What Is Noise Sensitivity?

Sensitivity refers to the degree of change in the response of a method to a change in the unit concentration or unit amount of the substance to be measured. It can be measured by the ratio of the response of the instrument or other indicators to the concentration or amount of the substance to be measured. description.

The displacement rate of the sensitivity indicator relative to the measured change. Sensitivity is a sign of measuring physical instruments, especially electrical instruments focus on improving the sensitivity of the instrument. The study of the sensitivity can deepen the understanding of the structure and principle of the instrument.

Chinese name: Sensitivity
Foreign name: Sensitivity

Solid: Description of unit change
Use: Physical instrument

Definition of sensitivity

English name: Sensitivity

The degree of response of a radio receiver to input radio waves is also called sensitivity; especially the quotient of the output power or other functions of this unit divided by the input power or other functions.

Sensitivity Truth Table

Sensitivity application

Sensitivity balance

On the pointer of the balance, there is a small, inconspicuous slider, which is used to adjust the sensitivity of the balance. The sensitivity moves upward, the sensitivity of the balance increases;

Test sensitivity experiment The sensitivity moves down, reducing the sensitivity of the balance.

It turns out that the beam of the balance together with the pointer is an object with a fixed rotation axis, and the rotation axis is the central knife edge O. When the masses in the two plates of the balance are not equal, the beam will be tilted by a certain angle . Let the masses in the two plates be m1, m2 (m1> m2), the mass of the beam is M, and the center of gravity O on the pointer is O from h Where the balance arm is L.

According to the moment balance condition M = 0

m2gLcos + Mghsin = m1gLcos

The above formula shows that under the condition that the quality of the two disks is constant (m1-m2), the larger the ratio L / M, the smaller h, the larger . That is, the higher the sensitivity of the balance.

Generally speaking, the L / M value of the balance cannot be adjusted. The height h of the center of gravity of the two beams can be changed by the position of the sensor : When the sensor moves upward, the center of gravity increases, h decreases, and the balance's Sensitivity increases; otherwise, sensitivity decreases.

The sensitivity of the balance should not be as high as possible. As h decreases and the center of gravity increases, the stability of the balance becomes worse. At this time, the smaller the gravity recovery moment Mghsin, the worse the stability becomes. Therefore, the balance should be designed with sensitivity and stability at the same time.

Sensitivity meter

The electric meter commonly used in the laboratory is magnetoelectric. Its structure is a rotatable coil installed in the magnetic field of a permanent magnet. When the current flows through the coil through the hairspring, the coil overcomes the resistance of the hairspring due to the interaction of the current and the magnetic field. The moment is deflected by an angle. When the magnetic induction strength, coil area, number of coil turns and hairspring strength are constant, the magnitude of the current is proportional to the angle of the coil deflection. The smaller the bias current, the higher the sensitivity. The full bias current of the meter head is generally 10 microamperes to several hundred milliamps.

If you want to measure weak current (10-6 10-10 amps) or small voltage (10-3 10-6 volts), you should increase the sensitivity of the meter, and use a high-sensitivity meter, that is, a sensitive ammeter.

The structure of the sensitive galvanometer includes three main parts, from which we can see the principle of improving sensitivity.

Magnetic field: Radial magnetic field generated by a permanent magnet.

Deflection part: The coil can be rotated in a magnetic field, and its upper and lower ends are tightened with a metal wire (tension wire), and the tension wire is also used as a current lead at both ends of the coil. Since the shaft and bearing of the ordinary electric meter are replaced with a tension wire, mechanical friction is avoided, and the sensitivity of the galvanometer can be greatly improved.

Reading part: The small mirror M is fixed on the coil. It reflects the light from the light source onto the ruler and forms a cursor. When the current passes through the coil, the small mirror M rotates with the coil through the angle, and the reflected light turns through the 2 angle. . The distance that the cursor moves on the scale is d = 2L, and l is the distance from the small mirror M to the scale. Since the deflection angle of the coil is proportional to the current Ig, the distance d at which the cursor is moved can measure the magnitude of the current Ig. Using the cursor as a "pointer" instead of the metal pointer of an ordinary electric meter is equivalent to lengthening the length of the pointer and further improving the sensitivity of the ammeter.

Sensitivity multimeter

The lower left corner of the multimeter head on the textbook is labeled "5000 / V", which indicates the sensitivity of the meter. Ig × Rv = U, Rv / U = 1 / Ig, so according to 5000 / V, we can know the full bias current of the meter head Ig = U / Rv = 1/5000 = 200 (A), the larger the value of Rv / U The smaller the full bias current of the meter head, the more sensitive the meter is. Therefore, the specifications of multimeters generally call this value sensitivity.

