Dumb question on noise source

Started by David Eather January 11, 2018
My test gear (along with most of my brain) have been packed away, so I can  
only ask this as a question rather than lash it up and know.

I have a +/- 5v source and a 6.8v zener diode as a noise source. I'm  
expecting about 0.3mV noise. My question is: will I get more noise by  
biasing with a resistor (about 3.3k) or biasing with a constant current  
source. TIA


-- 
I look forward to the day when a chicken can cross the road without having  
its motives questioned.
On Thursday, January 11, 2018 at 11:47:47 AM UTC-8, David Eather wrote:

> I have a +/- 5v source and a 6.8v zener diode as a noise source. I'm > expecting about 0.3mV noise. My question is: will I get more noise by > biasing with a resistor (about 3.3k) or biasing with a constant current > source. TIA
The zener won't care, but a 'constant current source' might be bandwidth-limited with an internal op amp. That would rather complicate the noise spectrum; resistor (noninductive, metal film) would be preferable.
David Eather wrote...
> > I have a +/- 5v source and a 6.8v zener diode as a noise source. I'm > expecting about 0.3mV noise. My question is: will I get more noise by > biasing with a resistor (about 3.3k) or biasing with a constant current > source. TIA
"zener" diodes are actually field-effect diodes below about 5 volts, and true avalanche diodes, with short microplasma breakdowns, above 7 volts. In the transition region, zener diodes have low Rac, low dV/dI, low noise and other "good" properties. So 8 volts or higher is a better voltage for high noise generation. The noise is related to current, whatever the source. You have to empirically determine the best parameters, and the diodes are all different. -- Thanks, - Win
On 01/11/2018 02:47 PM, David Eather wrote:
> My test gear (along with most of my brain) have been packed away, so I > can only ask this as a question rather than lash it up and know. > > I have a +/- 5v source and a 6.8v zener diode as a noise source. I'm > expecting about 0.3mV noise. My question is: will I get more noise by > biasing with a resistor (about 3.3k) or biasing with a constant current > source. TIA >
The current source will need to have very large gain to be effective, because if the zener is operating in the avalanche region (where you want it to be to get the most shot noise) it's operating in the steepest part of its I/V curve. A simple one or two transistor current source might not be that much better than a resistor, maybe worse depending on bandwidth. Reverse-biased BJTs apparently make better noise sources at low voltages than regular Zeners; well that's what everybody else seems to use, anyway.
Am 11.01.2018 um 22:24 schrieb bitrex:

> Reverse-biased BJTs apparently make better noise sources at low voltages > than regular Zeners; well that's what everybody else seems to use, anyway.
Probably you can expect less shunt capacitance for a BFR93 BE junction than for the average Z-Diode. That type actually works quite good to 2 GHz, at least for me. Gerhard
Am 11.01.2018 um 22:16 schrieb Winfield Hill:
> David Eather wrote... >> >> I have a +/- 5v source and a 6.8v zener diode as a noise source. I'm >> expecting about 0.3mV noise. My question is: will I get more noise by >> biasing with a resistor (about 3.3k) or biasing with a constant current >> source. TIA > > "zener" diodes are actually field-effect diodes below about > 5 volts, and true avalanche diodes, with short microplasma > breakdowns, above 7 volts. In the transition region, zener > diodes have low Rac, low dV/dI, low noise and other "good" > properties. So 8 volts or higher is a better voltage for > high noise generation. The noise is related to current, > whatever the source. You have to empirically determine > the best parameters, and the diodes are all different.
Where the TC goes through 0 at 6V or 7V , both effects are "equally strong". You can see the onset of avalanching already at low voltages. See for the BZX84 family, how the voltage noise gets more from 2V7, 3V3, 4V7, 6V8. That grows faster than the Zener voltage itself. All diodes were powered with 1 or 2K from a 14V NiMH battery. The battery adds nothing, as has been tested by using a 47 Ohm resistor instead of the Zener. It shows just the expected thermal noise. 0 dB is 1nV/sqrt Hz. < https://www.flickr.com/photos/137684711@N07/24411798996/in/album-72157662535945536/ > @David: A 3K3 resistor should do no harm. The noise depends on the current, more current is usually quieter with wider bandwidth. I have not done enough tests to be statistically convincing. Gerhard
On Fri, 12 Jan 2018 05:47:32 +1000, "David Eather" <eather@tpg.com.au>
wrote:

>My test gear (along with most of my brain) have been packed away, so I can >only ask this as a question rather than lash it up and know. > >I have a +/- 5v source and a 6.8v zener diode as a noise source. I'm >expecting about 0.3mV noise. My question is: will I get more noise by >biasing with a resistor (about 3.3k) or biasing with a constant current >source. TIA
Shouldn't matter. The zener dynamic impedance will be a lot less than 3.3K. You might expect something like 300 nV RMS per root Hz of measurement bandwidth. -- John Larkin Highland Technology, Inc picosecond timing precision measurement jlarkin att highlandtechnology dott com http://www.highlandtechnology.com
Winfield Hill wrote:
> David Eather wrote... >> >> I have a +/- 5v source and a 6.8v zener diode as a noise source. I'm >> expecting about 0.3mV noise. My question is: will I get more noise by >> biasing with a resistor (about 3.3k) or biasing with a constant current >> source. TIA > > "zener" diodes are actually field-effect diodes below about > 5 volts, and true avalanche diodes, with short microplasma > breakdowns, above 7 volts. In the transition region, zener > diodes have low Rac, low dV/dI, low noise and other "good" > properties. So 8 volts or higher is a better voltage for > high noise generation. The noise is related to current, > whatever the source. You have to empirically determine > the best parameters, and the diodes are all different. > >
Correct; even the same1N between various manufacturers, or the same 1N and manufacturer but different packages and/or power rating. Multiply those variations for a given voltage by the number of voltages you wish to characterize, and take a loan on the house!
On Fri, 12 Jan 2018 06:40:48 +1000, whit3rd <whit3rd@gmail.com> wrote:

> On Thursday, January 11, 2018 at 11:47:47 AM UTC-8, David Eather wrote: > >> I have a +/- 5v source and a 6.8v zener diode as a noise source. I'm >> expecting about 0.3mV noise. My question is: will I get more noise by >> biasing with a resistor (about 3.3k) or biasing with a constant current >> source. TIA > > bandwidth-limited with an internal op amp. That would rather > complicate the noise spectrum; resistor (noninductive, metal film) would > be preferable.
ok - and it simplifies things. A cc source might better deal with noise and fluctuatons on the power supply? -- I look forward to the day when a chicken can cross the road without having its motives questioned.
On Fri, 12 Jan 2018 07:16:19 +1000, Winfield Hill  
<hill@rowland.harvard.edu> wrote:

> David Eather wrote... >> >> I have a +/- 5v source and a 6.8v zener diode as a noise source. I'm >> expecting about 0.3mV noise. My question is: will I get more noise by >> biasing with a resistor (about 3.3k) or biasing with a constant current >> source. TIA > > "zener" diodes are actually field-effect diodes below about > 5 volts, and true avalanche diodes, with short microplasma > breakdowns, above 7 volts. In the transition region, zener > diodes have low Rac, low dV/dI, low noise and other "good" > properties. So 8 volts or higher is a better voltage for > high noise generation. The noise is related to current, > whatever the source. You have to empirically determine > the best parameters, and the diodes are all different. > >
more than 8 volts and up the current. gotcha. I was planing on a vca810 or for a low speed version a lm13700 as a variable gain stage. The worst possible outcome would be trusting the end user to make adjustments. -- I look forward to the day when a chicken can cross the road without having its motives questioned.