#4
---
I realized how much fun oscillators are and how Sine waves are so beautifulso I decided I'm going to up it a notch and take a shot at a 100 MHz oscillator.
I tried a Colpitts variation of the Hartley oscillator I posted earlier with the same MPF102 JFET, a diode, 1 MOhm Res, the tank, and its surrounding capacitors. Inductors and Caps had to get much smaller. So L is in the nH range and C is in the low pF.
Couldn't get the thing to oscillate at all no matter how hard I tried. I tried too many things to keep track of. I was suspecting the inductance is solow and the inductor Q is inadequate. I tried different types of air-core wire turns. Eventually I moved to the Toroid version but no luck.
Next, I decided a JFET isn't the thing for this and decided to move to BJT.I used a 2N3904. I played a lot with the number of turns and capacitor values until I reached a circuit similar to this:
https://www.dropbox.com/s/fp3dv13ahmu6pzh/Collpits.asc
My L is 3-5 turns on a 68-6 toroid. The value on the circuit is just a guess based on the frequency I'm getting. I had the following observations:
- I had a big 300p cap for C2 this killed the ability to oscillate (later Irealized it had to do with the amount of feedback. If you reverse C1 and C2 however, simulation shows the voltage swing is low. I am guessing it has to do with transistor gain. Increasing C1 should probably also mean decreasing R3. I am taking shots in the dark here, I will try to do some analysis later).
- Even with C2=300p, increasing C3 to 66p restored oscillation.
- After reducing C3 back to 33p, I was able to reduce C3 back to 33p and get oscillation.
- I kept on trying to reduce inductance to increase frequency but that camewith a price of reducing the voltage swing at the tank.
- I still do not have a unified theory of how the values of L and Cs affectfrequency and voltage swing but it seems I should strive for having higherL for better stability and voltage swing. This means I need to reduce the C's to as low as I can get.
- In the end I was able to reach 67 MHz as the max I could squeeze out of my circuit but it's not very stable.
- Even with everything laid out on a perf-board and with no power supply bypass you still can see 67 MHz of oscillation that is clean looking on a scope. Stability is another matter.
- Soldering and de-soldering is a pain when you have try different things.
- a plain vanilla 2N3904 can go long ways!
- I don't like JFETs. I love BJTs.
-------------------------------------------------
Next steps:
- Add more turns to my toroid
- Try a better a toroid core, may be a 2 or 6 Mix
- Reduce caps to the minimum possible.
- Hope for 100 MHz.