Low frequency noise source - Page 1
Forum section: electronics / projects

author: GK
author_rank: Super Contributor
timestamp: March 12, 2013, 07:58:31 am
content: Hello all.I need to build myself a primary white noise source with a DC - ~100kHz noise bandwidth. The down-to-DC requirement means that simple methods are ruled out due to 1/f ("flicker") noise. I know the very basics of how old school analogue DC> noise generators do it, I'm just having a real hard time finding a decent reference explaining it in detail. My current plan is to make a noise generator that is, for all sakes and purposes, purely white from 200kHz to 300kHz (or 300kHz to 400kHz?) and then down-convert to DC-100kHz with a 300kHz carrier. The IF filter will be a high order low pass (8 poles or more) with a-3dB frequency of ~100kHz.Unless I'm missing something, this seems an eminently doable/workable solution, but for one thing I'm not entirely sure how to adequately deal with image products, or how to adequately deal with them by filtering / bandwidth-limiting the noise source prior to down converting. Or do I even have to do such a thing? Any ideas? If the noise source prior to down-conversion is white in a 200kHz bandwidth instead (100kHz either side of the carrier frequency) wouldn't the images just harmlessly sum to an desired IF that is still white? I'd also really appreciate any hints to a decent reference/text on very low frequency IF noise generation techniques.The noise generator will be used for statistical and stochastic analog computer simulations up to 100s in durationCheers,Glen

author: amiq
author_rank: Regular Contributor
timestamp: March 12, 2013, 09:21:50 am
content: In this frequency range look at filtering a psuedo random binary sequence to produce the noise. IIRC the Art of Electronics (1st edition?) gives an example of one possible implementation.

author: branadic
author_rank: Super Contributor
timestamp: March 12, 2013, 11:57:48 am
content: What about that:http://www.electricdruid.net/index.php?page=projects.noisegenI have build it and it is perfectly working for me.

author: GK
author_rank: Super Contributor
timestamp: March 12, 2013, 12:19:35 pm
content: Sorry, pseudo randomness isn't desired. The best source of a purely random signal will always be an analogue one!Just to be clear on a few other things; there are purpose-made noise diodes out there (eg NC102) but they are at best limited to operation >0.1Hz due to flicker noise. A fair bit better than a run of the mill zener diode, but still not low enough for me.A commercial unit that does 1/f-free white noise down to DC is the Wandel and Goltermann RG-1 (specified DC-108 kHz, actually) and this unit uses the down-conversion technique (with a 300kHz carrier oscillator to the best of my knowledge). However, other than that, I have not been able to find any further data on this intriguing technique.And before someone mentions it, no, strictly speaking there is no such thing as DC noise, but it is given as the lower frequency limitation of instruments such as the RG-1 which, to the best of my knowledge, have a DC-coupled IF/output filter (and, presumably, no 1/f noise component as low as you could care to measure).EDIT:Oh, and before someone mentions that noise is specified in a bandwidth, and that trying to make a noise source down to "DC" is pointless as at such low bandwidths there wont be any voltage to measure - NO! The voltage difference for pure white noise in a 0.01Hz bandwidth is only 70dB below that in a 100kHz bandwidth (a frequency ratio of 10 million :1). The voltage amplitude goes by the square root of the frequency. For example if the noise in a 100kHz bandwidth is 1V rms, it will be 316uV rms in a 0.01Hz bandwidth (still a rather large signal, amplitude wise).

author: GK
author_rank: Super Contributor
timestamp: April 21, 2013, 01:37:55 am
content: Found my answers inDesign Fundamentals of Analog Computer Components, R. M. Howe - chapter 7.2 "Noise Generators" (see attached). Through Abebooks I'm slowly collecting a bookshelf of classic analog computing texts.Perhaps someone may find this information either interesting or useful. The diagrams on page 222 and the associated description are relevant to the thread topic. Those confused by the circuit block referred to with the antiquated term "chopper", just substitute all instances of the word with "mixer driven by a 400Hz carrier".

author: Unknown
author_rank: Unknown
timestamp: April 21, 2013, 01:14:55 pm
content: Not sure this will help as such, unless you have an interest to purchase the Wandel and Goltermann RG-1 on E-Bay, however it's located in Israel for $180 ! so need to ask shipping cost; otherwise look at the images, solid build boat anchor; see URL...http://www.ebay.com/itm/WANDEL-GOLTERMANN-NOISE-GENERATOR-RG-1-/120984327109?pt=LH_DefaultDomain_0&hash=item1c2b3a53c5Same company, has this Wave-Teh for $150, looks interesting range wise:http://www.ebay.com/itm/WAVETEK-MODEL-132-VCG-NOISE-GENERATOR-/120997681855?pt=LH_DefaultDomain_0&hash=item1c2c061abfWent through my collection of circuits, sourced from the Web, and this looks like it might do DC; unfortunately their was no technical information with the circuit, see below... also, after some more searching on this site I found a pdf with useful information on noise, URL:http://www.qsl.net/va3iul/Noise/Understanding%20Noise%20Figure.pdfGood luck.

author: branadic
author_rank: Super Contributor
timestamp: April 21, 2013, 05:13:26 pm
content: A few days ago I've made some noise measurements on an IR photo diode followed by a transimpedance amplifier. The question was how big is noise at the output of the amp with and without IR laser diode stimulation. I've used a modified version of Jim Williams low noise amp (AN83, but with 80dB, 40dB each stage, to directly feed the adc of a STM32F103). I found the noise to be white in the measurement range of 10-100kHz.This is close to the article in here:http://www.elektronikpraxis.vogel.de/design-tipps/analogtechnik/articles/372250/Some guys found this noise source to be a bit buggy, as stated here:http://www.multiforum.se/afu/viewtopic.php?t=1440&start=15&mforum=afuI think based on this material you could make your own noise source from dc-100kHz.

author: Unknown
author_rank: Unknown
timestamp: April 21, 2013, 07:57:29 pm
content: Thanks branadic... if this helps GK; I have copied the main page and converted into English via Google Translate ? now a pdf document as below.

author: GK
author_rank: Super Contributor
timestamp: April 21, 2013, 08:26:51 pm
content: Thanks for that! Good stuff!