Add multichannel LED driver post

2 files changed, 889 insertions, 6 deletions

diff --git a/content/posts/led-characterization/images/photodiode_sensitivity.svg b/content/posts/led-characterization/images/photodiode_sensitivity.svg
new file mode 100644
index 0000000..e845444
--- /dev/null
+++ b/content/posts/led-characterization/images/photodiode_sensitivity.svg
@@ -0,0 +1,874 @@
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diff --git a/content/posts/led-characterization/index.rst b/content/posts/led-characterization/index.rst
index 0f6e32b..ebcc16c 100644
--- a/content/posts/led-characterization/index.rst
+++ b/content/posts/led-characterization/index.rst
@@ -103,14 +103,13 @@ over the problem, there are several sources for imperfections:
         <figure class="side-by-side">
             <img src="images/driver_ringing_strong.jpg" alt="Strong ringing on the LED voltage waveform edge at about
             100% overshoot during about 70% of the cycle time.">
-            <figcaption>The shift register logic output of the multichannel LED driver directly driving a small mosfet's
-            gate through an inch or so of PCB trace caused extremely bad ringing at high driving
-            frequencies.</figcaption>
+            <figcaption>The LED strip being at the end of a couple meters of wire caused extremely bad ringing at high
+            driving frequencies.</figcaption>
         </figure><figure class="side-by-side">
             <img src="images/driver_ringing_weak.jpg" alt="Weak ringing on the LED voltage waveform edge at about 30%
             overshoot during about 20% of the cycle time.">
-            <figcaption>Adding a resistor dampened the ringing somewhat, but ultimately it cannot be eliminated
-            entirely.</figcaption>
+            <figcaption>Adding a resistor in front of the MOSFET gate to slow the transition dampened the ringing
+            somewhat, but ultimately it cannot be eliminated entirely.</figcaption>
         </figure>
     </figure>
 
@@ -371,6 +370,17 @@ The photodiode's response is strongly wavelength-dependent. In particular in the
 gets very poor down to about 20% at the edge to ultraviolet. This effect is strong enough to move the apparent location
 of the blue peak towards red.
 
+.. raw:: html
+    
+    <figure>
+        <img src="images/photodiode_sensitivity.svg" alt="A plot of photodiode sensitivity against wavelength relative
+        to peak sensitivity at 820nm.  The sensitivity rises from 20% at 380nm approximately linearly to 80% at 620nm,
+        then the rise rolls off.">
+        <figcaption>A plot of the photodiode's relative sensitivity in the visible spectrum. The sensitivity is
+        normalized against its peak at 820nm.
+        </figcaption>
+    </figure>
+
 The problem is that in order to remove this non-linearity, we would already have to know the wavelength of the measured
 light. Since I don't, I settled for a two-step process. First, a coarse wavelength calibration is done relative to the
 red peak and the short-wavelength edge of the blue peak. The photodiode measurements are then sensitivity-corrected
@@ -390,7 +400,6 @@ wavelength in nanometers.
         </figcaption>
     </figure>
 
-
 .. FIXME re-do these measurements, avoiding clipping
 .. FIXME re-do calibration using CCFL
 .. FIXME calibration for brightness imbalance due to wedge-shaped projection of spectrum