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Zener noise source (freq < 1MHz.)

Started by George Herold January 2, 2012
Zener noise source (freq < 1MHz.)

I was revisiting Zener's as noise sources today.
I first tried some 12V (1W*)  diodes. (I want to bias them from 15V
rails.)
They worked fine, but I found the noise varied by at least 6dB (a
factor of 2 in V peak)
for different diodes from the same strip.
(After testing 5 I gave up.)

So I fell back to an old circuit.

+15--+----+
     Z    |
     ^    1M
     |    |
     +----|-C+-(buffer)-------(      )
     |    +--|--C-+--(buffer)-(summer)
     1M   Z 100k  |
     |    ^  |   100k
-15--+----|  +----+
                  GND

Z are the zeners,  20V (0.5W),
^

You should wrap black tape around glass body ones.
(note to self: try some surface mount)

For C I used 1uF films but I bet ceramics will work fine too.  The
output of each buffer looks like a pulse into an RC filter.  (time
constant maybe 1uS depending on bias and buffer resistors)
The best thing is that all 20V/ 1/2 W zeners I've tried give the same
noise to maybe 10% (in voltage).  (Again exact number depend on the
R's used).
With the R's shown the noise is flat out to about
600 kHz, or so.

George H.


 *the 1 watters was all I had in the parts box.
On Mon, 02 Jan 2012 15:52:58 -0800, George Herold wrote:

