Various sense organs that give us information about the state of the surrounding world may be more or less sensitive to the phenomena they display, that is, they can reflect these phenomena with greater or lesser accuracy. The sensitivity of the sense organs is determined by the minimum stimulus that, under given conditions, is capable of causing a sensation.
The minimum strength of the stimulus that causes a barely noticeable sensation is called the lower absolute threshold of sensitivity. Irritants of lesser strength, the so-called subthreshold, do not cause sensations. The lower threshold of sensations determines the level of absolute sensitivity of this analyzer. There is an inverse relationship between absolute sensitivity and the threshold value: the lower the threshold value, the higher the sensitivity of this analyzer. This ratio can be expressed by the formula E = 1/P, where E is the sensitivity, P is the threshold value.
Analyzers have different sensitivities. In humans, visual and auditory analyzers have very high sensitivity. As the experiments of S.I. Vavilov, the human eye is able to see light when only 2–8 quanta of radiant energy hit its retina. This allows you to see a burning candle on a dark night at a distance of up to 27 km.
The auditory cells of the inner ear detect movements whose amplitude is less than 1% of the diameter of a hydrogen molecule. Thanks to this, we hear the ticking of the clock in complete silence at a distance of up to 6 m. The threshold of one human olfactory cell for the corresponding odorous substances does not exceed 8 molecules. This is enough to smell in the presence of one drop of perfume in a room of 6 rooms. It takes at least 25,000 times more molecules to produce a taste sensation than it does to create an olfactory sensation. In this case, the presence of sugar is felt in a solution of one teaspoon of it per 8 liters of water.
The absolute sensitivity of the analyzer is limited not only by the lower, but also by the upper sensitivity threshold, i.e. maximum strength stimulus, in which there is still an adequate sensation to the acting stimulus. A further increase in the strength of stimuli acting on receptors causes only pain sensations in them (such an effect is exerted, for example, by loud noise and blinding brightness).
The value of absolute thresholds depends on the nature of the activity, age, functional state of the organism, strength and duration of stimulation.
In addition to the magnitude of the absolute threshold, sensations are characterized by an indicator of a relative, or differential, threshold. The minimum difference between two stimuli that causes a barely noticeable difference in sensations is called the discrimination threshold, difference or differential threshold. The German physiologist E. Weber, testing a person's ability to determine the heavier of the two objects in the right and left hand, found that differential sensitivity is relative, not absolute. This means that the ratio of a barely noticeable difference to the magnitude of the initial stimulus is a constant value. The greater the intensity of the initial stimulus, the more it is necessary to increase it in order to notice a difference, i.e., the greater the magnitude of a barely noticeable difference.
The differential sensation threshold for the same organ is constant value and expressed the following formula: dJ/J = C, where J is the initial value of the stimulus, dJ is its increase, causing a barely perceptible sensation of a change in the value of the stimulus, and C is a constant. The value of the differential threshold for different modalities is not the same: for vision it is approximately 1/100, for hearing - 1/10, for tactile sensations - 1/30. The law embodied in the above formula is called the Bouguer-Weber law. It must be emphasized that it is valid only for medium ranges.
Based on the experimental data of Weber, the German physicist G. Fechner expressed the dependence of the intensity of sensations on the strength of the stimulus by the following formula: E = k * logJ + C, where E is the magnitude of sensations, J is the strength of the stimulus, k and C are constants. According to the Weber-Fechner law, the magnitude of sensations is directly proportional to the logarithm of the intensity of the stimulus. In other words, the sensation changes much more slowly than the strength of the stimulus grows. An increase in the strength of irritation in geometric progression corresponds to an increase in sensation in an arithmetic progression.
The sensitivity of analyzers, determined by the magnitude of absolute thresholds, changes under the influence of physiological and psychological conditions. A change in the sensitivity of the sense organs under the influence of the action of a stimulus is called sensory adaptation. There are three types of this phenomenon.
1. Adaptation as the complete disappearance of sensation in the process of prolonged action of the stimulus. It is a common fact that the sense of smell disappears distinctly shortly after we enter a room with an unpleasant odor. However, complete visual adaptation up to the disappearance of sensations under the action of a constant and motionless stimulus does not occur. This is due to the compensation of the immobility of the stimulus due to the movement of the eyes themselves. Constant voluntary and involuntary movements of the receptor apparatus ensure the continuity and variability of sensations. Experiments in which conditions were artificially created to stabilize the image relative to the retina (the image was placed on a special suction cup and moved along with the eye) showed that the visual sensation disappeared after 2–3 s.
