[sdiy] white noise from resistor?

G L Hearn georgehearn at btinternet.com
Wed Nov 5 16:09:11 CET 2008


I agree with Tom that noise sources are best done digitally.  In my latest
project I've done a noise source with shift registers and feedback (standard
TTL gates) that works very well, you could also use a PIC like Tom suggests
for a small efficient solution.  If you are using a pseudo random bit
sequence to generate the output on a pin don't forget to put a low pass
filter on the output else it won't be 'white' noise.  George

-----Original Message-----
From: synth-diy-bounces at dropmix.xs4all.nl
[mailto:synth-diy-bounces at dropmix.xs4all.nl] On Behalf Of Tom Wiltshire
Sent: 05 November 2008 14:06
To: Paul Perry
Cc: Synth-diy at dropmix.xs4all.nl
Subject: Re: [sdiy] white noise from resistor?


On 5 Nov 2008, at 13:33, Paul Perry wrote:

>
> I'm looking for a simple white noise source.
> The usual reverse breakdown transistor junction has a problem -  
> first, you
> need a fairly high voltage, and more importantly, level and quality  
> varies
> between transistors.
> In these days of very low leakage & high input impedance op amps,  
> could I
> get by with just amplifying the thermal noise of a high value  
> resistor?
> I know some of the EEs here can set me straight on this one..

Must it be an analogue noise source?

It seems to me that the significant problems that you mention with  
the usual reversed transistor junction technique make this a case  
where a digital solution is a clear favourite. The number of people  
that build various hardware shift-register sources suggests this too,  
although personally I'd favour software. I'm a recovering  
analogophile, so I don't say this lightly, but digital clearly has a  
place, and I think this is one of those occasions where a digital  
solution does a much better (cheaper/easier/more reliable) job than  
analogue.

I'll send you an 8-pin PIC that'll produce good quality white noise  
up to >40KHz for what it costs me if you can't find someone to  
program one up at your end.

Regards,
Tom

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