> Zener noise source (freq < 1MHz.) > > I was revisiting Zener's as noise sources today. I first tried some 12V > (1W*) diodes. (I want to bias them from 15V rails.) > They worked fine, but I found the noise varied by at least 6dB (a factor > of 2 in V peak) > for different diodes from the same strip. (After testing 5 I gave up.) > > So I fell back to an old circuit. > > +15--+----+ > Z | > ^ 1M > | | > +----|-C+-(buffer)-------( ) > | +--|--C-+--(buffer)-(summer) > 1M Z 100k | > | ^ | 100k > -15--+----| +----+ > GND > > Z are the zeners, 20V (0.5W), > ^ > > You should wrap black tape around glass body ones. (note to self: try > some surface mount) > > For C I used 1uF films but I bet ceramics will work fine too. The > output of each buffer looks like a pulse into an RC filter. (time > constant maybe 1uS depending on bias and buffer resistors) The best > thing is that all 20V/ 1/2 W zeners I've tried give the same noise to > maybe 10% (in voltage). (Again exact number depend on the R's used). > With the R's shown the noise is flat out to about 600 kHz, or so. > > George H. > > > *the 1 watters was all I had in the parts box.
IIRC, the noise in an avalanche diode is proportional to current. Could you have been falling prey to the variability in the Zener voltage and the low overhead between the diode and the rail? A constant-current sink would fix that... -- My liberal friends think I'm a conservative kook. My conservative friends think I'm a liberal kook. Why am I not happy that they have found common ground? Tim Wescott, Communications, Control, Circuits & Software http://www.wescottdesign.com
On Jan 2, 7:44=A0pm, Tim Wescott <t...@seemywebsite.com> wrote:
> On Mon, 02 Jan 2012 15:52:58 -0800, George Herold wrote: > > Zener noise source (freq < 1MHz.) > > > I was revisiting Zener's as noise sources today. I first tried some 12V > > (1W*) =A0diodes. (I want to bias them from 15V rails.) > > They worked fine, but I found the noise varied by at least 6dB (a facto=
r
> > of 2 in V peak) > > for different diodes from the same strip. (After testing 5 I gave up.) > > > So I fell back to an old circuit. > > > +15--+----+ > > =A0 =A0 =A0Z =A0 =A0| > > =A0 =A0 =A0^ =A0 =A01M > > =A0 =A0 =A0| =A0 =A0| > > =A0 =A0 =A0+----|-C+-(buffer)-------( =A0 =A0 =A0) > > =A0 =A0 =A0| =A0 =A0+--|--C-+--(buffer)-(summer) > > =A0 =A0 =A01M =A0 Z 100k =A0| > > =A0 =A0 =A0| =A0 =A0^ =A0| =A0 100k > > -15--+----| =A0+----+ > > =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 GND > > > Z are the zeners, =A020V (0.5W), > > ^ > > > You should wrap black tape around glass body ones. (note to self: try > > some surface mount) > > > For C I used 1uF films but I bet ceramics will work fine too. =A0The > > output of each buffer looks like a pulse into an RC filter. =A0(time > > constant maybe 1uS depending on bias and buffer resistors) The best > > thing is that all 20V/ 1/2 W zeners I've tried give the same noise to > > maybe 10% (in voltage). =A0(Again exact number depend on the R's used). > > With the R's shown the noise is flat out to about 600 kHz, or so. > > > George H. > > > =A0*the 1 watters was all I had in the parts box. > > IIRC, the noise in an avalanche diode is proportional to current. > > Could you have been falling prey to the variability in the Zener voltage > and the low overhead between the diode and the rail? =A0A constant-curren=
t
> sink would fix that... > > -- > My liberal friends think I'm a conservative kook. > My conservative friends think I'm a liberal kook. > Why am I not happy that they have found common ground? > > Tim Wescott, Communications, Control, Circuits & Softwarehttp://www.wesco=
ttdesign.com- Hide quoted text -
> > - Show quoted text -
Hi Tim, I just had 12V zeners +30k ohm across 15 volts. Zero correlation between zener voltage and noise. (I only tested 5.) George H.
"George Herold" <gherold@teachspin.com> wrote in message 
news:99290406-72e0-474c-9b20-a0f728df6c6b@r5g2000yqc.googlegroups.com...
On Jan 2, 7:44 pm, Tim Wescott <t...@seemywebsite.com> wrote:
