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(thermal) noise generator
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Fernando



Joined: Dec 30, 2006
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PostPosted: Sat Jun 06, 2020 10:13 am    Post subject: (thermal) noise generator Reply with quote  Mark this post and the followings unread

I have some 22M resistors so I thought about this when I found a thermal noise calculator this afternoon...

10 x 22M resistors should give 1mV noise
For 2mV you need 40 x 22M, which is less practical

I didn't tested it yet, it's just something I'd liked to try for ages. I will try soon.

https://www.daycounter.com/Calculators/Thermal-Noise-Calculator.phtml
https://analog.intgckts.com/noise/thermal-noise-of-a-resistor/

http://www.sengpielaudio.com/calculator-db-volt.htm


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Last edited by Fernando on Sat Jun 06, 2020 3:34 pm; edited 3 times in total
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AlanP



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PostPosted: Sat Jun 06, 2020 10:25 am    Post subject: Reply with quote  Mark this post and the followings unread

Might be easier to just use a 10 giga-ohm resistor. Never used them myself, though, so don't know their details.
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Fernando



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PostPosted: Sat Jun 06, 2020 10:27 am    Post subject: Reply with quote  Mark this post and the followings unread

AlanP wrote:
Might be easier to just use a 10 giga-ohm resistor. Never used them myself, though, so don't know their details.


yes I'm looking for sources of larger resistance now : )

maybe just a big chunk of some poor conduction material?

edit: OK, I see it's pretty easy to get 1 to 22 Gohm resistors... even 1 Tohm
With 44 Gohms (2 x 22Gohm) you get around 13mV of noise, not bad : )
With 1 Tohm you'd get 60mV, yumm! haha

I like avalanche noise but using resistors would be fun

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Fernando



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PostPosted: Sun Jun 07, 2020 6:11 pm    Post subject: Reply with quote  Mark this post and the followings unread

Would this be a reasonable start?


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Last edited by Fernando on Mon Jun 08, 2020 4:42 am; edited 1 time in total
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elektrouwe



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PostPosted: Mon Jun 08, 2020 4:38 am    Post subject: Reply with quote  Mark this post and the followings unread

Fernando wrote:
Would this be a reasonable start?

no [ educational mode on Wink ]. What would be the bandwidth given a 44Gohm source R and a 10pF input buffer capacity ? What happens if you load a 1mV voltage source with 1Tohm output impedance with an opamp with 1GOhm input impedance ?
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Fernando



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PostPosted: Mon Jun 08, 2020 4:48 am    Post subject: Reply with quote  Mark this post and the followings unread

elektrouwe wrote:
Fernando wrote:
Would this be a reasonable start?

no [ educational mode on ;-) ]. What would be the bandwidth given a 44Gohm source R and a 10pF input buffer capacity ? What happens if you load a 1mV voltage source with 1Tohm output impedance with an opamp with 1GOhm input impedance ?


Hey! thank you : )

I assume 10pF represents the input capacitance of the op amp...
A drastic R/C LP filter with -3dB at 0.36Hz? (44Gohm/10pF to ground)

Oh and yes the output impedance of such a resistor is giant too : (
even for a very high input imp op amp

disasta'
I trusted the unity gain buffer will take care of everything but ehmmm... no

How to pick up the noise with minimum degradation?
please enlight me : )

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Sven



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PostPosted: Mon Jun 08, 2020 9:20 am    Post subject: Reply with quote  Mark this post and the followings unread

Fernando wrote:
Would this be a reasonable start?


No. Noise is a voltage. To amplify this voltage both ends of the resistor need to be connected to an amplifier.
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Fernando



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PostPosted: Mon Jun 08, 2020 12:55 pm    Post subject: Reply with quote  Mark this post and the followings unread

Sven wrote:
Fernando wrote:
Would this be a reasonable start?


No. Noise is a voltage. To amplify this voltage both ends of the resistor need to be connected to an amplifier.


