CN105445549B - A Spectrum Measurement Method for Weak Electromagnetic Signals - Google Patents

Abstract

The invention discloses a weak electromagnetic signal spectrum measurement method, comprising the following steps: connecting an output port of a radio frequency signal to be measured to an input port of a receiving channel of a superheterodyne instrument, setting the state of the instrument that needs to perform signal measurement; performing a scan measurement to obtain a group spectrum data; isolate the RF signal under test and the input port of the superheterodyne instrument, perform a sweep measurement, obtain another group of spectrum data sets, and determine its threshold; find the absolute value of the error of the two groups of spectrum data, Determine the magnitude of the absolute value of the error and the threshold. If the absolute value of the error is less than the threshold, the spectrum measurement result of the RF input signal is the threshold, otherwise the spectrum measurement result is the absolute value of the error. The invention improves the measurement accuracy of weak signal level without reducing input attenuation or increasing low-noise amplification.

Description

A Spectrum Measurement Method for Weak Electromagnetic Signals

technical field

The invention relates to a method for measuring the spectrum of weak electromagnetic signals.

Background technique

Superheterodyne receiving or analyzing instruments are the most commonly used test instruments for spectrum measurement of electromagnetic signals. Among them, heterodyne spectrum analyzer has the advantages of simplicity, flexibility, wide frequency range and high sensitivity. It is the most widely used superheterodyne instrument. With the continuous development of radio technology, the requirements for the measurement sensitivity and dynamic range of spectrum analyzers for weak signals are getting higher and higher.

The minimum measurable signal level and dynamic range are important indicators to measure the weak signal measurement capability of the spectrum analyzer. The basic principle of a superheterodyne spectrum analyzer is shown in Figure 1.

Usually, the signal measured and displayed by the spectrum analyzer is equal to the input signal + the inherent noise of the instrument, so the minimum signal that can be measured is limited by the inherent noise of the instrument. Manufacturers often use the indicator showing the average noise level to specify the inherent noise of the instrument.

Displayed average noise level = -174dBm+F _dB +10log(RBW _noise /Hz).

Among them, F _dB is the noise figure of the whole machine, and RBW _noise is the equivalent noise bandwidth of the IF resolution bandwidth filter of the instrument.

In order to improve the accuracy of weak signal measurement, the existing technical solutions include: first, reducing the noise figure FdB of the instrument, that is, reducing the input attenuator or not attenuating, or adding a low-noise amplifier circuit inside or outside the instrument, thereby reducing the screen Displayed noise level improves weak signal level measurement capability.

Another typical existing technical means is to reduce the intermediate frequency resolution bandwidth RBW, and reducing the RBW will reduce the noise energy passing through the intermediate frequency filter, that is, reduce the noise level displayed on the screen. Although this method is only very effective for measuring sinusoidal signal levels, it also results in a significant reduction in measurement speed. If you want to measure the noise level of the input signal, such as the channel power or adjacent channel rejection ratio measurement of noise-like signals such as digital communication signals, reducing the RBW will also reduce the noise energy of the input signal passing through the IF filter. The signal-to-noise ratio still cannot be improved, nor can the measurement accuracy be improved.

Another existing technical means can also improve the measurement capability of weak signals by reducing the video filter bandwidth VBW or using multiple averaging or average detection and other smoothing measures, but these methods only reduce the measurement results. variance, does not improve the accuracy of weak signal level measurements, and does not reduce the displayed average noise level.

The existing technical means to reduce the noise figure of the instrument, such as reducing the attenuator and adding the low noise amplifier, not only greatly increase the hardware cost of the instrument, but also cause the input signal to enter when the input signal has high-level frequency components. The power inside the instrument is too large, and the internal circuit components are damaged. Even if the input signal power is within the acceptable level range of the instrument, it usually causes a certain nonlinear distortion, which reduces the measurement dynamic range, and the distortion component may also affect the signal measurement of small-level frequency components. Restricted by the current technology and manufacturing level, the noise figure of the instrument cannot be reduced to zero, that is, there will always be inherent noise. When the level of the input signal is approximately close to the inherent noise level of the instrument, the measurement accuracy of the signal level is worse.