2. Negative adaptation - dulling of sensations under the influence of a strong stimulus. For example, when we enter a brightly lit space from a semi-dark room, at first we are blinded and unable to distinguish any details around. After some time, the sensitivity of the visual analyzer decreases sharply and we begin to see. Another variant of negative adaptation is observed when the hand is immersed in cold water: in the first moments a strong cold stimulus acts, and then the intensity of sensations decreases.
3. Positive adaptation - increased sensitivity under the influence of a weak stimulus. In the visual analyzer, this is dark adaptation, when the sensitivity of the eyes increases under the influence of being in the dark. A similar form of auditory adaptation is silence adaptation.
Adaptation has a huge biological significance: it allows you to capture weak stimuli and protect the sense organs from excessive irritation in case of exposure to strong ones.
The intensity of sensations depends not only on the strength of the stimulus and the level of adaptation of the receptor, but also on the stimuli currently affecting other sense organs. A change in the sensitivity of the analyzer under the influence of other sense organs is called the interaction of sensations. It can be expressed both in an increase and in a decrease in sensitivity. The general pattern is that weak stimuli affecting one analyzer increase the sensitivity of another and, conversely, strong stimuli reduce the sensitivity of other analyzers when they interact. For example, accompanying the reading of a book with quiet, calm music, we increase the sensitivity and receptivity of the visual analyzer; too loud music, on the contrary, contributes to their lowering.
An increase in sensitivity as a result of the interaction of analyzers and exercises is called sensitization. The possibilities for training the sense organs and their improvement are very great. There are two areas that determine the increase in the sensitivity of the senses:
1) sensitization, which spontaneously leads to the need to compensate for sensory defects: blindness, deafness. For example, some deaf people develop vibrational sensitivity so strongly that they can even listen to music;
2) sensitization caused by activity, specific requirements of the profession. For example, high degree perfection is achieved by olfactory and gustatory sensations in tasters of tea, cheese, wine, tobacco, etc.
Thus, sensations develop under the influence of living conditions and the requirements of practical labor activity.
Adaptation, or adaptation, is a change in the sensitivity of the sense organs under the influence of the action of a stimulus.
Three varieties of this phenomenon can be distinguished.
1. Adaptation as the complete disappearance of sensation in the process of prolonged action of the stimulus. In the case of constant stimuli, the sensation tends to fade. For example, a light load resting on the skin soon ceases to be felt. The distinct disappearance of olfactory sensations shortly after we enter an atmosphere with an unpleasant odor is also a common fact. The intensity of the taste sensation weakens if the corresponding substance is kept in the mouth for some time and, finally, the sensation may die out altogether.
Full adaptation of the visual analyzer under the action of a constant and immobile stimulus does not occur. This is due to compensation for the immobility of the stimulus due to the movements of the receptor apparatus itself. Constant voluntary and involuntary eye movements ensure the continuity of the visual sensation. Experiments in which the conditions for image stabilization1 relative to the retina were artificially created showed that in this case, the visual sensation disappears 2-3 seconds after its occurrence, i.e. complete adaptation.
2. Adaptation is also called another phenomenon, close to the one described, which is expressed in the dulling of sensation under the influence of a strong stimulus. For example, when a hand is immersed in cold water, the intensity of sensation caused by a temperature stimulus decreases. When we move from a semi-dark room into a brightly lit space, we are at first blinded and unable to distinguish any details around. After some time, the sensitivity of the visual analyzer decreases sharply, and we begin to see normally. This decrease in the sensitivity of the eye to intense light stimulation is called light adaptation.
The described two types of adaptation can be combined with the term negative adaptation, since as a result of them the sensitivity of the analyzers decreases.
3. Adaptation is called an increase in sensitivity under the influence of a weak stimulus. This kind of adaptation, which is characteristic of certain types of sensations, can be defined as positive adaptation.
In the visual analyzer, this is dark adaptation, when the sensitivity of the eye increases under the influence of being in the dark. A similar form of auditory adaptation is silence adaptation.
