Quantum Noise

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Some noise measurements for LEDs and zener diodes

noise_test_rig1a_640x453.jpg
Schematic of a test rig by Christopher to test various semiconductor devices for electronic noise.
Posted by Christopher, DIYaudio.com member
10th June 2004

There has been many discussions on the forum over the years about how noisy various voltage references are. I can't really remember anyone pointing to any measurements or satisfactory theoretical arguments, but maybe it is my memory being bad. It seems to be generally agreed (maybe) that LEDs are less noisy then zener diodes, but I don't think anybody has tried to put figures on the difference. There has also been discussions about what LEDs are lesat noisy. The only claim I can remember reading on the forum is that green LEDs are the least noisy, but no argument was provided to back it up. Hence, I decided to see it it was possible to get some answers to these questions using only some simple DIY test equipment. I don't have the time or money to do a proper scientific study, but maybe this can be considered a somewhat systematic pilot study for further investigations into this matter. For now I have done some measurements on various types of LEDs and zener diodes and some of the results were quite unexpected and begging for further studies.

I didn't do this just because I had nothing better to do, because I had, but since I like the forum and it members so much I decided to put some time and money into doing this study (OK, I admit I also did it to still my scientific curiosity).

The study and the measurements so far are described in the attached document. Please note that the study is too small and uses too questionable equipment to be considered properly scientific, so no hard claims or conclusions can be drawn from it. There are hints at some interesting phenomea though. Had I know beforehand what the results would be, I would have used a slightly different selection of components for study, but that's usually how it is.

Attached Files (noise_measurements1.txt below)


File: noise_measurements1.txt

Noise measurements for LEDs and zener diodes
--------------------------------------------

(C) Christer at DiyAudio.com
You are allowed to copy and use this information for your
personal and non-commercial use.


DESCRIPTION OF TEST RIG
-----------------------

The test rig uses three current sources of approx. 1, 5 and 20 mA
built using low-noise BJTs (BC559) to feed the device under test
(DUT) alternatingly. The noise was measured using two op amps in
in non-inverting configuration cascaded, both having a gain of 34,
making a gain of 1156 in total. The first op amp is a very-low-noise
model (LT1115) and uses a gain resistor of only 10 Ohms in the feedback
network. The gain resistor is thus 330 Ohms which works since the op
amp is only expected to output very low-level signals. The second op
amp is a low-noise type (NE5534) with gain and feedback resistors of
100 Ohms and 3.3 kOhms. The output was measured using a PC soundcard
(Creative Audigy LS in 16-bit 44.1 kHz mode). Each measurement consists
of a 10 second capture of the soundcard input and the RMS value for
this 10 s. signal was computed. The program was calibrated (using a
sine wave and an oscilloscope) to give aprroximately correct voltage
readings and all measurements were divided by 1156 to give the equivalent
input RMS noise at the first op amp, ie. at the DUT. No extra filters
except what is on the soundcard were used.


TEST METHOD
-----------

A spectrum of LED types ranging from IR to blue and of approximately
the same type were measured. All LEDS were selected to have an max
If of at least 20 mA, since this current was used in the test. Further
four 0.5W types of zener diodes were tested, two of them (5.6 and 6.8 V)
were deliberately selected close to each other but such that the 5.6 V
diode should be expected to have true zener breakdown and the 6.8 V one
to have avalanche breakdown. The other two were selected to be far away
from this "transition region". Two 1.3W zeners were also tested to see
how the power rating affects noise figures.

For each type of DUT, two devices (denoted #1 and #2 and presumably
from the same batch) were tested at the three test currents 1, 5 and
20 mA and the equivalent noise at the DUT was measured and calculated
as described above. For each combination of device and current, five
10-second measurements were made.

For reference, the voltage drop at each test current
was also measured for one device of each type.


MEASUREMENTS
------------

All values are RMS values 

Idle noise:
----------------------------------------------------
Measured idle noise of amplifier with grounded input:
0.19 0.19 0.19 0.19 0.18 uV
(The theoretical max value was calculated to 0.16 uV for 
20kHz bandwidth  and 0.22 uB for 40 kHz bandwidth).

