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White Noise Generator Circuit That Will Work Down to 7VDC

Thread starter #1
I normally use the circuit below as a reliable white noise source. It works best on a power supply of 12VDC, but is usable down to about 10VDC.

However, I need a similar noise source, that also uses common parts, but can operate from a 9V battery. That means down to 7V to allow for the battery running out.

Can anyone suggest a circuit that will do this, based upon a device that has a lower voltage breakdown? This is a pocket-size device so size and parts count are relevant.
 

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crutschow

Well-Known Member
Most Helpful Member
#2
Try a low voltage zener diode (perhaps 4 or 5V) instead of the transistor.
 
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Thread starter #3
I inserted a 3.6V zener in the above circuit, started at 9V and cranked it up to 12V. Not much happening. Just to check. I reinserted the BC547. Miniscule output at 9V but at 12V there is 1V of white noise.

Given your suggestion, I am unsure why the zener is not working.
 
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#4
Your Zener is too good :)

Also, you should try the Zener at various currents because the noise is not the same at all currents, and the temperature dependence of the noise also varies by current.
This one-page paper explains in more detail: https://www.scribd.com/document/250639320/04236738

Another approach may be to use transistors with a lower emitter-base breakdown voltage, such as some UHF transistors. The MPS5179 / MMBT5179 has a rated reverse base-emitter breakdown of 2.5 volts as opposed to 4 volts or more for general purpose transistors.

Edit: The BFG310/XR has a reverse emitter-base breakdown rating of 2.0 volts and the BFG21W has a reverse emitter-base breakdown voltage of 1.0 volts.
 
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BobW

Active Member
#5
The best white noise generator circuit that I've ever used is the same as the one in the first post, but as noted, they need a reasonably high Vcc in order to get the transistor to avalanche. One time when I needed a white noise source to operate from a 5 volt logic supply, I tried a lot of other circuits using different noise sources including zeners and other diodes, but none came anywhere close to putting out the noise of the avalanche transistor circuit. What I ended up doing was build a charge pump circuit to boost the 5V supply to about 18V, and used that to power the avalanche transistor circuit. It doesn't take much to build a charge pump, because the avalanche transistor only draws a few microamps, and the rest of the circuit can operate from 5V. I used four 1N4148 diodes and four 0.1µF caps as a voltage quadrupler driven by a logic gate oscillator. Of course, you have to make sure that the oscillator switching noise gets filtered out and doesn't end up in the white noise signal.

If you do find something that works well at low voltage I'd be very interested in seeing the circuit.
 

AnalogKid

Well-Known Member
#6
An all-CMOS Linear Feedback Shift Register (LFSR) would run down to 3 V, but the body count probably is too high if space is tight.

A CMOS 555 would take very little static current to boost 7 V to 13 V. With such a low load current, it could run at something like 1 Hz, way below the bandwidth you probably are using.

ak
 
Thread starter #7
Yes, I have a CMOS noise generator cicuit that uses a CD4030 and two CD4006's. Unfortunately, that's more parts and board space than would suit this project.

I did try increasing the current through the zener, but this had no effect.

If anyone has a low-parts-count, 7VDC supply concept that will inject noise, ANY kind of broad spectrum noise, into a circuit I would be very keen to hear.
 

BobW

Active Member
#8
There are some kinds of Schottky detector diodes that have a very low reverse breakdown voltage. You could give them a try. For example, HSMS-2850 has a maximum PIV rating of 2 volts.
 
Thread starter #9
I tried a number of transistors I had on-hand. The most promising was a 2N2222 with a specified breakdown of 7.2V. Dropped into the first circuit shown above, and with the emitter resistor reduced to 47K, I obtained 1Vpp noise with a 9VDC supply, and 100mA with 8VDC. Lower then that, the output pretty much disappears, as it also does over about 10V supply given the current component values. I had hoped to go down to 7V though.

Fitting in with this outcome, instead of a 9V alkaline battery, I could use a Lithium type that has a flatter discharge curve. I'll probably stick with that solution, unless someone can suggest a better one that uses common parts.
 
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unclejed613

Well-Known Member
#11
i think the reason you got no results with a zener, is that below 6 volts, the device operates in zener mode, while devices rates above 6 volts actually function in avalanche mode (which generates the noise). you could go with a high frequency oscillator and a voltage multiplier to generate the voltage for the zener. make sure the oscillator operates well above 20khz. i recently picked up an ion chamber radiation detector from the early 60s, and interestingly enough, it's got a similar power supply for the 50V rail that biases the ion chamber. the radiation meter runs off a 1.5V battery.
 

schmitt trigger

Well-Known Member
#12
How about using germanium transistors? If I recall properly, their breakdown is lower. The 2N128 had a breakdown of 4.5 volts.
 

unclejed613

Well-Known Member
#13
that might work. germanium is also noisier than silicon.
 

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