Adaptive regulation of the level of sensitivity, depending on which stimuli (weak or strong) affect the receptors, is of great biological importance. Adaptation helps to catch weak stimuli through the sense organs and protects the sense organs from excessive irritation in case of unusually strong influences.
The phenomenon of adaptation can be explained by those peripheral changes that occur in the functioning of the receptor during prolonged exposure to a stimulus. So, it is known that under the influence of light, visual purple, located in the rods of the retina, decomposes. In the dark, on the contrary, visual purple is restored, which leads to an increase in sensitivity. The phenomenon of adaptation is also explained by the processes taking place in the central sections of the analyzers. With prolonged stimulation, the cerebral cortex responds with internal protective inhibition, which reduces sensitivity. The development of inhibition causes increased excitation of other foci, which contributes to an increase in sensitivity in new conditions.
The intensity of sensations depends not only on the strength of the stimulus and the level of adaptation of the receptor, but also on the stimuli currently affecting other sense organs. A change in the sensitivity of the analyzer under the influence of irritation of other sense organs is called the interaction of sensations.
The literature describes numerous facts of sensitivity changes caused by the interaction of sensations. Thus, the sensitivity of the visual analyzer changes under the influence of auditory stimulation.
Weak sound stimuli increase the color sensitivity of the visual analyzer. At the same time, a sharp deterioration in the distinctive sensitivity of the eye is observed when, for example, the loud noise of an aircraft engine is used as an auditory stimulus.
Visual sensitivity also increases under the influence of certain olfactory stimuli. However, with a pronounced negative emotional coloring smell, there is a decrease in visual sensitivity. Similarly, with weak light stimuli, auditory sensations are enhanced, and exposure to intense light stimuli worsens auditory sensitivity. There are known facts of increasing visual, auditory, tactile and olfactory sensitivity under the influence of weak pain stimuli.
A change in the sensitivity of any analyzer is also observed with subthreshold stimulation of other analyzers. So, P.P. Lazarev (1878-1942) obtained evidence of a decrease in visual sensitivity under the influence of skin irradiation with ultraviolet rays.
Thus, all our analyzer systems are capable of influencing each other to a greater or lesser extent. At the same time, the interaction of sensations, like adaptation, manifests itself in two opposite processes: an increase and a decrease in sensitivity. The general pattern here is that weak stimuli increase, and strong ones decrease, the sensitivity of the analyzers during their interaction.
The interaction of sensations is manifested in another kind of phenomena called synesthesia. Synesthesia is the occurrence under the influence of irritation of one analyzer of a sensation characteristic of another analyzer. Synesthesia is seen in a wide variety of sensations. The most common visual-auditory synesthesia, when, under the influence of sound stimuli, the subject has visual images. At various people there is no overlap in these synesthesias, however, they are fairly constant for each individual.
The phenomenon of synesthesia is based on the creation in last years color-musical devices that turn sound images into color ones. Less common are cases of auditory sensations when exposed to visual stimuli, taste sensations in response to auditory stimuli, etc. Not all people have synesthesia, although it is quite widespread. The phenomenon of synesthesia is another evidence of the constant interconnection of the analyzer systems of the human body, the integrity of the sensory reflection of the objective world.
An increase in sensitivity as a result of the interaction of analyzers and exercise is called sensitization.
The physiological mechanism of the interaction of sensations is the processes of irradiation and concentration of excitation in the cerebral cortex, where the central sections of the analyzers are represented. According to I.P. Pavlov, a weak stimulus causes an excitation process in the cerebral cortex, which easily irradiates (spreads). As a result of the irradiation of the excitation process, the sensitivity of another analyzer increases. Under the action of a strong stimulus, a process of excitation occurs, which, on the contrary, has a tendency to concentration. According to the law of mutual induction, this leads to inhibition in the central sections of other analyzers and a decrease in the sensitivity of the latter.
The lower threshold of sensations - the smallest amount of stimulus that causes a barely perceptible sensation. The upper threshold of sensations - the maximum value of the stimulus that the analyzer is able to perceive adequately. Sensitivity range - the interval between the lower and upper threshold of sensations.
Differential threshold - the smallest difference between stimuli, when the difference between them is still captured (Weber's law).