> On Mon, 02 Jan 2012 15:52:58 -0800, George Herold wrote: > > Zener noise source (freq < 1MHz.) > > > I was revisiting Zener's as noise sources today. I first tried some 12V > > (1W*) diodes. (I want to bias them from 15V rails.) > > They worked fine, but I found the noise varied by at least 6dB (a factor > > of 2 in V peak) > > for different diodes from the same strip. (After testing 5 I gave up.) > > > So I fell back to an old circuit. > > > +15--+----+ > > Z | > > ^ 1M > > | | > > +----|-C+-(buffer)-------( ) > > | +--|--C-+--(buffer)-(summer) > > 1M Z 100k | > > | ^ | 100k > > -15--+----| +----+ > > GND > > > Z are the zeners, 20V (0.5W), > > ^ > > > You should wrap black tape around glass body ones. (note to self: try > > some surface mount) > > > For C I used 1uF films but I bet ceramics will work fine too. The > > output of each buffer looks like a pulse into an RC filter. (time > > constant maybe 1uS depending on bias and buffer resistors) The best > > thing is that all 20V/ 1/2 W zeners I've tried give the same noise to > > maybe 10% (in voltage). (Again exact number depend on the R's used). > > With the R's shown the noise is flat out to about 600 kHz, or so. > > > George H. > > > *the 1 watters was all I had in the parts box. > > IIRC, the noise in an avalanche diode is proportional to current. > > Could you have been falling prey to the variability in the Zener voltage > and the low overhead between the diode and the rail? A constant-current > sink would fix that... > > -- > My liberal friends think I'm a conservative kook. > My conservative friends think I'm a liberal kook. > Why am I not happy that they have found common ground? > > Tim Wescott, Communications, Control, Circuits & > Softwarehttp://www.wescottdesign.com- Hide quoted text - > > - Show quoted text -
>Hi Tim, I just had 12V zeners +30k ohm across 15 volts. >Zero correlation between zener voltage and noise. >(I only tested 5.) > >George H.
He's talking about the 5% tolerance in Vz causing a large change in the actual voltage drop across the 30K resistor. Looks like you were shooting for 100uA but you can get anywhere from 80 to 120uA. a 20% swing. I don't know how strong a function of noise is to bias current but that may explain some of your findings.
On Jan 3, 1:44=A0am, Tim Wescott <t...@seemywebsite.com> wrote:
> On Mon, 02 Jan 2012 15:52:58 -0800, George Herold wrote: > > Zener noise source (freq < 1MHz.) > > > I was revisiting Zener's as noise sources today. I first tried some 12V > > (1W*) =A0diodes. (I want to bias them from 15V rails.) > > They worked fine, but I found the noise varied by at least 6dB (a facto=
r
> > of 2 in V peak) > > for different diodes from the same strip. (After testing 5 I gave up.) > > > So I fell back to an old circuit. > > > +15--+----+ > > =A0 =A0 =A0Z =A0 =A0| > > =A0 =A0 =A0^ =A0 =A01M > > =A0 =A0 =A0| =A0 =A0| > > =A0 =A0 =A0+----|-C+-(buffer)-------( =A0 =A0 =A0) > > =A0 =A0 =A0| =A0 =A0+--|--C-+--(buffer)-(summer) > > =A0 =A0 =A01M =A0 Z 100k =A0| > > =A0 =A0 =A0| =A0 =A0^ =A0| =A0 100k > > -15--+----| =A0+----+ > > =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 GND > > > Z are the zeners, =A020V (0.5W), > > ^ > > > You should wrap black tape around glass body ones. (note to self: try > > some surface mount) > > > For C I used 1uF films but I bet ceramics will work fine too. =A0The > > output of each buffer looks like a pulse into an RC filter. =A0(time > > constant maybe 1uS depending on bias and buffer resistors) The best > > thing is that all 20V/ 1/2 W zeners I've tried give the same noise to > > maybe 10% (in voltage). =A0(Again exact number depend on the R's used). > > With the R's shown the noise is flat out to about 600 kHz, or so. > > > George H. > > > =A0*the 1 watters was all I had in the parts box. > > IIRC, the noise in an avalanche diode is proportional to current.
Perhaps you meant inversely proportional to the square root of the current? It's actually a little bit more complicated than that. At low currents, the avalanche self-quenches from time to time, and you get spikes on the output as the diode capacitance charges up until another charge carrier shows up to start a new avalanche. At higher currents the avalanche hardly ever quenches, and the noise is just the statistical noise on a sustained avalanche process <snip> -- Bill Sloman, Nijmegen
"Tim Wescott"

