Độ nhạy, nhiễu âm, số nhiễu âm trong máy thâu thanh.

dientu75

14 Oct 202310:27

Summary

TLDRThe video discusses the impact of noise on the sensitivity of shortwave receivers, particularly under 30 MHz. Using the Icom IC-3 7851 as an example, the script explains how natural noise affects receiver performance, calculating noise power and noise figure. It highlights that while the receiver has excellent sensitivity, high atmospheric noise can overshadow it, making it difficult to pick up weak signals. The script also explores the difference between theoretical and practical noise levels, suggesting that the receiver's sensitivity may be less effective in high noise environments but becomes crucial at higher frequencies where natural noise is reduced.

Takeaways

📡 High-frequency interference in natural environments significantly affects receiver sensitivity, especially below 30 MHz.
🔊 Even with good sensitivity, receivers struggle to eliminate natural noise factors, impacting their performance.
🔍 Example: Using the Icom IC-7851 to illustrate how environmental noise affects signal reception.
📐 Calculations show the noise power at standard atmospheric temperature (290K) is significant.
⚙️ The source voltage needs to be calculated considering the impedance of the source and the load.
📊 Signal-to-noise ratio (SNR) at input: 20 times the logarithm of the input voltage divided by the noise voltage.
🔢 Example calculation of SNR for the Icom IC-7851 shows the noise figure may be around 5 dB, which is more realistic.
⚖️ The environmental noise level often exceeds the receiver's sensitivity, making it hard to receive weak signals.
🌐 Natural noise has a substantial impact on high-frequency bands, rendering good sensitivity less effective.
📈 On ultra-high frequencies (above 30 MHz), environmental noise reduces significantly, making receiver sensitivity more critical.

Q & A

What is the main topic of the video script?
-The main topic of the video script is the sensitivity and noise figure in shortwave radio receivers, specifically discussing the impact of natural noise on receiver sensitivity.
What is the significance of the 30 MHz frequency range mentioned in the script?
-The 30 MHz frequency range is significant because it represents the high-frequency band where the noise level in the natural environment is very high, affecting the performance of radio receivers.
What does the script mention about the noise figure of the Icom IC-37851 receiver?
-The script discusses the noise figure of the Icom IC-37851 receiver, suggesting that its sensitivity might be too high in the presence of high natural noise, making it difficult to filter out the natural noise and receive weak signals effectively.
What is the noise figure and how is it calculated in the context of the script?
-The noise figure is a measure of the noise performance of a radio receiver. In the script, it is calculated by considering the signal-to-noise ratio at the input and output of the receiver and using logarithmic calculations to determine the noise level in decibels (dB).
What is the role of the antenna in the context of noise and receiver sensitivity?
-The antenna plays a crucial role in picking up signals and noise from the environment. The script mentions that the noise power at the antenna is a significant factor in determining the receiver's noise figure and overall sensitivity.
What does the script imply about the performance of the Icom IC-37851 in high noise environments?
-The script implies that the Icom IC-37851 may not perform optimally in high noise environments, such as those with high levels of natural noise, due to its high sensitivity which could lead to the receiver being overwhelmed by noise.
What is the significance of the 5 dB noise figure mentioned in the script?
-The 5 dB noise figure is suggested as a more reasonable and realistic figure for the receiver's noise performance, considering the high levels of natural noise in the 30 MHz frequency range.
How does the script discuss the impact of natural noise on the receiver's ability to pick up weak signals?
-The script discusses that with a high level of natural noise, such as 414 microvolts, the receiver's sensitivity becomes less effective as it can only pick up noise, making it difficult to detect weak signals.
What is the conclusion about the Icom IC-37851's performance in the script?
-The conclusion is that while the Icom IC-37851 has good design specifications, its high sensitivity may not be useful in high noise environments, such as those with high levels of natural noise at the 30 MHz frequency range.
What alternative frequency range is suggested for better performance of the Icom IC-37851 in the script?
-The script suggests that the Icom IC-37851 may perform better at higher frequency ranges, such as the VHF band (above 30 MHz), where the level of natural noise is significantly reduced.

Outlines

00:00

🔍 Impact of Noise on Sensitivity in Shortwave Radios

The first paragraph discusses the impact of natural noise on the sensitivity of shortwave radios, specifically mentioning the high-frequency range below 30 MHz. It explains that even a highly sensitive receiver may not perform as expected due to the significant noise levels in the natural environment. The example of the Icom IC-3 7851 radio is used to illustrate the concept of noise power and its calculation at standard temperature conditions. The paragraph also introduces the noise figure concept and how it relates to signal-to-noise ratio, emphasizing the challenge of distinguishing weak signals from background noise.

05:00

📡 Reevaluating Noise Figure and Sensitivity in Radio Communication

The second paragraph delves deeper into the noise figure, correcting a previous assumption and explaining that the actual noise figure might be higher than initially thought. It discusses the calculation of noise figure in relation to the noise from the antenna and the receiver's internal noise. The paragraph also explores the implications of a high noise figure on the receiver's ability to pick up weak signals, suggesting that a noise figure of 5 dB might be more realistic and reasonable than an exceptionally low figure. It highlights the importance of considering natural noise sources and their impact on radio reception.

