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...
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.
Here is a link to my LTSpice File