> > IIRC, the noise in an avalanche diode is proportional to current. >
** That would be rule for noise current, not noise voltage. ... Phil
"George Herold"

Hi Tim, I just had 12V zeners +30k ohm across 15 volts.
Zero correlation between zener voltage and noise.
(I only tested 5.)


** Just tested a 12V / 400mW zener from 1uA to 10mA -  using a 10kohms 
series resistor and a high Z ( 680kohms) preamp with a gain of 250 times out 
to 50 kHz or so.

White noise snapped on at just under 1uA at 5mV, fell back to 2mV at 30uA 
rising again to 6mV at 100uA - then back to 5mV at 250uA and down to 2mV at 
1mA falling steadily to 0.2mV at 10mA.

Looks like the internal noise generator is a varying current source that is 
being shunted by the zener's falling dynamic resistance as average DC bias 
increases.

So there is a tug of war going on.


.... Phil




On Jan 2, 9:51=A0pm, "mook johnson" <m...@mook.net> wrote:
> "George Herold" <gher...@teachspin.com> wrote in message > > news:99290406-72e0-474c-9b20-a0f728df6c6b@r5g2000yqc.googlegroups.com... > On Jan 2, 7:44 pm, Tim Wescott <t...@seemywebsite.com> wrote: > > > > > > > On Mon, 02 Jan 2012 15:52:58 -0800, George Herold wrote: > > > Zener noise source (freq < 1MHz.) > > > > I was revisiting Zener's as noise sources today. I first tried some 1=
2V
> > > (1W*) diodes. (I want to bias them from 15V rails.) > > > They worked fine, but I found the noise varied by at least 6dB (a fac=
tor
> > > of 2 in V peak) > > > for different diodes from the same strip. (After testing 5 I gave up.=
)
> > > > So I fell back to an old circuit. > > > > +15--+----+ > > > Z | > > > ^ 1M > > > | | > > > +----|-C+-(buffer)-------( ) > > > | +--|--C-+--(buffer)-(summer) > > > 1M Z 100k | > > > | ^ | 100k > > > -15--+----| +----+ > > > GND > > > > Z are the zeners, 20V (0.5W), > > > ^ > > > > You should wrap black tape around glass body ones. (note to self: try > > > some surface mount) > > > > For C I used 1uF films but I bet ceramics will work fine too. The > > > output of each buffer looks like a pulse into an RC filter. (time > > > constant maybe 1uS depending on bias and buffer resistors) The best > > > thing is that all 20V/ 1/2 W zeners I've tried give the same noise to > > > maybe 10% (in voltage). (Again exact number depend on the R's used). > > > With the R's shown the noise is flat out to about 600 kHz, or so. > > > > George H. > > > > *the 1 watters was all I had in the parts box. > > > IIRC, the noise in an avalanche diode is proportional to current. > > > Could you have been falling prey to the variability in the Zener voltag=
e
> > and the low overhead between the diode and the rail? A constant-current > > sink would fix that... > > > -- > > My liberal friends think I'm a conservative kook. > > My conservative friends think I'm a liberal kook. > > Why am I not happy that they have found common ground? > > > Tim Wescott, Communications, Control, Circuits & > > Softwarehttp://www.wescottdesign.com-Hide quoted text - > > > - Show quoted text - > >Hi Tim, I just had 12V zeners +30k ohm across 15 volts. > >Zero correlation between zener voltage and noise. > >(I only tested 5.) > > >George H. > > He's talking about the 5% tolerance in Vz causing a large change in the > actual voltage drop across the 30K resistor. > > Looks like you were shooting for 100uA but you can get anywhere from 80 t=
o
> 120uA. =A0a 20% swing. =A0 I don't know how strong a function of noise is=
to
> bias current but that may explain some of your findings.- Hide quoted tex=
t -
> > - Show quoted text -
Hi Mook, OK here are the numbers from the 5 diodes I tested. Noise is measured with a SRS spectrum analyzer (which has this weird dBV units.) diode voltage noise (V) dbV 1 11.66 -58.3 2 11.60 -60.5 3 11.95 -64 4 11.95 -57.5 5 11.78 -64.0 George H.
On Jan 3, 1:15=A0am, "Phil Allison" <phi...@tpg.com.au> wrote:
> "George Herold" > > Hi Tim, I just had 12V zeners +30k ohm across 15 volts. > Zero correlation between zener voltage and noise. > (I only tested 5.) > > ** Just tested a 12V / 400mW zener from 1uA to 10mA - =A0using a 10kohms > series resistor and a high Z ( 680kohms) preamp with a gain of 250 times =
out
> to 50 kHz or so. > > White noise snapped on at just under 1uA at 5mV, fell back to 2mV at 30uA > rising again to 6mV at 100uA - then back to 5mV at 250uA and down to 2mV =
at
> 1mA falling steadily to 0.2mV at 10mA. > > Looks like the internal noise generator is a varying current source that =
is
> being shunted by the zener's falling dynamic resistance as average DC bia=
s
> increases. > > So there is a tug of war going on. > > .... Phil
Nice, Did you find a place with more low frequency noise? Here=92s some 'scope shots with bias current listed and noise density (at 35kHz) (All but the last made with zener feeding a TIA opamp circuit.) http://img707.imageshack.us/img707/6632/tek0000.png http://img855.imageshack.us/img855/2774/tek0001.png the following is starting to look asymmetrical. http://img708.imageshack.us/img708/3198/tek0002.png The one below has more noise at lower frequencies. http://img818.imageshack.us/img818/3083/tek0003.png It=92s hard to catch the character with a single =91scope shot. But you can see distinct levels in the following http://img545.imageshack.us/img545/7433/tek0004.png And now just occasional spikes up. http://img825.imageshack.us/img825/266/tek0005.png This last was made with diode grounded and cap from mid point between diode and bias resistor feeding a gain of 2 opamp circuit. http://img836.imageshack.us/img836/3199/tek0006.png Very distinct levels! George H.
On Tue, 03 Jan 2012 17:01:17 +1100, Phil Allison wrote:

> "Tim Wescott" > > >> IIRC, the noise in an avalanche diode is proportional to current. >> >> > ** That would be rule for noise current, not noise voltage.
That makes sense. -- My liberal friends think I'm a conservative kook. My conservative friends think I'm a liberal kook. Why am I not happy that they have found common ground? Tim Wescott, Communications, Control, Circuits & Software http://www.wescottdesign.com