Zener diode vs Noise diode for noise generation.

[DanioIO]

Hi,
Im thinking about making a simple noise generator to test some filters. Can I achieve  good performance with a simple "of the shelf" zener diode or should i buy a noise diode. Additionally i found some noise generating lamps are they good or should i stick to semiconductors?

[Zero999]

It does work. You'll need to experiment with different currents for maximum noise output. I found higher voltage zeners with a constant current source work best.

[Paul Price]

I would suggest first trying a reversed biased PNP transistor junction so that it avalanches, works just fine, the other solutions are costly, harder to find parts to implement. Use an wide band buffer to amplify the output.

A second easy solution if to set a wideband op-amp to a gain of 10000 to 100000 and reap the harvest of nose created there.
You will have to set a near-DC freq. cutoff in the feedback loop because the offset voltages and currents will otherwise saturate the output.

[theatrus]

I've used the reverse breakdown of a 2n4124 before - it has a conveniently low 5V breakdown.

[KJDS]

What frequency?

[Paul Price]

The frequency of cutoff is arbitrarily set, but .1Hz is probably a good cutoff point.


You get the op-amp noise amplified directly from the op-amp itself, it should be shielded and the power supply filtered well. There is no input signal to the op-amp. It's own internal electron song is the only music you will want to hear.

[T3sl4co1l]

Note that you can't get wideband noise in a single stage from 1k+ gain.  To cover the audio band, you need a 20MHz GBW amp to do that.

A quad of even a slacker jellybean amp (e.g. LM324?) offers more than enough GBW in a package to achieve this though.  Set each stage for the same gain.

ST's LM324 datasheet says 40nV/rtHz, which is line level (0.775Vrms) in the audio band (20kHz) for a total gain of 0.775V / (40nV/rtHz * sqrt(20,000 Hz) ) = 137k.  (Considering the LM324 minimum gain at DC is only 25k, you can see why multiple stages are a must!)  The 4th root is 19.23, the amount of gain per stage required.  So, four inverting-gain-of-20 stages will do just fine.  The actual bandwidth to expect is GBW / gain, or 1.3MHz / 19.23 = 67kHz, which should be more than enough.  In fact, the RMS output will be about 1.83 times too high at this natural bandwidth; use a 20kHz lowpass to fix this.

Particularly in older amps like LM741, beware of excess noise such as 1/f and popcorn noise.  I don't know offhand how much of this is a problem with other types.  Bipolar amps do tend to exhibit a lower 1/f knee than JFET amps, but it's still within the audio band.  For best results, calibrate the source.

All this assuming the filters you're testing are even audio band, which you didn't mention...

Tim