Therefore, the existing problem is: without reducing the input attenuation or increasing the low noise amplification, measure the instrument noise level in advance, and then delete the instrument noise level when the measurement is performed, so as to improve the weak signal power. Level measurement accuracy, further reduce the influence of instrument inherent noise on weak signal measurement, and improve weak and small signal level measurement accuracy.

SUMMARY OF THE INVENTION

In order to solve the above problems, the present invention proposes a method for measuring the spectrum of weak electromagnetic signals. This method improves the measurement accuracy of weak signal levels and further reduces the inherent noise of the instrument without reducing input attenuation or increasing low-noise amplification. Effects on weak signal measurements.

In order to achieve the above object, the present invention adopts the following technical solutions:

A weak electromagnetic signal spectrum measurement method, comprising the following steps:

(1) Connect the output port of the RF signal under test to the input port of the receiving channel of the superheterodyne instrument, and set the state of the instrument that needs to perform signal measurement;

(2) Perform a sweep measurement to obtain a set of spectrum data;

(3) Isolate the RF signal under test and the input port of the superheterodyne instrument, perform a sweep measurement, obtain another group of spectrum data sets, and determine its threshold;

(4) Obtain the absolute value of the error of the two sets of spectrum data, and determine the magnitude of the absolute value of the error and the threshold. If the absolute value of the error is less than the threshold, the spectrum measurement result of the RF input signal is the threshold value, otherwise the spectrum measurement result is the absolute value of the error.

In the step (3), the threshold confirmation method is: Z(i)=X(i)/M, the value of M depends on the size of the noise floor of the superheterodyne instrument, Z(i) is the threshold, X (i) is a set of spectral data with N elements obtained by scanning measurement after isolation, i=1, 2, . . . , N.

In the described step (3), the method for isolation includes any one of the following:

(1-1) Disconnect the RF signal under test from the input port of the superheterodyne instrument, and then connect the input port of the superheterodyne instrument to the matching load;

(1-2) Under the condition that the attenuation value of the input attenuator of the superheterodyne instrument is accurately known, the input attenuation of the superheterodyne instrument can be increased to the maximum.

The concrete method of described step (4) is:

(4-1) Calculate the absolute value of the error of the two sets of data, that is, S'(i)=|X(i)-Y(i)|; among them, Y(i) is the number of elements obtained by scanning and measuring before isolation is a set of spectral data of N, i=1, 2, ..., N;

(4-2) If S'(i)<threshold Z(i), then let S(i)=Z(i); otherwise, S(i)=S'(i);

(4-3) Take S(i) as the final spectrum measurement result for the RF input signal.

In the step (2), the spectrum data is the frequency power value.

In the step (3), the spectrum data is the frequency power value.

The beneficial effects of the present invention are:

(1) The measurement accuracy of weak signal level is improved without reducing input attenuation or increasing low-noise amplification, and the cost is low;

(2) On the basis of improving the weak signal measurement capability by reducing attenuation or increasing low-noise amplification, this method can still further reduce the influence of instrument inherent noise on weak signal measurement, and improve the measurement accuracy of weak and small signal level;

(3) Solving the problem of the accuracy of noise signal level measurement that cannot be solved by reducing the intermediate frequency resolution bandwidth (RBW) of the spectrum analyzer to improve the accuracy of sinusoidal signal level measurement;

(4) On the basis of reducing the video filter bandwidth VBW or adopting multiple averaging or adopting average detection and other smoothing measures, the method can still further improve the measurement accuracy of weak signal level;

(5) The present invention is not opposed to the above-mentioned prior art means, and the method proposed by the present invention can still be used to further improve the weak signal level measurement accuracy while using the above-mentioned technical means.

Description of drawings

Fig. 1 is the principle schematic diagram of superheterodyne spectrum analyzer of the present invention;

FIG. 2 is a schematic flow chart of the present invention.

Detailed ways:

The present invention will be further described below with reference to the accompanying drawings and embodiments.