Operational threshold - the amount of difference between signals at which the accuracy and speed of discrimination reach a maximum. The value of the operational threshold is 10-15 times greater than the value of the differential threshold.
Time threshold - the minimum duration of exposure to a stimulus required for a sensation to occur.
The latent period of the reaction - the time interval from the moment the signal is given to the moment the sensation occurs.
inertia - the time of disappearance of sensation after the end of exposure.
In order to effectively influence a person, it is necessary to take into account the characteristics of his analyzers, which are determined empirically (for example, a change in the rate of speech) or have already been determined and fixed in special literature. It is known, for example, that the inertia of vision in a normal person is 0.1-0.2 sec, so the duration of the signal and the interval between the appearing signals should not be less than the time of preservation of sensations, equal to 0.2-0.5 sec. Otherwise, the speed and accuracy of the response will slow down, since during the arrival of a new signal, a person will still have an image of the previous one.
In the process of communication - the feeling of a person by a person - there is also inertia, dictating its own "law": as long as you see that the perception of your "old" image is still fresh in your memory, do not strive to quickly and obsessively manifest yourself in a new quality: this is explained by the fact that an adequate reaction will not follow, and the more impressionable the person on whom the impact is made, the more inert it will react to changes.
Feelings and their adequacy, or, in other words, psychological possibilities information receiving person are most important in the activities of those people whose work requires a high degree of accuracy: engineers, doctors, etc.
The sensitivity of the analyzers is not constant and changes under the influence of physiological and psychological conditions. The sense organs have the property fixtures, or adaptation. Adaptation can manifest itself both as a complete disappearance of sensation during prolonged exposure to a stimulus, and as a decrease or increase in sensitivity under the influence of an irritant.
The intensity of sensations depends not only on the strength of the stimulus and the level of adaptation of the receptors, but also on the stimuli currently affecting other sense organs. A change in the sensitivity of analyzers under the influence of irritation of other sense organs is called the interaction of sensations. The interaction of sensations is manifested in an increase and decrease in sensitivity: weak stimuli increase the sensitivity of the analyzers, and strong ones decrease it.
The interaction of sensations is manifested in the phenomena of sensitization and synesthesia. Sensitization(lat. sensibilis - sensitive) - increased sensitivity of nerve centers under the influence of an irritant. Sensitization can develop not only through the use of adverse stimuli, but also through exercise. Thus, musicians develop high auditory sensitivity, tasters develop olfactory and gustatory sensations. Synesthesia - this is the emergence under the influence of irritation of some analyzer of a sensation characteristic of another analyzer. So, when exposed to sound stimuli, a person may experience visual images.
The world around us, its beauty, sounds, colors, smells, temperature, size and much more we learn through the senses. With the help of the sense organs, the human body receives in the form of sensations a variety of information about the state of the external and internal environment.
SENSATION is a simple mental process, which consists in reflecting the individual properties of objects and phenomena of the surrounding world, as well as internal states organism under the direct action of stimuli on the corresponding receptors.
The sense organs are irritated. It is necessary to distinguish between stimuli that are adequate for a particular sense organ and inadequate for it. Sensation is the primary process from which the knowledge of the surrounding world begins.
SENSATION is a cognitive mental process of reflection in the human psyche of individual properties and qualities of objects and phenomena with their direct impact on his senses.
The role of sensations in life and cognition of reality is very important, since they constitute the only source of our knowledge about the external world and about ourselves.
The physiological basis of sensations. Feeling occurs as a reaction nervous system to some stimulus. The physiological basis of sensation is a nervous process that occurs when a stimulus acts on an analyzer adequate to it.
The sensation has a reflex character; physiologically it provides the analyzer systems. The analyzer is a nervous apparatus that performs the function of analyzing and synthesizing stimuli that come from the external and internal environment of the body.
ANALYZERS- these are the organs of the human body that analyze the surrounding reality and single out certain or other types of psycho-energy in it.
The concept of analyzer was introduced by I.P. Pavlov. The analyzer consists of three parts:
The peripheral section is a receptor that converts a certain type of energy into a nervous process;
Afferent (centripetal) pathways that transmit the excitation that has arisen in the receptor in the higher centers of the nervous system, and efferent (centrifugal), along which impulses from the higher centers are transmitted to lower levels;
Subcortical and cortical projective zones, where the processing of nerve impulses from the peripheral regions takes place.
