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questions in Solid State Radio engineering

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Sorry i type mistake.
Please download the attachment and check what i need

as you see the attachment why we can't measure voltage of 100Kohm with the voltmeter with bandwidth 1Mhz?could you explain ?
Click to expand...
 

no one here can help me or i asked this question in wrong section(Electronic Elementary) instead of Analog?
 

Hi baby_1

I think you mean this?



Accuracy will be bad because the meter will measure the input signal and the noise from the resistor, not just the input signal.
 

thanks godfreyl
could you tell me how can we effect of Bandwidth of voltmeter of this problem? and how we can find the min accuracy(in this example 1mV) that a voltmeter have to with a thermal noise?
 

Did you see that resistor noise depends on measurement bandwidth? Higher bandwidth has more noise.

If we measure the noise of the 100K resistor, we find:
Noise = 40uV between 0 and 1 MHz (bandwidth = 1MHz)
Noise = 40uV between 1 and 2 MHz (bandwidth = 1MHz)

Noise = 4uV between 0 and 10 KHz (bandwidth = 10KHz)
Noise = 4uV between 10 and 20 KHz (bandwidth = 10KHz)

Noise = 5.7uV between 0 and 20 KHz (bandwidth = 20KHz)
 
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thanks a lot
could you tell me how we can find the noise of a resistor? and how can reduce their noise? what about Dc and Ac signal ?

- - - Updated - - -

thanks a lot
could you tell me how we can find the noise of a resistor? and how can reduce their noise? what about Dc and Ac signal ?
 

why we can't measure a 100Kohm with the voltmeter with bandwidth 1Mhz?
Click to expand...

Sorry i type mistake.
Please download the attachment and check what i need
Click to expand...

In other words, we are asked to find out the question in the quoted literature and tell you what the problem is?

P.S.:
could you tell me how we can find the noise of a resistor? and how can reduce their noise?
Click to expand...
That's in fact answered in the quoted literature:
- it gives a formula for resistor noise
- it clarifies that resistor noise can't be reduced (it doesn't discuss cryogenic cooling)

Nevertheless, you can reduce the effect of resistor noise by omitting resistors, if applicable.
 
Hello Fvm
Thanks for you reply
i want to know how can we find noise of a resistor practically?because the resistor not ideal
 

Resistors can have a certain amount of excessive noise with applied DC voltage, but "ideal" kT noise will be most likely dominant. In so far the quoted equation is giving the "practical" method.
 
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