Thank you.
One end to ground?
To both inputs of an op amp?
Or using two inputs of an inverting mixer? (two parallel inputs of same op amp)
Or using two op amps and mix them later?

any example or guide welcome
hope my ignorance is not too irritating 😅

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Fernando



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PostPosted: Fri Jun 12, 2020 5:33 am    Post subject: Reply with quote  Mark this post and the followings unread

It looks like having the large resistor to ground would be OK, ideally and generally at least.
The main problem, I guess, is still the low pass filter formed by the large resistor and the very small capacitance of the op amp (say 44Gohm and 5pF) That would filter down to 1.5Hz or so...

I found a couple of links, but still don't know how to interface the large resistor without the adverse effect of such a low pass filter... : (

https://www.st-andrews.ac.uk/~www_pa/Scots_Guide/iandm/part3/page2.html

https://electronics.stackexchange.com/questions/197072/johnson-noise-with-amplifier-circuit

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JovianPyx



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PostPosted: Fri Jun 12, 2020 5:54 am    Post subject: Reply with quote  Mark this post and the followings unread

While "thermal noise" is an interesting scientific topic, it has a varying effect depending on the material used. Carbon resistors will be one of the better noise generators hence why so many amplifier designers have stopped using them.

What you want is to pass current through the resistor. If the resistor is grounded, then there's no appreciable current flowing through the resistor. But this method of making a noise generator is not optimal. In fact, the noise in a diode or transistor is also caused by thermal physics, but the noise amplitude in some semiconductors is far larger than a resistor. The problem with transistor noise sources is that transistors have different noise characteristics per unit so one has to build a test jig and listen to different transistors until a "good" one is found. Problems can include weak noise signal and popcorn noise. Another problem of making analog noise sources is that even semiconductor noise is very low amplitude and thus needs a high gain amplifier to be useful. High gain amplifiers themselves are plagued by noise, but different kinds, the most common being mains hum pickup.

So if you just want decent noise, in the end, the best consistently high quality noise is generated digitally.

If this is just a tinkering experiment - well, have fun and let us know what the end result is Smile

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Fernando



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PostPosted: Fri Jun 12, 2020 6:08 am    Post subject: Reply with quote  Mark this post and the followings unread

Thank you.

Yes zener or transistor noise generators are great. And I use a socket or breadboard to find the right ones, etc.

It's just that I'd like to try with a huge resistor, for fun and find it interesting.
I didn't expect to be so problematic, but since it is I will learn in the way to it.
The "problem" is that I need some help : ) can't do it by myself alone

> What you want is to pass current through the resistor. If the resistor is grounded, then there's no appreciable current flowing through the resistor.

.

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Last edited by Fernando on Fri Jun 12, 2020 7:57 am; edited 1 time in total
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JovianPyx



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PostPosted: Fri Jun 12, 2020 7:04 am    Post subject: Reply with quote  Mark this post and the followings unread

Got it. Wink

Here is what I would try:

Code:

             +-------R4--+
  +V         |           |
  |          |  |\       |
 R1          |  | \      |
  |          |  |  \     |
  +---||--R3-+--+ - \    |
  |   C1        |    >---+-----> out
  |             |   /
 R2         +---+ +/
  |         |   | /
  |        GND  |/
 GND


Pardon the ASCII-matic...

R1 would be 10K
R2 would be a resistor in megohms range
R3 should be at least as large as R2
R4 should be G (gain) times R3 depending on how much gain you want/need.
C1 something like .01 uF
It is best if R2 is the ONLY carbon resistor and all the rest are metal film.
The opamp should be one with high input impedance, such as TL07x.
A small capacitor may be needed across R4 to prevent oscillation.

No idea how much gain would be appropriate. It's possible it won't do anything at all.

However, the idea is to pass current through R2 which would develop a noise signal across it's terminals. Given enough gain (G), you might hear the noise signal.

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Grumble



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PostPosted: Sun Jun 14, 2020 9:54 am    Post subject: Reply with quote  Mark this post and the followings unread

Make R1 also very high, like R2 and R3 because basically you have R1 in parallel with R2 and this wil lower your noise output.

Quote:
A small capacitor may be needed across R4 to prevent oscillation.

Be careful, since this too will lower your noise output.

Better to have a power resistor very close to R2 to heat it up, you are looking for thermal noise, so increase the temperature and keep it at a fixed level,

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