10:01

🌐 Adjusting Sensitivity for Different Frequency Bands

The final paragraph shifts the focus to the importance of adjusting the receiver's sensitivity based on the frequency band in use. It contrasts the challenges of high noise levels in the high-frequency bands with the benefits of lower noise levels in the very high-frequency bands above 30 MHz. The speaker thanks the audience for their attention and invites any questions, indicating a willingness to engage further on the topic of radio communication and sensitivity in the presence of natural noise.

Mindmap

Keywords

💡Sensitivity

Sensitivity in the context of the video refers to the ability of a receiver, such as the ICOM IC-37851 mentioned, to detect weak signals. It is a key concept because it determines how well the receiver can operate amidst natural noise. The script discusses the impact of noise on sensitivity, noting that even a highly sensitive receiver may not perform as desired if the environmental noise is too high.

💡Noise

Noise is any unwanted signal that interferes with the reception of a desired signal. The video script discusses natural noise and its significant impact on the performance of a receiver, especially at high frequencies below 30 MHz. It is a crucial concept as it affects the receiver's ability to detect signals and can limit the receiver's sensitivity.

💡ICOM IC-37851

The ICOM IC-37851 is a specific model of a receiver used as an example in the script. It is relevant because the video uses this model to illustrate the concepts of sensitivity and noise. The script mentions its performance characteristics and how it is affected by natural noise.

💡Frequency

Frequency is the rate of oscillation or cycle per unit time, measured in MHz (megahertz) in the video. It is important because different frequencies are more susceptible to noise, and the script specifically discusses the impact of noise on high frequencies below 30 MHz.

💡Signal-to-Noise Ratio (SNR)

The Signal-to-Noise Ratio is the level of a signal compared to the level of background noise. It is a critical metric in the video, as it determines the clarity of the received signal. The script calculates SNR and discusses how it is affected by the receiver's sensitivity and the noise level.

💡Noise Figure

Noise Figure is a measure of how much noise a receiver adds to the received signal. It is a key concept in the video, as it helps to understand the receiver's performance in the presence of noise. The script calculates the noise figure of the ICOM IC-37851 and discusses its implications on the receiver's sensitivity.

💡Microvolts (μV)

Microvolts are a unit of electrical potential, where one microvolt is one-millionth of a volt. In the video, microvolts are used to express the equivalent noise voltage that the receiver can detect, which is a measure of its sensitivity.

💡Decibels (dB)

Decibels are a logarithmic unit used to express the ratio of two values of a physical quantity, often power or intensity. In the script, decibels are used to quantify the noise level and the difference in noise figures, which helps in understanding the relative noise impact on the receiver's performance.

💡Natural Noise

Natural noise refers to the background noise that occurs naturally in the environment, such as atmospheric noise from the sky. The video script discusses how natural noise can significantly affect the receiver's sensitivity and performance, especially at certain frequencies.

💡Antenna Noise Temperature

Antenna Noise Temperature is a measure of the noise generated by an antenna. In the video, it is used to calculate the noise power and to understand the impact of the antenna on the overall noise figure of the receiver.

💡Bandwidth

Bandwidth in the context of the video refers to the range of frequencies that a receiver can process. It is relevant because the script mentions a bandwidth factor in the calculation of noise power, which affects the overall noise figure and the receiver's sensitivity.

Highlights

Discusses the impact of environmental noise on sensitivity in high-frequency ranges below 30 MHz.

Explains how even with good sensitivity, natural environmental noise can hinder device performance.

Uses the Icom IC-7851 as an example to illustrate the effect of noise power from the sky at a standard temperature of 290K (17°C).

Calculates the noise power using the Boltzmann constant multiplied by temperature and bandwidth, resulting in 8.4 x 10^-18 W.

Determines the voltage at the source, considering the source impedance and load impedance, which are both 50 ohms.

Uses the formula for power (voltage squared divided by impedance) to find the source voltage, resulting in 4.1 x 10^-8 V or 4.1 µV.

Discusses the signal-to-noise ratio (SNR) at the input, calculated as 20 times the logarithm of the input voltage divided by noise voltage.

Mentions the Icom IC-7851's sensitivity below 30 MHz, capable of receiving signals down to 0.16 µV.

Calculates the SNR as 20 times the log of 4 divided by 0.4 µV, resulting in 12 dB.

Introduces the concept of noise figure, defined as the difference between the SNR at the input and output.

States that the Icom IC-7851's noise figure is 2 dB, which is unusually low and possibly underestimated.

Suggests a more realistic noise figure of 5 dB, considering a potential measurement error or higher actual sensitivity.

Explains the ratio of noise power to signal power, with the noise figure calculated as 10 times the log of the noise ratio.

Highlights the natural environmental noise being significantly higher than the device noise, impacting signal reception.

Concludes that despite the Icom IC-7851's good design specifications, high-frequency natural noise limits its sensitivity.

Notes that the device's sensitivity becomes more crucial in ultra-high-frequency ranges above 30 MHz where natural noise is reduced.