Saturday, May 5, 2012

RF isn't Black Magic

Most people in the Embedded Electronics world view discrete RF Design as "Black Magic". I would agree RF is complicated, but it isn't Black Magic. The magic is that when you are dealing with signals in the RF frequency range the "small things" matter; my little experiment below proved that to me yet again!

I was inspired by VK2ZAY's LC Test Oscillator video and wanted to build one of my own. After a couple hours playing with LTSPICE, I soldered up a board and powered it up expecting to see a 50MHz sine wave on the output and I got nothing...
Now is when the learning started! I first checked all my JFETs DC Biasing and I was getting the expected current through all of the source resistors, so I knew my JFETs were On and ready to start amplifying. I then started to suspect my inductor. I was just using a small power inductor meant more for switching power supplies than a RF Oscillator. After kicking open my RF Circuit Design Book (which is an excellent book on discrete RF design) I had a hand wound air core 220nH inductor made for the LC Tank circuit and still nothing.....

I started to suspect my low cost capacitors I bought from a Thailand based supplier, so I bought an LCR Meter and checked them. They all measured to the right value...... Why isn't my RF Osc. Oscillating?

The last thing I tried was changing the capacitor in the LC Tank circuit from a through hole ceramic cap to two 1206 surface mount 470pf caps in series to get 235pF. I powered up the circuit and there was a beautiful 24MHz signal showing up on my scope. So the surface mount cap in the LC Tank circuit fixed my dead RF Oscillator, but why?

Just for kicks I hooked up a  through hole 10-120pF variable capacitor in parallel with the two series 470pF caps thinking I should now be able to adjust the frequency of the LC Tank circuit a bit. When I adjusted the variable cap the frequency stayed at 24MHz... that is when the light bulb came on. The through hole caps I was using weren't working because their lead inductance was preventing them from being "pure" capacitance in the LC Tank circuit. It makes sense now, I am dealing with a small amount of inductance in the tank (220nH) and any added inductance in the capacitor's leads are preventing the capacitor from working in the Tank circuit.
This was a pretty fun project. I've build opamp based oscillators before, but there is something satisfying about building something up with only discrete Transistors and passives and getting it to work!

Here is a link to my LTSpice File


  1. For interest, you should try removing the inductor and replace it with a crystal of 24MHz to give you stability ans see if it still works

  2. Sorry this would effect the biasing due to the configuration, and an inductor would still be needed. Still fun stuff to look at though.