Measured idle noise of amplifier with 100 Ohm source resistor:
0.26 0.25 0.24 0.26 0.26 uV
(The theoretical max value was calculated to 0.20 uV for 
20kHz bandwidth  and 0.28 uB for 40 kHz bandwidth).


LEDs:
----------------------------------------------------
(All LEDs of brand Everlight)

IR204/P1 (IR):
#1 @ 1mA:  3.7  3.7  3.7  3.7  3.7  uV  
#1 @ 5mA:  0.67 0.66 0.65 0.66 0.66 uV
#1 @ 20mA: 0.24 0.23 0.24 0.23 0.23 uV
#2 @ 1mA:  3.8  3.8  3.7  3.7  3.7  uV  (Vf = 1.05 V)
#2 @ 5mA:  0.65 0.64 0.64 0.64 0.64 uV  (Vf = 1.11 V)
#2 @ 20mA: 0.24 0.25 0.23 0.24 0.22 uV  (Vf = 1.17 V)

EL202HD (red):
#1 @ 1mA:  0.31 0.32 0.31 0.31 0.32 uV
#1 @ 5mA:  0.26 0.26 0.27 0.27 0.27 uV
#1 @ 20mA: 0.39 0.36 0.37 0.36 0.37 uV
#2 @ 1mA:  0.39 0.37 0.38 0.38 0.35 uV  (Vf = 1.82 V)
#2 @ 5mA:  0.32 0.30 0.30 0.30 0.31 uV  (Vf = 1.89 V)
#2 @ 20mA: 0.41 0.40 0.41 0.41 0.46 uV  (Vf = 2.09 V)

EL204ID (red-orange):
#1 @ 1mA:  0.31 0.30 0.31 0.31 0.31 uV
#1 @ 5mA:  0.25 0.26 0.26 0.26 0.24 uV
#1 @ 20mA: 0.41 0.41 0.48 0.40 0.41 uV
#2 @ 1mA:  0.35 0.31 0.29 0.30 0.32 uV  (Vf = 1.64 V)
#2 @ 5mA:  0.25 0.26 0.27 0.26 0.30 uV  (Vf = 1.74 V)
#2 @ 20mA: 0.40 0.40 0.39 0.40 0.41 uV  (Vf = 1.90 V)

EL204YD (yellow):
#1 @ 1mA:  0.42 0.30 0.29 0.29 0.28 uV
#1 @ 5mA:  0.28 0.26 0.25 0.33 0.27 uV
#1 @ 20mA: 0.42 0.39 0.39 0.40 0.40 uV
#2 @ 1mA:  0.31 0.30 0.31 0.30 0.31 uV  (Vf = 1.78 V)
#2 @ 5mA:  0.28 0.47 0.28 0.26 0.25 uV  (Vf = 1.87 V)
#2 @ 20mA: 0.34 0.34 0.35 0.34 0.34 uV  (Vf = 2.02 V)

EL204GD (green):
#1 @ 1mA:  0.68 0.50 0.50 0.47 0.46 uV
#1 @ 5mA:  0.35 0.30 0.28 0.28 0.29 uV
#1 @ 20mA: 0.36 0.35 0.35 0.35 0.35 uV
#2 @ 1mA:  0.46 0.46 0.44 0.44 0.41 uV  (Vf = 1.82 V)
#2 @ 5mA:  0.36 0.33 0.32 0.33 0.32 uV  (Vf = 1.92 V)
#2 @ 20mA: 0.39 0.40 0.39 0.41 0.40 uV  (Vf = 2.12 V)

EL204UBD (blue):
#1 @ 1mA:  4.6 4.5 4.6 4.5 4.6 uV
#1 @ 5mA:  3.2 3.2 3.2 3.2 3.2 uV
#1 @ 20mA: 2.8 2.8 2.7 2.7 2.7 uV
#2 @ 1mA:  4.4 4.4 4.3 4.2 4.3 uV  (Vf = 3.26 V)
#2 @ 5mA:  3.1 3.2 3.2 3.1 3.2 uV  (Vf = 3.44 V)
#2 @ 20mA: 2.9 2.8 2.8 2.8 2.7 uV  (Vf = 3.69 V)


Zeners:
---------------------------------------------------------
(All zeners of brand Temic.)