What is the sensitivity of a voltmeter converted to an ammeter with Ig = 1000 microamps and Rg = 1000 ohms? Yes 1 / Ig = 1000 / V

Sensitivity oscilloscope

Send the square wave signal generator with a frequency of 1KHz and a 0.5V square wave to the "X input" of the oscilloscope. If the horizontal trace length is displayed on the screen, it is not less than 7.8 divisions in the J2458 oscilloscope In the 325-2 type oscilloscope is not less than 6.3 divisions, and in the J2459 type oscilloscope is not less than 5 divisions, the X-axis sensitivity of the oscilloscope is qualified. Otherwise, find the cause and replace the failed component.

Sensitivity amplifier

For amplifiers, sensitivity generally refers to the voltage level of the signal applied to the input when the rated output power or voltage is reached, so it is also referred to as input sensitivity. For speakers, sensitivity refers to applying 1W of input power to the speakers. How many decibels of sound pressure can be produced at a distance of 1 meter ahead.

Sensitivity TV

The sensitivity of the TV refers to the minimum signal voltage value that needs to be input from its antenna when the fluorescent screen of the machine displays a good image, that is, the TV's ability to receive weak electromagnetic wave signals. The level of sensitivity is usually expressed by "microvolts" or "millivolts". The smaller the value, the stronger its ability to receive weak signals, that is, the higher the TV's receiving sensitivity. High-sensitivity TVs are good for long-distance viewing.

The sensitivity of the TV mainly depends on the performance design of the circuit of the TV's image channel part (high frequency head and IF amplifier part). If the gain of the channel part is high and the noise is small, the sensitivity of the TV is high. Domestic transistor black and white TVs are generally around 100 V, integrated circuit black and white TVs have a sensitivity of about 150 V, and crystal color TVs have a sensitivity of about 200 V.

To determine the sensitivity of a TV, the easiest way is to use a contrast method, that is, to use several TVs of the same model and specifications to receive the same TV station signal or test signal at the same location, and then shorten the length of the antenna ) Or remove the antenna, set the contrast and brightness knobs to a moderate position, and the color saturation knob to a minimum position, and judge its sensitivity by observing the noise point when there is no signal on the fluorescent screen. Generally, there are many black and white noise points that are jumping, which means that the sensitivity is high; the noise points are thin and small, and the sensitivity is low. For a TV with many noise points, if a TV signal arrives, this noise point will disappear immediately and a clear image will be presented. If there is still noise in the background of the image after the signal, it indicates that the signal-to-noise ratio of the aircraft is poor, the relative sensitivity is not good, and it is not ideal.

Sensitivity headphones

Sensitivity In general, the sensitivity of the headset reflects the amount of input power required for the same loudness. The higher the sensitivity of the headphones, the smaller the input power required, and the louder the sound output under the same power source. For portable devices such as Walkman, sensitivity is a very important indicator. Generally speaking, the sensitivity of Walkman headphones is higher than that of monitor-grade headphones, which is about 110db, so for a Walkman, this value is naturally larger and better.

Sensitivity microphone

Sensitivity is the ratio of the output voltage to the input sound pressure of the microphone under unit sound pressure excitation, and its unit is mV / Pa. To be consistent with the measurement of the level in the circuit, the sensitivity can also be expressed in decibels. The early decibels are expressed in units of dBm and dBV: 0dBm = 1mW / Pa, that is, the 1mW power output brought by a 1Pa input sound pressure to a 600 load is defined as 0dB; 0dBV = 1V / bar, generated at 1bar input sound pressure The 1V voltage output is defined as 0dB. The current decibel is expressed in the unit dB: 0dB = 0.775V / Pa, that is, the 0.775V voltage output of the microphone at 1Pa input sound pressure is defined as 0dB (thus the level measurement of the microphone sound pressure-voltage conversion is unified into the circuit The commonly used reference unit is 0dB = 0.775V).

Obviously, no matter how the sensitivity is expressed, we can convert it to dB, provided the line input is unified to the unit Pa.

For example: The sensitivity of NEUMANN U89 microphone is 8mV / Pa, which can be directly obtained from 20lg [(0.008V / Pa) ÷ (0.775V / Pa)] and its sensitivity is about -40dB.

Another example: the sensitivity of the AKG C414 microphone is -60dBV. From 0dBV = 1V / bar = 10V / Pa, first find the output voltage of -60dBV at 1Pa sound pressure X: 20lg [(XV / Pa) ÷ (10V / Pa)] = -60 gives X = 0.01 (V), that is, its sensitivity is 10mV / Pa. According to the formula 20lg [(0.01V / Pa) ÷ (0.775V / Pa)], the sensitivity is about -37dB.

