Forums » General Electronics Discussion

Hi guys,

I recently bread boarded a Colpitts oscillator using a 7MHz crystal, it works great on the scope and I get a really nice sine wave on the scope so I thought I'd try my luck on circuit lab but I haven't had too much luck. I was wondering if there was a crystal component because I just made a crystal equivalent circuit but it hasn't given me anything. Could someone check this out. I'm following the exact same schematic as the AARL Experimental methods in RF designs book has.

-Dan

Check your capacitor values from base to ground. I'm pretty sure you want like 390pf. not 390ppppf.

Also check the values in the crystal circuit. 15mH and 15F are not going to resonate a 7MHz.

Oh, I see, you have a bunch of unit errors, did you think you could use SPICE units?

Follow the CircuitLab unit rules and things will act better. Those 390pF caps need to be entered as "390p". Also the big capacitor in the crystal should be 0.015p.

Also, CurcuitLab doesn't model any noise, so oscillators have a hard time starting up. Try coupling a 1KHz 0.01 volt square wave through a 5pF capacitor to the emitter.

Given that, the circuit does oscillate, but the results are very fuzzy and funny-- CircuitLab may be modelling a VHF oscillation. Try putting a 20 ohm resistor in the base and maybe in the emitter leads.

Aha! it needs to have the power hooked up! And the right units!

Now it oscillates: See:

https://www.circuitlab.com/circuit/77drea/colpitts/