BZX55/C2V7 (0.5W 2.7V):
#1 @ 1mA:  1.1  1.1  1.1  1.1  1.1  uV
#1 @ 5mA:  1.0  0.88 0.85 0.86 0.87 uV
#1 @ 20mA: 1.0  0.81 0.72 0.72 1.1  uV
#2 @ 1mA:  1.2  1.1  1.1  1.1  1.1  uV  (Vr = 2.03 V)
#2 @ 5mA:  0.91 0.88 0.87 0.86 0.85 uV  (Vr = 2.50 V)
#2 @ 20mA: 1.1  0.80 0.77 0.73 0.71 uV  (Vr = 3.02 V)

BZX55/C5V6 (0.5W 5.6V):
#1 @ 1mA:  5.3  5.3  5.3  5.3  5.3  uV
#1 @ 5mA:  2.9  2.9  2.9  2.9  2.9  uV
#1 @ 20mA: 1.7  1.6  1.6  1.6  1.6  uV
#2 @ 1mA:  5.3  5.3  5.3  5.3  5.3  uV  (Vr = 5.68 V)
#2 @ 5mA:  2.9  2.9  2.9  2.9  2.9  uV  (Vr = 5.77 V)
#2 @ 20mA: 1.8  1.6  1.6  1.6  1.6  uV  (Vr = 5.81 V)

BZX55/C6V8 (0.5W 6.8V):
#1 @ 1mA:  16   16   16   16   16   uV
#1 @ 5mA:  21   21   21   21   21   uV
#1 @ 20mA:  5.8  5.5  5.5  5.5  5.6 uV 
#2 @ 1mA:  25   25   25   25   25   uV  (Vr = 6.93 V)
#2 @ 5mA:  13   13   13   13   13   uV  (Vr = 6.96 V)
#2 @ 20mA:  4.6  4.7  4.5  4.5  4.4 uV  (Vr = 7.00 V)
(Rechecked both devices due to their inconsistent 
behaviour for 1 and 5mA).

BZX55/C12   (0.5W 12V):
#1 @ 1mA:  0.35 0.37 0.37 0.39 0.39 uV
#1 @ 5mA:  0.30 0.28 0.28 0.28 0.30 uV
#1 @ 20mA: 0.24 0.25 0.25 0.26 0.25 uV
#2 @ 1mA:  0.32 0.33 0.32 0.33 0.32 uV  (Vr = 11.32 V) 
#2 @ 5mA:  0.26 0.26 0.27 0.32 0.26 uV  (Vr = 11.37 V)
#2 @ 20mA: 0.25 0.26 0.28 0.24 0.30 uV  (vr = 11.42 V)

BZX85/C2V7 (1.3W 2.7V):
#1 @ 1mA:  0.77 0.77 0.77 0.77 0.76 uV
#1 @ 5mA:  0.62 0.61 0.63 0.61 0.60 uV
#1 @ 20mA: 0.55 0.55 0.54 0.55 0.55 uV
#2 @ 1mA:  0.78 0.78 0.78 0.78 0.78 uV  (Vr = 1.30 V)
#2 @ 5mA:  0.62 0.62 0.61 0.62 0.62 uV  (Vr = 1.61 V)
#2 @ 20mA: 0.57 0.56 0.57 0.56 0.56 uV  (Vr = 1.92 V)

BZX85/C12 (1.3W 12V):
#1 @ 1mA:  0.49 0.53 0.48 0.50 0.52 uV
#1 @ 5mA:  0.54 0.55 0.58 0.46 0.48 uV
#1 @ 20mA: 0.44 0.35 0.38 0.36 0.33 uV
#2 @ 1mA:  0.42 0.43 0.46 0.48 0.41 uV  (Vr = 9.84 V)
#2 @ 5mA:  0.40 0.35 0.35 0.37 0.29 uV  (Vr = 9.89 V)
#2 @ 20mA: 0.34 0.33 0.31 0.30 0.31 uV  (Vr = 9.94 V)

-----------------
#1 @ 1mA:  
#1 @ 5mA:  
#1 @ 20mA: 
#2 @ 1mA:  
#2 @ 5mA:  
#2 @ 20mA: