decreases, the lower limit value decreases, and the dotted line of the noise level moves downward. The range in which the dynamic range can be set for the frequency interval f ₂ −f ₁ is the range shaded in FIG. 2B. If you set the dynamic range within this range, RB
It means that the value of W is unique.

The RBW value obtained as described above is also stored in the parameter value storage unit 36. RBW like this
2 required for the determination of, or the data necessary for obtaining this is stored in the spectrum analyzer in advance. The lower limit value of the bandwidth VBW of the low-pass filter (video filter) 27 is VBW ≧ RBW / 10 from the empirical rule.
, Which is determined by the accuracy of the measured value. On the other hand, the resolution (RBW) of the spectrum analyzer must not drop because the low-pass filter 27 has passed, so VBW should be set to R
It is averaged by making it narrower than BW. Therefore, VBW < R
It becomes BW. Under this condition, VBW = RBW in order to minimize the sweep time.

Further, as the reference level, the maximum peak is 2
The input attenuator 16 is controlled so that the peak values of the four fundamental waves 41 and 42 coincide with the upper end of the display surface. These VB
W and the reference level are also stored in the parameter value storage unit 36. The control unit 31 automatically determines such various parameters. After that, in the case of measuring the intermodulation distortion, in step S ₂ in FIG. 1, the display center frequency f _c , the frequency span, and the R stored in the parameter storage unit 36.
Parameters such as BW, VBW, and basic level are set for the corresponding units under the control of the control unit 31.
Also, the sweep time T _s is the frequency span (Hz) / {RBW
(Hz) x min (RBW, VBW) (Hz) x 0.
5} (sec) is calculated and set. min (RBW, VBW) represents the smaller one of RBW and VBW. In addition, each time the measurement is performed, the buffer memory 29
To do it.

In the above, the present invention is applied to the measurement of third-order distortion.
However, it can also be applied to the measurement of 5th and 7th distortion.
You. In the case of 5th and 7th distortion, change in input signal level
The level changes 5 times and 7 times respectively. This signal
To measure with a spectrum analyzer, use the spectrum analyzer
The riser frequency span is the difference between the fundamental frequencies f_{twenty one}-F₁Of 7
It is enough to double or 9 times. The intercept points are (I
_P+ (ΔL / 4), I _P+ (ΔL / 4)), (I_P+ (Δ
L / 6), I_P+ (ΔL / 6)).

[0026]

As described above, according to the present invention, the amount of intermodulation distortion is obtained by slightly changing the input attenuation amount and detecting the state where there is almost no change in the amount of intermodulation distortion before and after the change. Therefore, it is possible to reliably detect even a relatively small intermodulation distortion amount, and similarly, it is possible to obtain the intercept point even when the intermodulation distortion amount is small.

Further, according to the invention of claim 3, an appropriate RBW is automatically set, and it is possible to perform the measurement in a short time without the trouble of repeating the setting of the RBW by trial and error until four peak values clearly appear. You can

[Brief description of drawings]

FIG. 1 is a flowchart showing a processing procedure in an embodiment of the invention of claim 2;

FIG. 2 is a diagram showing a relationship between a noise level (dynamic range) and a frequency interval using RBW as a parameter.

FIG. 3 is a block diagram showing a hardware configuration example of intermodulation distortion measurement by a spectrum analyzer.

4A is a diagram showing an example of a fundamental wave and a third-order distortion displayed on a display screen, FIG. 4B is a diagram showing a fundamental wave level and a third-order distortion level before entering an input attenuator, and C is a diagram. The figure which shows the fundamental wave level and the 3rd-order distortion level to which the input attenuation amount ATT was given, D is the figure which shows the relationship between the input attenuation amount, the fundamental wave level, and the 3rd-order distortion level, E is the fundamental wave penetration of an object. It is a figure which shows the 3rd-order distortion amount characteristic with respect to an output characteristic and a fundamental wave input.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Osamu Aoyama 1-32-1, Asahimachi, Nerima-ku, Tokyo Within Advantest, Inc. In Stock Company Advantest (72) Inventor Yoshiaki Miyamae 1-32-1 Asahi-cho, Nerima-ku, Tokyo Stock Company Advantest (72) Inventor Toshiharu Kasahara 1-32-1 Asahi-cho, Nerima-ku, Tokyo Stock Association Company Advantest (72) Inventor Hiroaki Takaoku 1-32-1, Asahi-cho, Nerima-ku, Tokyo Stock company Advantest

Claims (3)

[Claims] 1. A first step of increasing an input attenuation amount by a predetermined value and measuring an intermodulation distortion amount at that time, and then changing the input attenuation amount by a minute value sufficiently smaller than the predetermined value to perform intermodulation at that time. A second step of measuring the distortion amount, and a third step of obtaining a variation amount of the inter-modulation distortion amount measured in the second step with respect to the inter-modulation distortion amount measured in the first step.
And a fourth step of repeating the first to third steps until the change amount becomes a predetermined value or less, and an intermodulation distortion amount in the input signal when the change amount becomes a predetermined value or less. And a fifth step of setting the amount of modulation distortion, and a method of measuring intermodulation distortion using a spectrum analyzer. 2. A sixth step of measuring a fundamental wave level in the intermodulation distortion amount obtained in the fifth step, and an intermodulation distortion amount obtained in the fifth step, which is based on a change in the fundamental wave level. A seventh step of obtaining an intercept point by obtaining an intersection point between the intermodulation distortion amount variation characteristic straight line and the fundamental wave output level variation characteristic line with respect to the fundamental wave input level variation, which passes through the fundamental wave level obtained in the sixth step. The intermodulation distortion measuring method according to claim 1, further comprising: 3. The intermodulation so as to satisfy the relational characteristic between the adjacent carrier frequency interval of the input signal and the noise level or the dynamic range of the input signal, using the passband width RBW of the bandpass filter of the spectrum analyzer as a parameter. The step of determining the RBW from the frequency interval between two fundamental waves with respect to distortion and automatically setting the RBW in the bandpass filter is included before the first step. Method for measuring intermodulation distortion by using a spectrum analyzer.