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  • Author Author: jc2048
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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 shabaz

    My Tektronic TBS2xxx scope had a reasonable spectrum display. Not magic, but (almost) workable in that frequency range.

    I gave it away to a makerspace here in Belgium, so that avenue is closed.

     

    A SA is a rather niche instrument. If you need it, you need it.

    I'm restructuring my lab to only keep decent basic instruments (and freeing up storage space while rebasing).

    Anything that didn't fit that need, and was still in prime condition, I gave to STEM initiatives.

  • in reply to shabaz

    Too.. much... pressure...  Must... comply... image

     

    I recreated the circuit but much to my chagrin had to substitute a 3k3 resistor as I did not have a 2k7.  Here is the beauty shot.

    image

    For comparison to Jon's original post, here is 100ms followed by a screen grab at 2us.

    image

    image

     

    Now a look at the spectrum analyzer view.  The center frequency is 10kHz with a span of 20kHz.  RBW is 20Hz.  Circuit supply voltage is 12V.

    image

    Below the circuit supply voltage is reduced to 6V to get an idea of the floor and there is about a 20dBV drop.

    image

    Now the span is set 0 to 10MHz, showing the droop with frequency.

    image

    Look right?  I'll leave this set up for a while in case someone wants me to run a different look.

  • in reply to shabaz

    Interesting!

    But yes, RedPitaya doesn't show RBW.

  • in reply to fmilburn

    Hi Frank,

     

    Wow, very quick work!

    The results are very useful not only for seeing the circuit behavior but also to just see just how well the GW Instek 'scope handled it - that's amazing, extremely hard/impossible on other contemporary 'scopes.

    About the icing on the cake would be if it can also do averaging in SA mode. If it can, it will just clean up things a bit more, but to be honest it's even great as-is.

  • in reply to Jan Cumps

    Hi Jan,

     

    Good idea, nothing worse than having equipment idle gathering dust, when it could be benefitting others.

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