White noise generator and layout tips? - Page 1
Forum section: electronics / projects

author: homeless_peep
author_rank: Contributor
timestamp: August 04, 2017, 10:06:11 am
content: Hello,for my current project I need a white noise generator as part of a (mostly) digital circuit (dead simple 40xx logic). My specs are quite loose here, except for the "peak to peak" (ie, 95% value) amplitude of the resulting white noise signal, which should be something like 8-10Vpp.I thought I'd give thermal noise a shot and went with a large resistance (10M), connected to the input of an amplifier and ground. I used 3 TL084 to get a total gain of about 80k (98dB- x100, x100, x8/adjustable), while at the same time bandlimiting the whole thing (by choosing the gain alone- no external filtering) to roughly 70Hz-20kHz.First question is, are my calculations of about 2.5Vrms correct? The amplifiers use small resistance values, so their noise contribution should be negligible compared to the amplified 10M one.Second question, how should I handle such high gain stages in terms of layout? The only other analog circuit on the board is a DAC0800 that uses the 4th OPAMP of the package as an output buffer. Its accuracy is not critical, but I would like to keep it clean sounding if possible.

author: GK
author_rank: Super Contributor
timestamp: August 04, 2017, 12:25:01 pm
content: The TL0XX has about 10pF of input capacitance. That alone shunting your 10M limits your upper bandwidth corner to ~1600 Hz.

author: Kalvin
author_rank: Super Contributor
timestamp: August 04, 2017, 12:39:00 pm
content: How about using a digital white noise generator? Will probably give much better performance and is not susceptible to external disturbances. The digital system is easy to tune by varying the clock rate, and it is also repeatable yet random enough for typical testing purposes. There are also zener-based noise generators which will provide easily wider bandwidths compared to your resistor-based approach because they suffer far less from the capacitance loading / roll-off problem. For example, in this discussion are some nice ideas to get started:https://electronics.stackexchange.com/questions/255050/how-to-build-a-noise-generator-based-entirely-on-logic-gates

author: homeless_peep
author_rank: Contributor
timestamp: August 04, 2017, 01:06:11 pm
content: How do you calculate that? Could you elaborate?A digital approach will increase cost and space, and will also need some kind of clocking. The (analog) white noise output is desirable in my application (other than being used for other functions).This is not aimed at testing, but rather random output generation at audio rates.How do I calculate the noise levels on the zener/transistor based approaches? any references?

author: Kalvin
author_rank: Super Contributor
timestamp: August 04, 2017, 01:16:02 pm
content: Here is one approach for zener-based white noise generator, suitable for audio frequencies too:http://www.edn.com/design/analog/4420926/White-noise-source-flat-from-1Hz-to-100kHzIt uses two 9V batteries in order to get proper bias voltage acorss the zener diode. By experimenting you should be able to use lower voltage zener for good results**. Another common approach is to reverse bias the base to emitter junction of a NPN transistor and use this as a noise diode.** Edit: As you already have +/- 12VDC available, you can use the design as presented without problems.

author: Kalvin
author_rank: Super Contributor
timestamp: August 04, 2017, 01:33:05 pm
content: Quote from: homeless_peep on August 04, 2017, 01:06:11 pmI guess you could use a simple, cheap 8-pin AVR- or PIC-based microcontroller for generating the white noise if you want to go with digital implementation. Of course you need to find or create software for that and be able to program the device, but after that you will have a simple white noise generator with constant performance which doesn't require external clock. There are quite some options on how to generate white noise using analog or digital techniques.