Colpitts oscillators

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There are. Some reading material:

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But you're wrong, Joerg... the crystal ISN'T in parallel resonance, it's detuned so that it looks like an inductance (at the OSCILLATOR resonant frequency).

...Jim Thompson

Yeah, I know what you mean -- Mathys belabors this point as well. While it's a legitimate one, my question is... what would you like to call a crystal oscillator that oscillates at a frequency where the tank presents (approximately) its highest impedance? The term "parallel resonant crystal oscillator" is indicative of electrically what's going on *overall*, even though, yeah, the crystal itself doesn't have a physical parallel resonance.

"Jim Thompson" wrote in message news: snipped-for-privacy@4ax.com...

Let me guess -- the guy lived down the street from you? :-)

Just kidding -- as far as I'm aware, Colpitts seems more popular than Butler only because it's older (better known) and might save a component or two... and of course because a lot of people really don't need as much accuracy out of their oscillators as they think they do (especially the digital guys... as a teen I was using those "drop crystal and two caps on these here two pins" microcontroller circuits, and I'm sure plenty of those weren't the world's most robust designs, but they did "just work" well enough...)

I got into this game way back when transistor performance was marginal. A 2-transistor Butler with a harmonic selecting tank just simply works every time, and provides for an easy logic-level output.

...Jim Thompson

From a certain point of view, yeah, it's theft, in that money is taken from you by force (or at least threat of being tossed in jail...). However, you are free -- at least in this country -- to denounce your citizenship, leave and find yourself an uninhabited island and live 100% tax-free. I suppose that if you win the lottery or something (and go for the "lump sum" payout) this might be a viable option. :-)

Most people -- including me -- don't think it's inherently unreasonable for the government to take some of their money in return for the services provided. The sticky point, of course, is whether or not people feel the services they're getting justify the amount taken... or at least that they feel the money is being spent reasonably efficiently...

Who's going to pay to keep them institutionalized? Oh yeah, taxes again... crud...

That, and the kewl blue unis, was why I picked the AF. :-)

Cheers! Rich

There's a simple answer, actually.

Just change the rules so that the only ones who have to pay any given tax are the ones who voted for it. >;->

Cheers! Rich

AN 419.

I usually use dual rails, but with we wrote this app note AD wanted a single supply solution.

Steve Swift

I know people call some oscillator circuits "parallel resonant". But even there the crystal does not oscillate at its highest impedance point. That frequency is usually referred to as the crystals anti-resonance frequency. No significant energy would passed through the crystal at this frequency. But yet this is necessary for all these socalled "parallel resonance" oscillators. The work somewhat above series resonance, but way below the anti-resonance frequency. I just mentioned it because the terminology is misleading. The crystal merely operates a bit higher in frequency because its load is designed to be capacitive. The fF motional capacitance is just too dominant.

Joop

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Plenty I guess. That's why I made the remark. The term just does not decribe what is actually taking place. See it as a (perhaps futile) attempt to stop people using the phrase "crystal in its parallel mode".

Joop

Yes, agreed.

I buy that.

OK... but the frequency difference between anti-resonant and resonant points is going to be some single-digit kHz typically, won't it? So the crystal operates "somewhere" between those two frequencies, giving, say,

Then, why do they call it "parallel resonant mode"? Crystals can be run at their series resonance as well; maybe you're denying parallel resonance because there's no traditional LC tank?

Thanks, Rich

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But they've got to try. A friend of ours has stage 4, also colon, spread to the outside. After lots of chemo she is actually improving, to the surprise of her oncologist. You can never know.

The capacitors form an impedance matching divider. You always want the impedance seen by the EB junction of the transistor to be much less than the transistors impedance. This ensures that the operation is determince by the inductors and capacitors and not by the transistor.

If you aren't using AGC, you are always making the transistor or something else distort to provide the limiting. If you do use AGC, you can make the transistor run in the linear mode buy setting the AGC's "set point" to a low enough amplitude.

No, experience teaches you what not to try. The theory lets you compute the ideal part values. Then, real life requires you to trim from that point because the parts are never ideal.

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It should be noted that a fair number of graduates who enter the Ph.D. program do the same.

One of the quickest ways to upset a friend in college was to ask him/ her what she planned to do for his/her grad-level. I learned after a while that it was simply a question you simply didn't ask (most) people. The the thought that they might actually have to generate original ideas was greatly disturbing.

-Le Chaud Lapin-

That's an interesting way of looking at it; thanks for the insight.

---Joel

Yes that PDF seems to be correct. It is just a bit wide with respect to pointing out the area of where "parallel resonance" takes place.

For fun I have put together a little simulation which might give some insight.

The same crystal is used three times. It is a 10.24MHz "parallel mode" xtal of which I measured the motional parameters.

The first section shows a typical Pierce "parallel resonance oscillator" feedback network (a typical microcontroller oscillator). Assuming an ideal amplifier for now, the oscillator will work at it

The second section is the bare crystal. A series resonant oscillator will work close to the frequency where I(C6) peaks.

The third section is the same as the second, but I put a capacitor in series to shift the frequency up a bit. I have selected 15pF since this corresponds closely to 2 x 33pF in series. Note it series resonance frequency is the same as the "parallel resonance point" of the Pierce circuit. Surprise? I think not. The crystal does not change "mode". It simply is the same resonator as it always has been. It just acts inductive at frequencies a bit above its resonance frequency, which happens when the load it sees in an oscillator circuit is capacitive.

Joop