The operation steps of the weak signal (measured radio frequency signal) spectrum measurement method proposed by the present invention are shown in Figure 2:

Step 1: Correctly connect the output port of the RF signal under test to the input port of the receiving channel of the superheterodyne instrument;

Step 2: Set the state of the instrument that needs to perform signal measurement;

Step 3: Perform a sweep measurement to obtain a set of spectral data Y(i) with N elements, i=1, 2,  , N;

Step 4: Isolate the RF signal under test and the input port of the superheterodyne instrument.

Step 5: Perform a scan measurement to obtain another set of spectral data X(i) with N elements, i=1, 2, . . . , N;

Step 6: Perform mathematical operations on the spectral data X(i) and Y(i) twice to obtain S(i);

Step 7: take S(i) as the final spectrum measurement result for the RF input signal;

Wherein, a group of spectrum data X(i) in step 3, X(i) is a group of frequency power values, and the unit is W.

Wherein, another group of spectrum data Y(i) in step 5, Y(i) is another group of frequency power values, and the unit is W.

The rules for performing mathematical operations on the two spectral data X(i) and Y(i) in step 6 are:

Step 1: Find a set of thresholds Z(i) according to X(i), Z(i)=X(i)/M, the value of M depends on the noise floor of the superheterodyne instrument, usually the value is an arbitrary constant between 10 and 1000.

Step 2: Find the absolute value of X(i)-Y(i), S'(i)=|X(i)-Y(i)|;

Step 3: If S'(i)<Z(i), then S(i)=Z(i); otherwise S(i)=S'(i).

The isolation method in step 4 can be any one of the following two methods, or other effective isolation methods. The greater the isolation, the better.

Method 1: Disconnect the RF signal under test from the input port of the superheterodyne instrument, and then connect the input port of the superheterodyne instrument to the matching load.

Method 2: When the attenuation value of the input attenuator of the superheterodyne instrument is accurately known, the input attenuation of the superheterodyne instrument can be increased to the maximum.

Although the specific embodiments of the present invention have been described above in conjunction with the accompanying drawings, they do not limit the scope of protection of the present invention. Those skilled in the art should understand that on the basis of the technical solutions of the present invention, those skilled in the art do not need to pay creative work. Various modifications or deformations that can be made are still within the protection scope of the present invention.

Claims (3)

1. a weak electromagnetic signal spectrum measurement method is characterized in that: comprise the following steps: (1) Connect the output port of the RF signal under test to the input port of the receiving channel of the superheterodyne instrument, and set the state of the instrument that needs to perform signal measurement; (2) Perform a sweep measurement to obtain a set of spectrum data; (3) Isolate the RF signal under test and the input port of the superheterodyne instrument. The isolation method is to disconnect the RF signal under test and the input port of the superheterodyne instrument, and then connect the input port of the superheterodyne instrument. Connect the matched load or increase the input attenuation of the superheterodyne instrument to the maximum when the attenuation value of the input attenuator of the superheterodyne instrument is accurately known, and perform a sweep measurement to obtain another set of spectrum data sets. , determine its threshold; (4) Obtain the absolute value of the error of the two groups of spectrum data, and judge the size of the absolute value of the error and the threshold value. If the absolute value of the error is less than the threshold value, the spectrum measurement result of the radio frequency input signal is the threshold value, otherwise the spectrum measurement result is the absolute value of the error; In described step (2), spectrum data is frequency power numerical value; In the step (3), the spectrum data is the frequency power value. 2. a kind of weak electromagnetic signal spectrum measurement method as claimed in claim 1 is characterized in that: in described step (3), the confirmation method of threshold value is: Z(i)=X(i)/M, M takes The value depends on the noise floor of the superheterodyne instrument, Z(i) is the threshold, X(i) is a set of spectral data with N elements obtained by scanning and measuring after isolation, i=1, 2,… , N. 3. a kind of weak electromagnetic signal spectrum measurement method as claimed in claim 1 is characterized in that: the concrete method of described step (4) is: (3-1) Find the absolute value of the error of the two sets of data, that is, S'(i)=|X(i)-Y(i)|; among them, Y(i) is the number of elements obtained by scanning and measuring before isolation is a set of spectral data of N, i=1, 2, ..., N; (3-2) If S'(i)<threshold Z(i), then let S(i)=Z(i); otherwise, S(i)=S'(i); (3-3) Take S(i) as the final spectrum measurement result for the RF input signal.