I'm trying to simulate a low frequency Colpitts oscillator in LTSpice using an NPN darlington and a crystal, and I'm having a lot of trouble getting it to start up. Does anyone have a working one of these? I'm just using a capacitor to simulate the crystal, the parameters I am working with are something like:
motional capacitance: 5.3ff ESR = 11k ESI ~ 4500 henries EPR = 100 meg EPC = 1.5pf
Oh, and you can make simulated oscillators start faster by putting a current source in parallel with the inductor. Make it 1 uA or something, and drop it to zero near the start of the simulation. That's the SPICE equivalent of plucking a guitar string.
Try it with 1 nA when you're done, to check for startup problems.
Thank you. Usually most oscillators I design get going simply by setting the option "Start external supply voltages at 0 volts" or whatever - that gives them enough kick. It doesn't seem to be happening in this case.
Does the crystal stabilized Colpitts need any other modifications from the standard version? One can just swap the inductor for a crystal and it should work the same, right?
Select "skip initial operating point" in the transient analysis, or it will never start. Or goose it as Phil suggests.
XOs are terrible to sim in time domain. The sims take forever and there's no reasonable way to measure the frequency to PPM resolution. Once I get one running, I cut over to AC loop analysis to fine-tune things.
Post your netlist so people can play with it.
John Larkin Highland Technology, Inc
lunatic fringe electronics
If it doesn't start, the gain/phase criteria isn't met.
I don't speak buzzword (Colpitts, Pierce, etc) oscillators because they're rarely used with microchips... most of my clocks are simply a crystal around an 'HCU04 or equivalent, and my PLL VCO's are AGC'd differential configuration.... starting with my MC1648 from the mid '60's. ...Jim Thompson
| James E.Thompson | mens |
| Analog Innovations | et |
I'm inherently lazy... so I will spend days developing Spice gimmicks to save a few aggravating minutes time doing tweak, tweak, tweak... and Spice models... that's how I got into the Spice behavioral modeling game ;-)
Here's one of several "tanks" I have built into my symbol libraries...
I'm actually attempting to use the Darlington buffer inside a LM13700 section to build a Colpitts crystal oscillator, which will then be 2 quadrant multiplied by an external signal within the transconductance amp itself...