or
"More of the stuff they don't teach you in school."
It's one thing to model an oscillator. It's quite another to make it work. Making oscillators work sometimes seems like an arcane art, conducted in dark basements by strange, bearded old men, surrounded by boiling cauldrons of vile smelling potions and such.
This is not really true. I don't have a beard.
There are several things that can make an oscillator work, or work in a strange manner. I'll address just one of the common issues here....moding.
Any resonant circuit can exibit two (and sometimes MORE) modes of operation. The two fundamental modes are series and parallel resonance. If you were to look at a seemingly simple solenoid inductor...let's say one of 1mH or so...on a network analyzer swept between a few hertz and a few GHz....you would be AMAZED at what a rat's nest you'd see! There are MULTIPLE resonances, both series and paralel, some harmonically related, some not. This is primarily due to PARASITIC capacitance...the capacitance that naturally exists between turns of an inductor. Capacitors also exhibit some parasitic inductance, which can also be a problem at UHF frequencies, but generally less of an issue for most basic oscillator designs.
The main trick for successful oscillator design is to be sure the oscillator only operates in ONE mode. This can be especially difficult to do with CRYSTAL oscillators, where the Series and Parallel resonant modes may be separated by a minuscule fraction of the nominal frequency. However, the clue to taming a crystal can be applied to any oscillator.....by assuring that your DESIRED mode has the Highest Q. For a SERIES resonant oscillator....using either a crystal or an LC circuit, you achieve this by using LOW load impedances. A crystal with a HIGH load impedance is almost guaranteed to operate in the PARALLEL mode, so you want to be sure both the source and the load impedances of your crystal are LOW.
If you WANT parallel resonance, then the opposite is true...you want your source and load impedances to be very high. This, again, applies to both crystals and LC's.
If you have an oscillator that tends to jump around in frequency...the FIRST thing to look for is possible MODE jumping. ALTER your load and source impedances and see what happens.
End of lesson one.
eric
"More of the stuff they don't teach you in school."
It's one thing to model an oscillator. It's quite another to make it work. Making oscillators work sometimes seems like an arcane art, conducted in dark basements by strange, bearded old men, surrounded by boiling cauldrons of vile smelling potions and such.
This is not really true. I don't have a beard.
There are several things that can make an oscillator work, or work in a strange manner. I'll address just one of the common issues here....moding.
Any resonant circuit can exibit two (and sometimes MORE) modes of operation. The two fundamental modes are series and parallel resonance. If you were to look at a seemingly simple solenoid inductor...let's say one of 1mH or so...on a network analyzer swept between a few hertz and a few GHz....you would be AMAZED at what a rat's nest you'd see! There are MULTIPLE resonances, both series and paralel, some harmonically related, some not. This is primarily due to PARASITIC capacitance...the capacitance that naturally exists between turns of an inductor. Capacitors also exhibit some parasitic inductance, which can also be a problem at UHF frequencies, but generally less of an issue for most basic oscillator designs.
The main trick for successful oscillator design is to be sure the oscillator only operates in ONE mode. This can be especially difficult to do with CRYSTAL oscillators, where the Series and Parallel resonant modes may be separated by a minuscule fraction of the nominal frequency. However, the clue to taming a crystal can be applied to any oscillator.....by assuring that your DESIRED mode has the Highest Q. For a SERIES resonant oscillator....using either a crystal or an LC circuit, you achieve this by using LOW load impedances. A crystal with a HIGH load impedance is almost guaranteed to operate in the PARALLEL mode, so you want to be sure both the source and the load impedances of your crystal are LOW.
If you WANT parallel resonance, then the opposite is true...you want your source and load impedances to be very high. This, again, applies to both crystals and LC's.
If you have an oscillator that tends to jump around in frequency...the FIRST thing to look for is possible MODE jumping. ALTER your load and source impedances and see what happens.
End of lesson one.
eric