From: "Tom Faloon"
Subject: Re: reverse-biased diode noise source?
Date: Mon, 16 Sep 2002 19:21:40 +0100
NNTP-Posting-Date: Mon, 16 Sep 2002 18:18:30 +0000 (UTC)
X-Newsreader: Microsoft Outlook Express 5.00.2615.200
Nice one Phil,
I like it!
Phil Hobbs wrote in message
> Now that amplifiers are so quiet, it's easy to make a well-calibrated
> low frequency noise source by shining the light from a LED on a
> photodiode--_NOT_ a phototransistor or photodarlington. Drive the LED
> from a quiet current source (e.g. a battery and metal film resistor or a
> well-designed current source with a big voltage drop across its (metal)
> sense resistor). Provided the incident light is really unmodulated, the
> photocurrent exhibits exactly full shot noise. Shot noise is white and
> Gaussian to very high accuracy, so you can calibrate the noise power
> spectral density by measuring the DC and applying the shot noise
> i sub N = sqrt(2*e*I sub dc) (1-Hz BW)
> where e is the charge on the electron (1.602e-19 coulombs).
> This method is much better than the Zener approach, because it's based
> on fundamental physics rather than device parameters that have to be
> calibrated from unit to unit. Since it's easy to generate a
> photocurrent that has exactly full shot noise--shine a flashlight on
> your photodiode--it's easy to test by comparison whether your LED drive
> current source is quiet enough.
> Phil Hobbs
> Tom Faloon wrote:
> > Hi Walter,
> > Any silicon or germanium rectifier diode, transistor base emitter
> > or zener diode will act as a noise generator when reverse biased to
> > breakdown (avalanche) region.
> > However, this is generally an unwanted effect, and manufacturers do not
> > specify characteristics.
> > It is possible to buy diodes with published characteristics, but these
> > specialist devices,
> > and cost and minimum order quantity put them in reach of business users
> > only.
> > You can obtain useful output from everyday devices, but you will not be
> > to predict results accurately in advance, or assume repeatability from
> > component to component.
> > In the simplest application, simply reverse connect the diode to a
> > via a current limiting resistor, increase the supply voltage until
> > current flows,
> > and a noise voltage will then be produced across the diode.
> > You can expect a fairly 'white but not perfect' noise spectrum over a
> > frequency range.
> > Non perfection is most likely to be due to secondary noise functions in
> > diode, the
> > most pronounced being flicker noise. (1/f) which will be present below
> > 100 kHz.
> > The bandwidth will depend on the components and layout used. (Junction
> > capacitance, lead inductance, PCB track capacitance & inductance.)
> > Bandwidths into the GHz region are possible.
> > The noise level generally increases with reverse current. Level will
> > on the device used
> > As a rough figure think fractions of a nanoVolts / root Hz. (Perhaps X
> > 100)
> > Useful devices.
> > A standard rectifier diode. (Not always practical, because most diodes
> > a fairly high breakdown voltages.)
> > Base emitter junction of a bipolar transistor is more practical.
> > breakdown is often around 5V)
> > (Leave the collector unconnected.
> > Zener diode, but use devices above about 6 or 7 Volts.
> > (We call all these devices Zener diodes, but in fact they operate in two
> > different modes. Devices operating below about 5 Volts rely on the Zener
> > effect. Higher
> > voltage devices use the avalanche effect.)
> > Tom Faloon
> > remove 'z' from address to email direct.
> > http://www.faloon.co.uk
> > **********************************************
> > Walter Harley wrote in message
> > news:email@example.com...
> > > I see many references to the fact that a reverse-biased zener diode
> > > functions as a wideband noise source. However, I can't find much
> > > information about how to put this into practice. How much noise does
> > > generate, for how much current? Does it have to be in avalanche, or
> > > does it generate noise even at leakage-current levels? Is there any
> > > difference between different diodes, e.g., different voltages? What
> > > the frequency spectrum like?
> > >
> > > Thanks,
> > > -walter