The analyzer constitutes the initial and most important part of the entire path of nervous processes, or the reflex arc.
Reflex arc = analyzer + effector,
An effector is a motor organ (a certain muscle) that receives a nerve impulse from the central nervous system (brain). The relationship of the elements of the reflex arc provides the basis for the orientation of a complex organism in environment, the activity of the organism depending on the conditions of its existence.
For a sensation to arise, the work of the entire analyzer as a whole is necessary. The action of the stimulus on the receptor causes the appearance of irritation.
Classification and varieties of sensations. There are various classifications of the sense organs and the sensitivity of the body to stimuli entering the analyzers from outside world or from within the body.
Depending on the degree of contact of the sense organs with stimuli, contact (tangential, gustatory, pain) and distant (visual, auditory, olfactory) sensitivity are distinguished. Contact receptors transmit irritation through direct contact with objects that affect them; such are the tactile, taste buds. Distant receptors respond to irritation * that comes from a distant object; distantreceptors are visual, auditory, olfactory.
Since sensations arise as a result of the action of a certain stimulus on the corresponding receptor, the classification of sensations takes into account the properties of both the stimuli that cause them and the receptors that are affected by these stimuli.
Behind the placement of receptors in the body - on the surface, inside the body, in muscles and tendons - sensations are emitted:
Exteroceptive, reflecting the properties of objects and phenomena of the outside world (visual, auditory, olfactory, gustatory)
Interoceptive, containing information about the state of internal organs (hunger, thirst, fatigue)
Proprioceptive, reflecting the movements of the organs of the body and the state of the body (kinesthetic and static).
According to the system of analyzers, there are such types of sensations: visual, auditory, tactile, pain, temperature, taste, olfactory, hunger and thirst, sexual, kinesthetic and static.
Each of these varieties of sensation has its own organ (analyzer), its own patterns of occurrence and function.
A subclass of proprioception, which is sensitivity to movement, is also called kinesthesia, and the corresponding receptors are kinesthetic, or kinesthetic.
Independent sensations include temperature, which is a function of a special temperature analyzer that performs thermoregulation and heat exchange of the body with the environment.
For example, the organ of visual sensations is the eye. The ear is the organ of perception of auditory sensations. Tactile, temperature and pain sensitivity is a function of organs located in the skin.
Tactile sensations provide knowledge about the measure of equality and relief of the surface of objects, which can be felt during their palpation.
Pain signals a violation of the integrity of the tissue, which, of course, causes a protective reaction in a person.
Temperature sensation - a sensation of cold, heat, it is caused by contact with objects that have a temperature higher or lower than body temperature.
An intermediate position between tactile and auditory sensations is occupied by vibrational sensations, signaling the vibration of an object. The organ of vibrational sense has not yet been found.
Olfactory sensations signal the state of the food's suitability for consumption, clean or polluted air.
The organ of taste sensations is special cones sensitive to chemical irritants located on the tongue and palate.
Static or gravitational sensations reflect the position of our body in space - lying, standing, sitting, balancing, falling.
Kinesthetic sensations reflect the movements and states of individual parts of the body - arms, legs, head, body.
Organic sensations signal such states of the body as hunger, thirst, well-being, fatigue, pain.
Sexual sensations signal the body's need for sexual release, providing pleasure due to irritation of the so-called erogenous zones and sex in general.
In terms of data modern science The accepted division of sensations into external (exteroceptors) and internal (interoceptors) is insufficient. Some kinds of sensations can be considered externally internal. These include temperature, pain, taste, vibration, musculo-articular, sexual and static di and amich n and.
General properties of sensations. Sensation is a form of reflection of adequate stimuli. However, different types of sensations have not only specificity, but also common properties for them. These properties include quality, intensity, duration, and spatial localization.
Quality is the main feature of a certain sensation that distinguishes it from other types of sensations and varies within a given type. So, auditory sensations differ in pitch, timbre, loudness; visual - by saturation, color tone and the like.
The intensity of the sensations is quantitative characteristic and are determined by the strength of the stimulus and the functional state of the receptor.
The duration of a sensation is its temporal characteristic. it is also determined by the functional state of the sense organ, but mainly by the duration of the stimulus and its intensity. During the action of the stimulus on the sense organ, the sensation does not occur immediately, but after a while, which is called the latent (hidden) period of sensation.
