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The Art of Noise

jc2048

22nd May 2021

 

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I've just been playing with a very simple noise generator and that's the result.

The digital phosphor of the oscilloscope presents multiple traces of the waveform as though it were sketched with a pencil or crayon.

 

In case it's of any interest, here's the circuit:

 

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The internet is full of variations, with either a Zener, reverse-biased LED, or reverse-biased base-emitter junction as the avalanche 'noise diode'.

 

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For these traces I used a reverse-biased base-emitter junction (the transistor on the left with two pins connected - the other transistor is working as a common-emitter amplifier)

and it produced a very good level of noise out on a 12V supply.

 

At lower frequencies (the scope is AC-coupled for these), the noise looks quite noise-like. Like this:

 

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but if I go back to the start and show you a single trace of the first waveform, we see this

 

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so we're now looking at a time scale where we can see the avalanching stopping and starting.

 

Update 4th June 2021

 

What started as a throwaway blog about an interesting/amusing waveform seems to have taken on a life of its own.

 

Thanks for all the useful comments below. They've had the useful effect of forcing me to rethink some of this.

 

1. The original circuit that I copied had an integrator (low-pass filter) in the feedback of the common-emitter stage

[the 27k and the 10uF] to set the DC bias voltage. That means that the resulting overall transfer characteristic will

be high-pass [the inverse]. I should have considered that a bit more at the time. If I simulate it with this following

circuit (I've substituted a simple signal generator, with an 8V DC offset on it, for the 'noise diode' which is the input to the circuit)

 

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I get this for the response

 

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That won't be entirely right - the simulation is small-signal and the generator is working large-signal - but it lets us

see in a rough way what's happening and understand it in general terms. It falls off from a few hundred Hertz downwards.

That means I'm going to see very little of the low frequency noise as the circuit will integrate it away. Indeed, if I had wanted

to use the generator circuit for audio, it would probably have been better if I had moved the low end cut-off down by

another decade [100uF might do instead of the 10uF].

 

At the top end, that plot shows quite nicely that the response is determined largely by the amplifier transistor running

out of steam and that will depend to some extent on the individual transistor.

 

2. In my reply to Shabaz in the comments, I suggested that seeing the noise decline as I filtered it with the 'scopes

noise filtering meant that it wasn't white noise. That is quite wrong and I need to correct it. Since random noise level

is a function of bandwidth, if I restrict the bandwith, I lessen the noise seen. That doesn't mean that it's necessarily

'white' noise [equal power per unit of bandwidth], but it most certainly doesn't mean the opposite [that it's not].

 

In Motchenbacher and Fitchen, they suggest that avalanche noise is white but that there is also excess noise

in the form of multistate noise that is predominently l/f. This is in a section comparing the noise performance

of zener and avalanche diodes, though I presume a reverse-biased base-emitter junction is similar.

Parents

  • Very nice art of noise!

     

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    I tried it with BC547 and 4.7K,47K and boom, there is the noise generated. It starts coming after exact 12V and I like the way it works. At the output, starts at 0V, slowly rises, and when input is 12V, noise starts generating and the voltage level settles down. When turned off, the capacitor takes it full negative and slowly comes up to 0 at full discharge. Btw, this is the first circuit I am trying with the new RedPitaya scope!

     

  • in reply to navadeepganeshu

    Your RedPitaya looks an interesting piece of kit. Can you use it as a spectrum analyser to look at how the noise is distributed in the frequency domain?

  • in reply to jc2048

    Yeah! For the first time, I am using a spectrum analyzer,....haha. I tried with the same noise at 15V now just to enhance it.

    The frequency of noise is all over around 0 to 35kHz and is rapidly, randomly changing. This is what I caught:

     

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    In the below waterfall chart, we see peak(red) is almost random in between 0-10kHz

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    Is that what is supposed to be? Correct me if I am wrong in interpreting this.

  • in reply to navadeepganeshu

    Hi Navadeep,

     

    I think you're seeing the correct thing, and it looks flat on the second screenshot, which is great. However, you should check it really is functioning by disconnecting your 12V supply and seeing the level of noise drop. That's so you can see your 'noise floor'.

     

    On the first screenshot, you can see on the far left side that the output is flat, for 0.05 MHz (50 kHz) or so. Therefore, when you look at a smaller frequency span on the second screenshot, that too corroborates that it is flat there.

    The actual values that you are seeing on the vertical axis are labelled 'dBm' but not much can be determined from that alone, since dBm is a power value, and depends on what resistance is being used, and although normally 50 ohm is common, I don't think your instrument has that (it might, I don't know) and might expect an external 50 ohm load. Anyway, for the purposes of establishing spectrum characteristics for the noise, that is not important.

     

    Incidentally the values in dBm are different between the first and second screenshot, due to 'resolution bandwidth'. The Red Piyaya software shows a spectrum, but it's not a spectrum analyzer, and so I think it's not exposing such stuff (at least, I can't see it on the screenshot labels/controls), so the vertical axis values are changing due to resolution bandwidth changes when you make adjustments to the frequency span it seems (unless you made some other settings changes elsewhere in-between, that are not visible on the screenshot). But like I mentioned, it's unimportant for your scenario where you're looking at the noise shape.

     

    EDIT: Screenshot from GW Instek MDO-2072EX MSO / MDO Oscilloscope - Review  which shows the RBW setting that spectrum analyzers normally have (the MDO-2072EX is primarily a 'scope, but, unusually operates like a spectrum analyzer from that review, which is extremely uncommon in most 'scopes).

    image

  • in reply to shabaz

    I wish that someone with a genuine spectrum analyser chimes in. The circuit is easy enough....

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