More... Reverse Biased Diodes

I am still looking for some other options for a better method of handling my tuning for my next QRSS prototype. I decided to revisit the bread board and take some additional measurements.

I was wondering if a series red LED + 1N5817 (both are reversed biased) might give me the range that I need within a 0 to 5vdc window. (I will be using a PIC 16F628A for pulse width modulation so 5vdc is my upper range.)

The results are promising based upon the graph below. My design goal is about 12-20pF. I think that I need about 18pF based upon some earlier prototypes.

This picture shows you what a pain it is try to measure these parts and then swap out new combinations. At some point I will design a little prototyping adapter for my LC meter  so that I can do this right on the front of the meter without any jumpers.

I tried one other combination that provided a very granular range.
Reverse biased in series 1N5817 + Red Led + 1N5817
0-5vdc = 6-18pF

I have four different variations that I have tested over the past couple of days that will get me into the ball park to differing degrees. (granular tuning vs. tuning range vs. parts count)
  1. Reverse Biased 1N5817... a bit too much capacitance by it's self
  2. Parallel 10-30pF variable cap + rev biased Red LED
  3. Series rev biased 1N5817 + rev biased Red LED
  4. Series rev biased 1N5817 + rev biased Red LED + rev bias 1N5817
Option 2 provides the best flexibility
Options 3 & 4 provides more granular tuning

I will need to ponder this a bit to determine which path I like the best.

73 de NG0R