General laws of sensations. General patterns of sensations are sensitivity thresholds, adaptation, interaction, sensitization, contrast, synesthesia.
Sensitivity. The sensitivity of the sense organ is determined by the minimum stimulus that, under specific conditions, becomes capable of causing a sensation. The minimum strength of the stimulus that causes a barely noticeable sensation is called the lower absolute threshold of sensitivity.
Irritants of lesser strength, the so-called subthreshold ones, do not cause sensations, and signals about them are not transmitted to the cerebral cortex.
The lower threshold of sensations determines the level of absolute sensitivity of this analyzer.
The absolute sensitivity of the analyzer is limited not only by the lower, but by the upper threshold of sensation.
The upper absolute threshold of sensitivity is called the maximum strength of the stimulus, at which there is still an adequate sensation for a certain stimulus. A further increase in the strength of stimuli acting on our receptors causes only a painful sensation in them (for example, a super-loud sound, dazzling brightness).
The difference of sensitivity, or sensitivity to discrimination, is also in inverse relationship to the value of the discrimination threshold: the larger the discrimination threshold, the smaller the difference in sensitivity.
Adaptation. The sensitivity of analyzers, determined by the magnitude of the absolute thresholds, is not constant and changes under the influence of a number of physiological and psychological conditions, among which the phenomenon of adaptation occupies a special place.
Adaptation, or adaptation, is a change in the sensitivity of the sense organs under the influence of the action of a stimulus.
There are three types of this phenomenon:
Adaptation as a continuous disappearance of sensation in the process of prolonged action of the stimulus.
Adaptation as a dulling of sensation under the influence of a strong stimulus. The described two types of adaptation can be combined with the term negative adaptation, since it results in a decrease in the sensitivity of the analyzers.
Adaptation as an increase in sensitivity under the influence of a weak stimulus. This type of adaptation, inherent in some types of sensations, can be defined as positive adaptation.
The phenomenon of increasing the sensitivity of the analyzer to the stimulus under the influence of mindfulness, orientation, installation is called sensitization. This phenomenon of the sense organs is possible not only as a result of the use of indirect stimuli, but also through exercise.
The interaction of sensations is a change in the sensitivity of one analyzer system under the influence of another. The intensity of sensations depends not only on the strength of the stimulus and the level of adaptation of the receptor, but also on the stimuli that affect other sense organs at that moment. Change in the sensitivity of the analyzer under the influence of irritation of other senses. the name of the interaction of sensations.
In this case, the interaction of sensations, as well as adaptations, will turn out to be in two opposite processes: an increase and a decrease in sensitivity. The main regularity here is that weak stimuli increase, and strong ones decrease, the sensitivity of the analyzers by their interaction.
A change in the sensitivity of the analyzers can cause the action of all-round signal stimuli.
If you carefully, carefully peer, listen, savor, then the sensitivity to the properties of objects and phenomena becomes clearer, brighter - objects and their properties are much better distinguished.
The contrast of sensations is a change in the intensity and quality of sensations under the influence of a previous or accompanying stimulus.
With the simultaneous action of two stimuli, a simultaneous contrast occurs. Such a contrast can be well traced in visual sensations. One and you yourself figure on a black background will seem lighter, on a white - darker. A green object on a red background is perceived as more saturated. Therefore, military objects are often masked so that there is no contrast. This should include the phenomenon of consistent contrast. After a cold, a weak warm stimulus will seem hot. The sensation of sour increases the sensitivity to sweet.
Synesthesia of feelings is the occurrence of a floor by an outpouring of an irritant of one analyzer of nidchutgiv. which are specific to another analyzer. In particular, during the action of sound stimuli, such as aircraft, rockets, etc., a person has visual images of them. Or whoever sees a wounded person also feels pain in a certain way.
The activities of the analyzers will be in interaction. This interaction is not isolated. It has been proven that light increases hearing sensitivity, and weak sounds increase visual sensitivity, cold washing of the head increases sensitivity to red, and the like.