Sensitive Ray Inspection

Sensitivity is the most important index for evaluating the quality of radiographs, and it marks the ability to find defects in radiographic inspection. There are two types of sensitivity: absolute sensitivity and relative sensitivity. Absolute sensitivity refers to the smallest defect size that can be found on the ray film in the direction of ray penetration. The relative sensitivity is expressed as the percentage of the smallest defect size that can be found in the thickness of the transillumination weldment. Since it is not possible to know in advance the minimum defect size along the direction of the ray penetration, an artificial "defect" of known size, an image quality meter, must be used for this purpose.

Sensitive clay

The ratio of the unconfined compressive strength of the clay in the undisturbed state to the unconfined compressive strength of the test immediately after reshaping. It represents a characteristic measure of the sensitivity of clay to perturbative remodeling.

Sensitivity camera chip

Sensitivity is one of the important parameters of the chip, and it has two physical meanings. One refers to the photoelectric conversion capability of an optical device, which has the same meaning as the response rate. That is, the sensitivity of the chip refers to the output signal voltage (current) per unit exposure within a certain spectral range. The unit can be nanoamperes / lux (nA / LUX), volts / watt (V / W), and volts / lux (V / LUX), Volt / lumen (V / lm). The other is the lowest radiant power (or illuminance) that the device can sense, which has the same meaning as the detection rate. Units can be expressed in Watts (W) or Lux.

Sensitivity , precision, and accuracy

Instrument sensitivity :

Sensitivity refers to the ability of the instrument to measure the smallest value. The smaller the minimum amount measured, the higher the sensitivity of the instrument. Such as the sensitivity of the balance, the smaller the number of milligrams, even the smaller the maximum mass required to deflect the balance pointer from the equilibrium position to the scale. Another example is the number "20k / V" marked on the dial of a multimeter, which indicates sensitivity. Its physical meaning is that when a voltage of 1V is applied across the meter, the total internal resistance Rv (the sum of the internal resistance of the meter head and the additional voltage) required to make the pointer fully biased is 20k. The larger the number, the higher the sensitivity. This is because U = IgRv, that is, Rv / U = 1 / Ig. Obviously, when Rv / U is larger, it means that the full bias current Ig is smaller, that is, the smaller the minimum current that the meter can measure, the higher the sensitivity ^{[1 ]} .

The sensitivity of the instrument is not as high as possible, because the sensitivity is too high, the stability of the measurement is worse, or even difficult to measure, that is, the accuracy is poor. Therefore, under the premise of ensuring measurement accuracy, the sensitivity is not easy to be too high. Sensitivity is generally not relevant for balances and electrical instruments, but for rulers, calipers, and spiral micrometers ^[1] .

Instrument precision :

Instrument precision, also known as precision, generally refers to the smallest graduation value of the instrument. For example, the minimum graduation of a meter ruler is 1mm, and its precision is 1mm. The minimum graduation of a mercury thermometer is 0.2 ° C, and its accuracy is 0.2 ° C. The smaller the minimum division value of the instrument, the higher the accuracy and the higher the sensitivity. For example, a thermometer with a minimum graduation of 0.1 ° C has higher sensitivity and precision than a thermometer with a minimum graduation of 0.2 ° C ^[1] .

Under normal use, the accuracy of the instrument is high, and the accuracy is also high. This shows that the accuracy of the instrument is a prerequisite for a certain degree of accuracy. What kind of accuracy is also required to adapt to what kind of accuracy. This is why people often use precision to describe accuracy together ^[1] .

However, the accuracy of the instrument does not fully reflect its accuracy. For example, a voltmeter of a certain specification has deteriorated internal additional voltage, which reduces its actual accuracy, but the accuracy remains the same. It can be seen that there is a difference between accuracy and accuracy. General instruments have accuracy problems ^[1] .

Instrument accuracy :

The accuracy of the instrument generally refers to the percentage value of the maximum relative error that occurs when its pointer is fully biased under specified conditions. The accuracy of an electric meter is 2.5, which means that the relative error does not exceed 2.5% of the full deviation, that is, its absolute error = range × accuracy. For a DC ammeter with a range of 0.6A, its maximum absolute error = 0.6A × 2.5% = 0.015A ^[1] .

Obviously, when the same measured value is measured with different ranges of the same meter, its maximum absolute error is different. Therefore, when using an electric meter, there is a problem of selecting an appropriate range. Accuracy is generally said for electrical instruments, and does not matter for other instruments ^[1] .