Hi everyone, I wish to measure noise voltage and noise current density, specially on transistors  and DOA's, how can this be performed?

Also, I would like to know the noise figure for different source impedances, from the definition of noise figure, it  is  calculated as SNR at the input divided by SNR at the output, so my idea is to have a low source impedance connected to a resistor substitution box, and change the source impedance whilst measuring SNR at the input and at the output and then just plot the results. I have both an AP Sys 1 and an HP8903A.


JohnRoberts

Hi everyone, I wish to measure noise voltage and noise current density, specially on transistors  and DOA's, how can this be performed?

Also, I would like to know the noise figure for different source impedances, from the definition of noise figure, it  is  calculated as SNR at the input divided by SNR at the output, so my idea is to have a low source impedance connected to a resistor substitution box, and change the source impedance whilst measuring SNR at the input and at the output and then just plot the results. I have both an AP Sys 1 and an HP8903A.
The definition sounds a little creative, or poorly phrased.

Active devices have NF at different impedances that are wrt perfect noiseless active devices (i.e. a NF of 1dB is 1 dB noisier than perfect).

Not easy to measure, but low noise transistors often provide published curves for constant NF at different impedances.

 JR

John Roberts
http://circularscience.com
Tune it, or don't play it...

The definition sounds a little creative, or poorly phrased.


Thats the definition Í've seen in all text-books, even wiki https://en.wikipedia.org/wiki/Noise_figure

I understand what Noise figure is, what I want to know is how to measure it if im not using a low noise transistor.

JohnRoberts

I still do not know how to measure/generate a NF curve.

https://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=17&ved=2ahUKEwi-lIqOrtPfAhUPRa0KHXUeBLUQFjAQegQICRAC&url=https%3A%2F%2Ftoshiba.semicon-storage.com%2Finfo%2Fdocget.jsp%3Fdid%3D63511&usg=AOvVaw2WilIq5KUio3koFDBllFC8

Page 21 of this application note show some typical contours of constant noise figure.  This is useful in the course of circuit design to establish the current density for an active device in a gain stage.  If you know the source  resistance, look across for the operating current that delivers a good (low)  NF.

JR
John Roberts
http://circularscience.com
Tune it, or don't play it...

I still do not know how to measure/generate a NF curve.

https://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=17&ved=2ahUKEwi-lIqOrtPfAhUPRa0KHXUeBLUQFjAQegQICRAC&url=https%3A%2F%2Ftoshiba.semicon-storage.com%2Finfo%2Fdocget.jsp%3Fdid%3D63511&usg=AOvVaw2WilIq5KUio3koFDBllFC8

Page 21 of this application note show some typical contours of constant noise figure.  This is useful in the course of circuit design to establish the current density for an active device in a gain stage.  If you know the source  resistance, look across for the operating current that delivers a good (low)  NF.

JR

Thats a great App note John, many thanks!, I will make some experiments and I'll report back.
« Last Edit: January 04, 2019, 12:45:22 AM by Dualflip »

ruffrecords

The Wiki definition is wrong. NF does not require a signal  in order to be defined or measured so references to S/N in the definition are not necessary. However, that is not important when it comes to measuring it. The usual way to do this is to attached a high gain amplifier to the circuit you wish to test in order to obtain noise levels you can measure. This works only if the stage being measured has a reasonable gain such that its NF dominates the overall NF. You simply terminate the input with your desired resistance and measure the output noise. You also need to accurately know the gain of the stage being tested plus the gain of the high gain amp.

The EIN (dBu) = output noise (dBu)  - high gain dB - stage gain dB

If Vnoise (dBu) is the calculated Johnson noise in the input terminating resistor then:

NF = EIN - Vnoise

Cheers

Ian
« Last Edit: January 05, 2019, 03:35:03 AM by ruffrecords »
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'The only people not making mistakes are the people doing nothing'

abbey road d enfer

I've always found NF to be of little practical interest in audio, except maybe when selecting transistors from datasheets.
EIN voltage is relatively easy to assess. Build the circuit, measure its gain, tehn short the input and measure output noise, you get the EIN voltage by dividing.
EIN current can only be determined by calculation, loading the input with a resistor of known value R and extracting the current noise value, using the quadratic combination rule. (EIN)²=(en)²+(R.in)²+4kTRf
The measurement sensitivity is when the R.in term is high, but It must be verified that the BW is not actually changed, due to capacitive input impedance.
You need to make sure the noise measurement BW is well defined.
Strictly, the BW should be a brickwall. Since it's hardly possible in practice, you need to know the equivalent noise BW of your measurement test set. For example, a single-pole (-6dB/octave) response ar 15.7kHz is equivalent to a 20kHz brickwall.
The 22kHz filters in the AP and HP are sharp enough to introduce little error, but the measurements need a better level of accuracy.
In order to avoid fluctuations in the reading (due in particular to 1/f noise, but also to the LF content of Johnson noise) it is safe to use the 400Hz HPF and adjust the math in consequence.
Who's right or wrong is irrelevant. What matters is what's right or wrong.
"The important thing is not to convince, but to give pause for thought." (B. Werber)
Star ground is for electricians.

I've always found NF to be of little practical interest in audio, except maybe when selecting transistors from datasheets.
EIN voltage is relatively easy to assess. Build the circuit, measure its gain, tehn short the input and measure output noise, you get the EIN voltage by dividing.
EIN current can only be determined by calculation, loading the input with a resistor of known value R and extracting the current noise value, using the quadratic combination rule. (EIN)²=(en)²+(R.in)²+4kTRf
The measurement sensitivity is when the R.in term is high, but It must be verified that the BW is not actually changed, due to capacitive input impedance.
You need to make sure the noise measurement BW is well defined.
Strictly, the BW should be a brickwall. Since it's hardly possible in practice, you need to know the equivalent noise BW of your measurement test set. For example, a single-pole (-6dB/octave) response ar 15.7kHz is equivalent to a 20kHz brickwall.
The 22kHz filters in the AP and HP are sharp enough to introduce little error, but the measurements need a better level of accuracy.
In order to avoid fluctuations in the reading (due in particular to 1/f noise, but also to the LF content of Johnson noise) it is safe to use the 400Hz HPF and adjust the math in consequence.

Thanks abbey, thats what I needed to know.


 

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