Bob Nelson
Most often, spectrum analyzers are used to measure very small signals. These may be known signals that need to be measured, or unknown signals that need to be detected. In any case, to improve this process, you should be aware of methods for increasing the sensitivity of the spectrum analyzer. In this article, we will discuss the optimal settings for measuring low level signals. In addition, we will discuss the use of noise correction and analyzer noise reduction functions to maximize instrument sensitivity.
Average self-noise and noise figure
The sensitivity of the spectrum analyzer can be found in its specifications. This parameter can be either average level own noise ( DANL), or noise figure ( NF). The average noise floor is the amplitude of the spectrum analyzer's noise floor over a given frequency range with a 50 ohm input load and 0 dB input attenuation. This parameter is usually expressed in dBm/Hz. In most cases, averaging is performed on a logarithmic scale. This reduces the displayed average noise level by 2.51 dB. As we will learn from the discussion below, it is this noise reduction that distinguishes the average noise floor from the noise figure. For example, if the analyzer specification specifies an average noise floor of 151 dBm/Hz with an IF filter bandwidth ( RBW) 1 Hz, then using the analyzer settings you can reduce the device's own noise level to at least this value. Incidentally, a CW signal that has the same amplitude as the spectrum analyzer noise will be measured 2.1 dB above the noise floor due to the summation of the two signals. Similarly, the observed amplitude of noise-like signals will be 3 dB higher than the noise floor.
The analyzer's inherent noise has two components. The first of them is determined by the noise figure ( NF ac), and the second is thermal noise. The thermal noise amplitude is described by the equation:
NF=kTB,
where k= 1.38×10–23 J/K - Boltzmann's constant; T- temperature (K); B is the bandwidth (Hz) in which the noise is measured.
This formula determines the thermal noise energy at the input of a spectrum analyzer with a 50 Ω load. In most cases, the bandwidth is reduced to 1 Hz, and at room temperature the calculated value of thermal noise is 10log( kTB)= -174 dBm/Hz.
As a result, the value of the average level of intrinsic noise in the 1 Hz band is described by the equation:
DANL = –174+NF ac= 2.51 dB. (one)
Besides,
NF ac = DANL+174+2,51. (2)
Note. If for the parameter DANL rms power averaging is used, the term 2.51 can be omitted.
Thus, the value of the average level of self-noise –151 dBm/Hz is equivalent to the value NF ac= 25.5 dB.
Settings affecting the sensitivity of the spectrum analyzer
The gain of the spectrum analyzer is equal to one. This means that the screen is calibrated against the analyzer's input port. Thus, if a signal with a level of 0 dBm is applied to the input, the measured signal will be equal to 0 dBm plus/minus the instrument's error. This must be taken into account when using an input attenuator or amplifier in the spectrum analyzer. Turning on the input attenuator causes the analyzer to increase the equivalent gain of the IF stage to maintain the calibrated level on the screen. This, in turn, raises the noise floor by the same amount, thus maintaining the same signal-to-noise ratio. This is also true for an external attenuator. In addition, it is necessary to recalculate to the passband of the IF filter ( RBW) greater than 1 Hz by adding the term 10log( RBW/one). These two terms allow you to determine the noise floor of the spectrum analyzer when different meanings attenuation and resolution bands.
Noise level = DANL+ attenuation + 10log( RBW). (3)
Adding a preamp
The built-in or external preamplifier can be used to reduce the spectrum analyzer's inherent noise. Typically, datasheets will list a second value for the average noise floor, including the built-in preamp, and all of the above equations can be used. When using an external preamplifier, a new average noise floor can be calculated by cascading the noise figure equations and calculating the gain of the spectrum analyzer equal to one. If we consider a system consisting of a spectrum analyzer and an amplifier, we get the equation:
NF system = NF predus+(NF ac–1)/G predus. (4)
Using value NF ac= 25.5dB from the previous example, 20dB preamp gain and 5dB noise figure, we can determine the overall system noise figure. But first you need to convert the values to a ratio of powers and take the logarithm of the result:
NF system= 10log(3.16+355/100) = 8.27 dB. (five)
Now you can use Equation (1) to find a new value for the average noise floor with an external preamplifier by simply replacing NF ac on the NF system, calculated in equation (5). In our example, the preamp significantly reduces DANL-151 to -168 dBm/Hz. However, this is not given for free. Preamplifiers tend to have a lot of non-linearity and a low compression point, which limits the ability to measure high level signals. In such cases, the built-in preamp is more useful as it can be turned on and off as needed. This is especially true for automated control and measuring systems.
