Very low frequency band-limited white noise - Page 1
Forum section: electronics / projects

author: ricko_uk
author_rank: Super Contributor
timestamp: June 24, 2021, 01:08:04 am
content: Hi,is there either a simple way/circuit, an IC, or a low cost off-the-shelf piece of equipment that can generate white noise at very low frequencies between 0.5Hz (preferably even 0.1Hz) and 20Hz (even up to 100Hz max but most needed ones are up to 20Hz)?Thank you

author: Circlotron
author_rank: Super Contributor
timestamp: June 24, 2021, 01:37:25 am
content: You may be able to use this audio editor. Not sure if it goes as low a frequency as you want.https://www.audacityteam.org/

author: NiHaoMike
author_rank: Super Contributor
timestamp: June 24, 2021, 01:48:17 am
content: Microcontroller generating random numbers using a LFSR and/or grabbing the LSB from the ADC and outputting them via PWM, then a LPF to remove the PWM carrier if desired. (Made something similar to simulate noisy sensors at work.)

author: David Hess
author_rank: Super Contributor
timestamp: June 25, 2021, 07:23:10 pm
content: I suspect your only option is an LFSR or digital method followed by filtering because pure analog techniques at such low frequency will suffer from flicker noise. Digital generation at such a low frequency is not particularly demanding.

author: ricko_uk
author_rank: Super Contributor
timestamp: June 26, 2021, 07:57:03 pm
content: Thank you all!

author: jonpaul
author_rank: Super Contributor
timestamp: June 27, 2021, 11:03:39 am
content: National Semi (TI) had a shift reg PRN gen for white/pink noise.MM5837N No idée if still avail.the output BW depends on the clock so any BW is possible with a slow clock.https://pdf1.alldatasheet.com/datasheet-pdf/view/9285/NSC/MM5837N.htmlhttps://www.schematicsforfree.com/archive/file/Audio/Circuits/Musician/Misc/Circuits/Mm5837N%20Noise%20Generator%20Applications.pdfEnjoy,Jon

author: ricko_uk
author_rank: Super Contributor
timestamp: June 27, 2021, 01:49:16 pm
content: Thank you Jon!

author: jonpaul
author_rank: Super Contributor
timestamp: June 27, 2021, 04:22:08 pm
content: Hello again: I have built pink and white noise gens from that IC decades ago.Very simple to use and just an RC filter on the output.I see them on epay ~ $3-5Bon Chance,Jon(EE since 1968, seen it all)

author: DrG
author_rank: Super Contributor
timestamp: June 27, 2021, 05:01:48 pm
content: Can't recommend the MM5837N as it is horrible. When it recycles it has a distinct *click*. I remember my disappointment when getting one at the shack, only to hear that click.You may be interested in checking out this fellow's pagehttps://electricdruid.net/white-noise-source/. Some years ago, he presented a program to replace the chip using a PIC12F675. You may need to get into the guts of the LFSR [lots of info online] or add some external circuitry to get exactly what you want, but I have used that program (actually did a quick translation for a PIC10F202 - see mpasm code below). All I had at the time were 202s and it works quite well for masking noise.Hope it helps.Code:;----------------------------------------------------------------------; Translation of a white noise generator from PIC12F675 to PIC10F202; Original code from: https://electricdruid.net/white-noise-source/;by Tom Wiltshire; GP2 pin 3 (on 8p pdip has the output to be consistent with the; MM5837 - other output pins are possible;----------------------------------------------------------------------LIST P=10F202include "p10f202.inc"__CONFIG _MCLRE_OFF & _CP_OFF & _WDT_OFF;; RAM VariablesCBLOCK 0x08; LFSR 1, a 31-bit registerLFSR1_1LFSR1_2LFSR1_3LFSR1_4; LFSR 2, a 23-bit registerLFSR2_1LFSR2_2LFSR2_3; Temporary storageTEMPENDC;----------------------------------------------------org 0x000movwf OSCCAL; note: use default OPTION reg valuesmovlw b'11011100' ; for GP2 outOPTION;**** Setup GPIO inputs/outputs ****movlw b'00000000'movwf GPIOmovlw b'00001011' ; GP2 as output (pin 3 on 8p pdip)TRIS GPIO; Set up initial values of the registersmovlw0x45movwfLFSR1_1movlw0x57movwfLFSR1_2movlw0x9FmovwfLFSR1_3movlw0xF2movwfLFSR1_4movlw0xD7movwfLFSR2_1movlw0xC8movwfLFSR2_2movlw0x79movwfLFSR2_3MainLoop:; 31-bit LFSR with taps at 31 and 28swapfLFSR1_4, W; Get bit 28movwfTEMPrlfLFSR1_4, W; Get bit 31xorwfTEMP, F; Shift the XORd bit into carryrlfTEMP, F; Shift the registerrlfLFSR1_1, FrlfLFSR1_2, FrlfLFSR1_3, FrlfLFSR1_4, F; Output the noise bitmovfLFSR1_1, WmovwfGPIO; 21-bit LFSR with taps at 21 and 19rrfLFSR2_3, W; Get bit 19movwfTEMPrrfTEMP, FswapfLFSR2_3, W; Get bit 21xorwfTEMP, F; Shift the XORd bit into carryrrfTEMP, F; Shift the registerrlfLFSR2_1, FrlfLFSR2_2, FrlfLFSR2_3, F; Output the noise bitmovfLFSR2_1, WmovwfGPIOgotoMainLoopEND

author: jonpaul
author_rank: Super Contributor
timestamp: June 28, 2021, 05:55:31 am
content: We used the Nat IC MM5837N for acoustic measurement, and no issues as the spectra were averaged.The "click" is inherent in any digital PRS generator.The OP can determine if the "click" is an issue. A LPF will reduce the HF components of the "click"Bon Chance,Jon

author: merlinb
author_rank: Contributor
timestamp: June 28, 2021, 08:56:32 am
content: Quote from: ricko_uk on June 24, 2021, 01:08:04 amHere's a classic way to generate true white pseudo-random noise from near DC to about 100kHz. Pretty old technology, but if you happen to have the logic ICs lying around then it's quick to put together. I can't remember exactly what the repetition time is, about 5 seconds or something and not easily detected by ear.You can of course add filters after the noise output to band-limit the noise if you wish.

author: DrG
author_rank: Super Contributor
timestamp: June 28, 2021, 02:11:37 pm
content: Quote from: jonpaul on June 28, 2021, 05:55:31 amYes, I agree with you that the OP can determine what fits his needs.The MM5837 has a 17 stage LFSR which repeats every few seconds and that is the audible click I was referring to.The MM5837 had no external clock pin like the pin-compatible AMI S2688 (also obsolete). You can easily change the external clocking speed with the PIC approach (at least the 675 one mentioned) by changing the external oscillator, but also by using delays in the main loop of even the 202. This, it would seem to me, to be important for getting what the OP has described.The comments section in the page that I cited contains a few improvements and discussion.Taken together and having used both the MM5837 and the PIC software LFSR for white noise, I am going to stay with my opinion of not recommending the MM5837 but, instead, recommending that the PIC is a better choice.We may disagree and I am fine with that.

author: ricko_uk
author_rank: Super Contributor
timestamp: June 28, 2021, 04:14:09 pm
content: Thank you all very much!I am thinking of trying them all and see which works best.Much appreciated infos and suggestions!!