So far, we have discussed how the IF filter bandwidth, attenuator, and preamplifier affect the sensitivity of a spectrum analyzer. Most modern spectrum analyzers have methods for measuring their own noise and correcting the measurement results based on the acquired data. These methods have been used for many years.
Noise Correction
When measuring the characteristics of a certain device under test (DUT) with a spectrum analyzer, the observed spectrum is the sum of ktb, NF ac and input signal TU. If the DUT is turned off and a 50 Ohm load is connected to the analyzer input, the spectrum will be the sum ktb And NF ac. This trace is the analyzer's own noise. IN general case noise correction is to measure the spectrum analyzer's own noise with a large average and store this value as a "correction trace". You then connect the device under test to the spectrum analyzer, measure the spectrum, and record the results in the "measured trace". The correction is done by subtracting the "correction trace" from the "measured trace" and displaying the results as a "result trace". This trace is a "DOT signal" with no additional noise:
Resulting trace = measured trace - correction trace = [DOT signal + ktb + NF ac]–[ktb + NF ac] = TR signal. (6)
Note. All values were converted from dBm to mW before subtraction. The resulting trace is in dBm.
This procedure improves the display of low-level signals and allows for more accurate amplitude measurements by eliminating the error associated with the spectrum analyzer's inherent noise.
On fig. 1 shows a relatively simple method for correcting noise by applying trace math. First, the spectrum analyzer noise floor is averaged with input loading, the result is stored in trace 1. Then the DUT is connected, the input signal is captured, and the result is stored in trace 2. Now you can use math - subtracting two traces and putting the results in trace 3. How You see, noise correction is especially effective when the input signal is close to the spectrum analyzer's noise floor. High-level signals contain much less noise, and the correction does not have a noticeable effect.
The main disadvantage of this approach is that each time you change the settings, you have to turn off the device under test and connect a 50 ohm load. The method to obtain a "correction trace" without turning off the DUT is to increase the attenuation of the input signal (eg by 70 dB) so that the noise of the spectrum analyzer significantly exceeds the input signal, and store the results in the "correction trace". In this case, the "correction trace" is given by the equation:
Correction trace = TR signal + ktb + NF ac+ attenuator. (7)
ktb + NF ac+ attenuator >> TU signal,
we can omit the "signal TR" term and state that:
Correction trace = ktb + NF ac+ attenuator. (8)
By subtracting the known value of the attenuator from formula (8), we can get the original "correction trace" that was used in the manual method:
Correction trace = ktb + NF ac. (9)
In this case, the problem is that the "correction trace" is only valid for the current instrument settings. Changing settings such as center frequency, span, or IF filter bandwidth makes the values stored in the "correction trace" incorrect. The best approach is to know the values NF ac at all points of the frequency spectrum and the application of the "correction trace" at any setting.
Noise Reduction
The Agilent N9030A PXA signal analyzer (Figure 2) has a unique noise reduction (NFE) feature. The noise figure of the PXA signal analyzer over the entire frequency range of the instrument is measured during manufacture and calibration. This data is then stored in the instrument's memory. When the user turns on the NFE, the meter calculates a "correction trace" for the current settings and stores the noise figure values. This eliminates the need to measure the intrinsic noise of the PXA, as was done in the manual procedure, which greatly simplifies noise correction and saves time spent on measuring instrument noise when changing settings.
In any of the described methods, thermal noise is subtracted from the "measured trace" ktb And NF ac, which allows you to get results below the value ktb. These results may be reliable in many cases, but not in all. Confidence may decrease when the measured values are very close to or equal to the instrument's inherent noise. In fact, the result will be an infinite value in dB. A practical implementation of noise correction typically involves introducing a threshold or graduated subtraction level near the instrument's own noise floor.
Conclusion
We have considered some methods for measuring signals low level using a spectrum analyzer. At the same time, we found that the sensitivity of the measuring device is affected by the bandwidth of the IF filter, the attenuation of the attenuator and the presence of a preamplifier. For additional increase instrument sensitivity, methods such as mathematical noise correction and noise reduction can be applied. In practice, a significant increase in sensitivity can be achieved by eliminating losses in external circuits.
